RF amplifier on a chip. High-frequency amplifiers on microcircuits. Fig.10. RF amplifier stage

Ministry of Education of the Russian Federation

Moscow Institute of Physics and Technology
(State University)

Department of Radio Engineering

Bipolar RF Amplifier
transistor

Laboratory work
on the course of radio engineering

Moscow 2003

UDC 621.396.6

Bipolar transistor radio frequency amplifier.
Laboratory work at the rate of Radio Engineering / Comp.
. - M.: MIPT, 2003. - 24 p.

state university), 2003

1. Introduction 4

2. Cascade on a bipolar transistor with OE 5

2.1. Schematic diagrams of the cascade 5

2.2. Parameters and characteristics of cascade 6

2.3. Selection of cascade parameters in multichannel
amplifier 11

3. Self-excitation URC 13

4. Cascode 15

4.1. Schematic diagrams 15

4.2. Parameters and characteristics of circuit 16

5. Experimental evaluation of the output and input
impedance cascade URF 17

6. Task 19

6.1. Circuits under study 19

6.2. Calculation of cascades 20

6.3. Measurements and research 21

References 23

1. Introduction

Radio frequency amplifiers (URCh) are widely used in various devices. Most often they are used as input blocks of radio receivers for frequency filtering of a useful signal from interference and increasing its amplitude. In such cases, the central frequency of the signal spectrum, as a rule, significantly exceeds the spectrum width, and then the URF acts as an active bandpass filter. There are a significant number of schemes similar to the URF, containing a different number of amplifying elements and frequency-selective circuits. The RFP may contain a single stage, or it may be multi-stage.

URC is usually described by the following parameters and characteristics:

– resonant (central) frequency of the amplified section of the input voltage spectrum,

– resonant gain https://pandia.ru/text/78/219/images/image003_71.gif" width="23" height="23 src=">

– bandwidth https://pandia.ru/text/78/219/images/image005_58.gif" width="40" height="23">

– input impedance https://pandia.ru/text/78/219/images/image007_51.gif" width="81" height="21">

– output impedance https://pandia.ru/text/78/219/images/image009_42.gif" width="97" height="21">

– amplitude-frequency and phase-frequency characteristics (AFC and PFC).

The purpose of this laboratory work is to theoretically study, calculate, assemble on an individual board and experimentally investigate the simplest variants of the RF. These are a resonant cascade on a bipolar transistor connected according to a common emitter (CE) circuit, a casco circuit on two transistors with one oscillatory circuit, and a two-stage UFC formed by a series connection of the named cascades.

2. Cascade on a bipolar transistor with OE

2.1. Schematic diagrams of the cascade

On fig. 1a) shows a circuit diagram of the cascade of a resonant amplifier based on a bipolar transistor with an OE with a partially switched-on circuit as a collector load and with a series supply of the collector circuit. On fig. 1b) a diagram of a similar cascade with a parallel supply of the collector circuit is given.

https://pandia.ru/text/78/219/images/image012_34.gif" width="21" height="25"> into the power source, and the variable component is sent past the source through the capacitor. This reduces unwanted feedback between several cascades of URF, powered from a single source As an impedance, a choke (a coil with a large inductance), a resistor, or a series connection of a choke and a resistor is used.

2.2. Parameters and characteristics of the cascade

The parameters and characteristics of any radio engineering device that describe its properties are usually found by compiling and analyzing the equivalent circuit of this device. For the RF cascade, we use an equivalent circuit for alternating current, containing models of the signal source, RE, and load. Let's imagine the signal source as a simple voltage generator with EMF and internal resistance ..gif" width="125" height="24 src="> where R– loop switching coefficient, – equivalent loop resistance, – generalized frequency detuning, https://pandia.ru/text/78/219/images/image024_23.gif" width="17" height="13 src=">1 - intrinsic quality factor of the circuit, is the resonant frequency, is the loop loss resistance connected in series with the inductance .

Let us first describe the properties of the cascade with an ideal transistor, in which the -parameters do not depend on the frequency and are equal: and https://pandia.ru/text/78/219/images/image033_15.gif =">.jpg" width="397" height="85 src=">

Based on the analysis of this scheme, it is easy to show that the considered cascade has:

– resonant frequency https://pandia.ru/text/78/219/images/image037_13.gif" width="99" height="43"> (1)

where is the transconductance of the transistor,

– resonant gain https://pandia.ru/text/78/219/images/image044_11.gif" width="91" height="23 src=">

is the input impedance

– output impedance to the left of points https://pandia.ru/text/78/219/images/image048_11.gif" width="73" height="23 src=">

– AFC and PFC are given by the dependences of the modulus and argument of expression (1) on frequency.

At the same time, for a real transistor, the -parameters depend on the frequency. In this work, we will take into account only the so-called first approximation of this dependence, which is valid for frequencies not exceeding several values of the upper limiting frequency of the current amplification of the transistor and has the following form:

https://pandia.ru/text/78/219/images/image053_10.gif" width="156" height="45 src=">

https://pandia.ru/text/78/219/images/image055_10.gif" width="157" height="45 src=">

Here is the time constant of the forward https://pandia.ru/text/78/219/images/image058_8.gif" width="128" height="23"> is the volume resistance of the base, is the time constant of the reverse base-collector transition. This approximation corresponds to the physically clear U-shaped equivalent circuit of the transistor (Giacolletto's circuit), when using it, the equivalent circuit of the cascade takes the form shown in Fig. 3.

In this circuit, in the frequency range of using the URF, you can ignore the resistor https://pandia.ru/text/78/219/images/image063_9.gif" width="32" height="23 src=">. KT315 at a frequency of 1 MHz, a capacitance of the order of three picofarads has an impedance of 50 kOhm, and the value is units of MΩ..gif" width="49" height="23">

In view of the above, the results of the analysis of the circuit shown in Fig. 3 are reduced to the following.

The output conductivity of the part of the cascade located to the left of the K line – E, found, for example, as a result of using the Norton theorem, is equal to

https://pandia.ru/text/78/219/images/image067_8.gif" width="181" height="47 src=">

Therefore, the output circuit of the cascade in this case is shunted by the resistor output resistance of the transistor and the output capacitance, the values \u200b\u200bof which depend on the parameters of the transistor, the output resistance of the signal source and frequency..gif" width="25" height="23 src=">.gif" width ="43" height="21"> we have the order of tens of kOhm and the order of several, and with the order of units of kOhm we get the order of (fractions-units) kOhm, and https://pandia.ru/text/78/219/images/image063_9 .gif" width="32" height="23">.

From the equivalent circuit of the cascade shown in Fig..gif "width="32" height="23">, is equal to where is the impedance of the loaded output circuit " width="136" height="23 src=">.gif" width="29" height="23 src="> Multiplying and dividing the expression for by the complex expression, we get where ,

It follows that the input impedance of the cascade between the B–E points is given by the circuit shown in Fig. 4a), where https://pandia.ru/text/78/219/images/image090_6.gif" width="19" height="21 src="> is a parallel connection of resistances and https://pandia.ru/ text/78/219/images/image094.jpg" width="265" height="97">Fig. four

For transistors with very low resistances, the elements and are practically the input resistive resistance of the entire cascade. In the case of large values or in the presence of an additional resistor https://pandia.ru/text/78/219/images/image100_5.gif" width="45 height=15" height="15"> the cascade parameters can be found by appropriate recalculation circuit shown in fig. 4a), into the circuit shown in fig. 4b), according to the formulas

https://pandia.ru/text/78/219/images/image102_5.gif" width="184" height="43 src=">

where

(In passing, we note that for the frequencies< относительная расстройка имеет знак минус и величина сопротивления https://pandia.ru/text/78/219/images/image010_42.gif" width="24 height=17" height="17">-circuit of any previous stage by the input impedance of the subsequent stage, the resonant frequency and gain of the shunted stage fall, and the bandwidth expands. At the same time, a properly designed cascade should provide the specified values of the entire amplifier and the maximum gain of each cascade https://pandia.ru/text/78/219/images/image108.jpg

To provide the required bandwidth of the cascade, the quality factor of its loaded circuit should be equal to https://pandia.ru/text/78/219/images/image111_4.gif" width="61" height="23 src=">.gif" width ="20" height="23 src="> must satisfy the condition

https://pandia.ru/text/78/219/images/image114_3.gif" width="20" height="21"> and coefficients of connection to the circuit from the output side of the UE stage and from the input side of the 2nd stage, respectively .

The resonant amplification of the cascade from its input to the input of the 2nd cascade is equal to

. (3)

From expressions (2) and (3) under the condition, the required (optimal) values of the connection coefficients are found

https://pandia.ru/text/78/219/images/image119_4.gif" width="101" height="28 src="> (4)

where

A cascade with these connection factors is sometimes called optimally matched. The value of the maximum resonant amplification of the matched stage is equal to

https://pandia.ru/text/78/219/images/image124_4.gif" width="133" height="43 src=">.gif" width="176" height="43 src=">. gif" width="103" height="24"> - the total capacitance of the circuit, providing the resonant frequency of the cascade, equal to the inductance of the coil of the circuit. From these relationships, we obtain the following formulas for determining the capacitance values and https://pandia.ru/text/78 /219/images/image133_3.gif" width="119" height="24">

3. Self-excitation URC

Self-excitation of the UFC occurs when there is a positive feedback in it. There are three channels for such communication. One of them is the connection of cascades through a common power source To reduce this connection, the cascades are “uncoupled” using filter elements and https://pandia.ru/text/78/219/images/image138_3.gif" width="41" height ="23 src=">

Let us consider the conditions under which the self-excitation of the UFC arises precisely because of the named capacitance. For the first time they were found by the Russian scientist Vladimir Ivanovich Siforov back in the era of tube radio engineering. showed that a single cascade of a resonant URF can only be excited if there is an inductive component in its input impedance. Such a component appears, for example, in the presence of a second oscillatory circuit at the input of the cascade. A similar situation arises in a multi-stage UFC, in which the role of the input circuit of each cascade, starting from the second, is played by the output circuit of the previous cascade.

On fig. 6 shows a simplified equivalent circuit of a cascade with two identical circuits, which are represented in it by two-poles with impedances these circuits (taking into account their shunting by the transistor). UE is represented by a current generator. Capacitance is the throughput capacitance of the cascade.

Let's break the wire of the circuit at a point and apply a harmonic input voltage to the cascade Under the influence of the sum of the input and output voltages, a feedback current will flow through the through capacitance. At high cascade gains, the contribution of the input voltage can be neglected and assumed that https://pandia.ru/text/78/219/images/image148_2.gif" width= "29" height="21 src="> If the initial voltage phases and are equal, and the amplitude of the connection voltage exceeds the amplitude https://pandia.ru/text/78/219/images/image150_2.gif" width="111" height="23">when we also have At this frequency, the impedances of both circuits are inductive. If at the resonant frequency the voltages and are in antiphase (the shift is equal to https://pandia.ru/text/78/219/images/image154_2.gif" width="17" height="21"> the phase shift between voltages and is already the phase shift between the vectors and is equal to and the shift between the vectors and is equal As a result, the phase shift between the voltages and turns out to be equal to zero, that is, the feedback becomes purely positive. point https://pandia.ru/text/78/219/images/image160_2.gif" width="21" height="24">

For the stability of the URF, it is necessary that the voltage amplitude be less than the voltage amplitude

Thus, expression (7) indicates ways to combat self-excitation due to the presence of the pass capacitance of the RE. These are the corresponding limits on the quantities and

4. Cascode scheme

4.1. Schematic diagrams

The cascode circuit is designed to increase the self-excitation resistance of the URCH, which is achieved by a significant decrease in its throughput capacitance compared to the minimum achievable throughput capacitance of a separate UE. Examples of cascode circuits with serial and parallel DC power supply are given in fig. 7.

4.2. Parameters and characteristics of the circuit

As can be seen from these figures, the AC load of the 1st transistor connected according to the OE circuit is the input impedance of the 2nd transistor connected according to the common base (CB) circuit. Since the value of such an impedance is very small compared to the output impedance of the 1st transistor (then the 1st transistor of the cascode circuit practically operates in the short circuit mode at its output, and the 2nd transistor operates in idle mode at its input. In addition, we have

If we now consider both transistors of the cascode circuit as a single RE, then under the indicated conditions, its -parameters are related to the similar parameters of the 1st and 2nd transistors by the following relations

https://pandia.ru/text/78/219/images/image171_2.gif" width="32" height="23 src=">.gif" width="55" height="23 src=">

https://pandia.ru/text/78/219/images/image175_2.gif" width="96" height="23 src=">.gif" width="21" height="23"> cascode scheme, estimating the degree of feedback through the through capacitance turns out to be much less than that of a single transistor connected according to the OE circuit. This makes the cascode circuit more resistant to self-excitation.

In addition, due to the smallness of the value, the input impedance of the cascode circuit is equal to the 1st transistor,
and the output impedance is https://pandia.ru/text/78/219/images/image180_2.gif" width="37" height="21">.gif" width="19 height=21" height="21 "> unloaded circuit. The theoretical calculation of these quantities is cumbersome and inaccurate; therefore, we will describe the procedure for their experimental evaluation.

Values https://pandia.ru/text/78/219/images/image075_6.gif" width="33" height="21 src=">.gif" width="27" height="23 src="> determined by the total capacity of the circuit + FROM P, where FROM P is the known capacitance of a suspended capacitor pre-installed in the circuit. We calculate the value according to the formula

https://pandia.ru/text/78/219/images/image182_2.gif" width="72" height="43 src="> (8)

where .

Capacity change FROM P we tune the cascade to the required resonant frequency and measure its bandwidth. After that, we connect the capacitive branch of the circuit to the collector of the transistor partially, as shown in the equivalent circuit of this inclusion in fig. 8a). At the same time, we choose the capacitance values and so that, taking into account the known capacitance, the resonant frequency of the cascade is https://pandia.ru/text/78/219/images/image186_2.gif" width="15" height="16">= ( 0.2–0.8) In the linear mode of operation of the cascade, we measure its bandwidth;

d) assuming that for all transistors h 21E = 100, find their initial base currents I bn = I book / h 21E,

e) select the current flowing through the voltage divider, composed of resistors R 1 and R 2, equal I d = (50–100) I bn, find values R 1 and R 2, taking into account also the condition that the base potential of the transistor VT3 relative to the ground should be equal to ( U ken + 0.6 V),

f) find the values R R, R b1, R f, R b2.

6.2.2. AC calculation:

a) take two coils with equal inductances (40–60) μH at the checkout, measure their inductances at https://pandia.ru/text/78/219/images/image024_23.gif" width="17" height="13 src="> L;

b) set the preliminary value of the coefficient of partial connection of the 1st circuit p= (0.25–0.33), determined by the ratio of its capacities;

c) calculate the capacitance values of both circuits;

d) choose the capacitance of the remaining capacitors of the circuit of the order of (0.01–1) μF, thereby ensuring the required smallness of their impedance at the resonant frequency.

6.3. Measurements and research

6.3.1. Study of single cascades

On the student's individual board, assemble a cascade on a transistor with an OE, connecting its circuit to the transistor completely, connecting points 3 and 4 using a coupling capacitor FROM R. Assemble the cascode circuit, leaving its input (point 6) free. Measure real values I book and U ken of both cascades and check their compliance with the specified values. If necessary, achieve compliance with an accuracy of (10–25)% by changing the values R b1, R b2, R 1 and R 2.

By connecting to the input of the 1st stage (points 1 and 2) a radio frequency harmonic voltage generator with an amplitude of not more than 20 mV, and to points 5 and 2 a voltmeter, measure the resonant frequency of this stage and check its compliance with the calculated value https://pandia.ru /text/78/219/images/image018_26.gif" width="20" height="21 src="> on the frequency response and phase response of the cascade on a transistor with an OE.

6.3.2. Study of a two-stage URF

Using the measurement results in clause 6.3.1, materials
2.3 and formulas (4)–(6), calculate the parameters of a matched cascade on a transistor with an OE loaded with a cascode circuit. In this case, the required bandwidth of the 1st stage should be set equal to its bandwidth when the circuit is fully turned on and in the absence of connection to the 2nd stage.

Assemble the described two-stage amplifier. In the presence of its self-excitation, take measures to eliminate the generation.

For a stable two-stage amplifier in the linear mode of its operation, measure the resonant gain and bandwidths of the 1st stage and the entire amplifier as a whole.

When preparing for the test at home and preparing the report:

a) master the derivation of calculation formulas (4), (8), (9)–(11),

b) compare the obtained values of all measured quantities with theoretically expected ones.

Bibliography

1. Fundamentals of radio electronics. - M .: Radio and communication, 1990.
2. , radio receivers. In 2 hours - M .: Sov. radio, 1961.–1963.

Laboratory work

High frequency amplifiers (UHF) are used to increase the sensitivity of radio receivers - radios, televisions, radio transmitters. Placed between the receiving antenna and the input of a radio or television receiver, these UHF circuits amplify the signal coming from the antenna (antenna amplifiers).

The use of such amplifiers allows you to increase the radius of reliable radio reception, in the case of radio stations (transceivers - transceivers) either increase the operating range, or, while maintaining the same range, reduce the radiation power of the radio transmitter.

Figure 1 shows examples of UHF schemes often used to increase the sensitivity of radio equipment. The values of the elements used depend on the specific conditions: on the frequencies (lower and upper) of the radio band, on the antenna, on the parameters of the subsequent cascade, on the supply voltage, etc.

Figure 1 (a) shows broadband UHF circuit according to the scheme with a common emitter(OE). Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

It should be recalled that in the reference data for transistors, limiting frequency parameters are given. It is known that when assessing the frequency capabilities of a transistor for a generator, it is enough to focus on the limiting value of the operating frequency, which should be at least two to three times lower than the limiting frequency indicated in the passport. However, for an RF amplifier connected according to the OE scheme, the limiting passport frequency already needs to be reduced by at least an order of magnitude or more.

Fig.1. Examples of circuits of simple high-frequency amplifiers (UHF) on transistors.

Radio elements for the circuit in Fig. 1 (a):

R1=51k(for silicon transistors), R2=470, R3=100, R4=30-100;
C1=10-20, C2=10-50, C3=10-20, C4=500-Zn;

Capacitor values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Transistor stages, as is known, connected according to the common emitter (CE) circuit, provide a relatively high gain, but their frequency properties are relatively low.

Transistor stages connected in a common base (CB) circuit have less gain than OE transistor circuits, but their frequency properties are better. This allows you to use the same transistors as in OE circuits, but at higher frequencies.

Figure 1 (b) shows broadband high frequency (UHF) amplifier circuit on a single transistor according to the scheme with a common base. In the collector circuit (load), the LC circuit is turned on. Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radio elements for the circuit in Fig. 1 (b):

R1=1k, R2=10k. R3=15k, R4=51 (for supply voltage ZV-5V). R4=500-3 k (for supply voltage 6V-15V);
C1=10-20, C2=10-20, C3=1n, C4=1n-3n;
T1 - silicon or germanium RF transistors, for example. KT315. KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for VHF frequencies. Capacitors such as KLS, KM, KD, etc.

Coil L1 contains 6-8 turns of PEV 0.51 wire, brass cores 8 mm long with M3 thread, tap from 1/3 of the turns.

Figure 1 (c) shows another broadband scheme UHF on one transistor, included according to the scheme with a common base. An RF inductor is included in the collector circuit. Depending on the transistor used, this circuit can be successfully applied up to frequencies of hundreds of megahertz.

Radioelements:

R1=1k, R2=33k, R3=20k, R4=2k (for supply voltage 6V);
C1=1n, C2=1n, C3=10n, C4=10n-33n;
T1 - silicon or germanium RF transistors, for example, KT315, KT3102, KT368, KT325, GT311, etc.

Capacitor and circuit values are given for MW, HF frequencies. For higher frequencies, such as the VHF band, the capacitance values must be reduced. In this case, chokes D01 can be used.

Capacitors such as KLS, KM, KD, etc.

Coils L1 - chokes, for the SV range it can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Larger gain value can be obtained through the use multi-transistor circuits. These can be various circuits, for example, based on the OK-OB cascode amplifier based on transistors of different structures with series supply. One of the options for such a UHF scheme is shown in Fig. 1 (d).

This UHF scheme has significant amplification (tens and even hundreds of times), but cascode amplifiers cannot provide significant amplification at high frequencies. Such schemes, as a rule, are used at the frequencies of the LW and MW bands. However, with the use of microwave transistors and careful design, such circuits can be successfully used up to frequencies of tens of megahertz.

Radioelements:

R1=33k, R2=33k, R3=39k, R4=1k, R5=91, R6=2.2k;
C1=10n, C2=100, C3=10n, C4=10n-33n. C5=10n;
T1 - GT311, KT315, KT3102, KT368, KT325, etc.
T2 - GT313, KT361, KT3107, etc.

Capacitor and circuit values are for MW frequencies. For higher frequencies, such as the HF band, the capacitance values and the loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the MW range contains 150 turns of PELSHO 0.1 wire on 7 mm frames, trimmers M600NN-3-SS2.8x12.

When setting up the circuit in Fig. 1 (d), it is necessary to select resistors R1, R3 so that the voltages between the emitters and collectors of the transistors become the same and amount to 3V at a circuit supply voltage of 9 V.

The use of transistorized UHF makes it possible to amplify radio signals. coming from antennas, in television ranges - meter and decimeter waves. In this case, antenna amplifier circuits built on the basis of circuit 1(a) are most often used.

Antenna amplifier circuit example for frequency range 150-210 MHz shown in Fig. 2 (a).

Fig.2.2. Scheme of the antenna amplifier of the MV range.

Radioelements:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470, R9=110, R10=75;
C1=15, C2=1n, C3=15, C4=22, C5=15, C6=22, C7=15, C8=22;
T1, T2, TZ - 1T311(D, L), GT311D, GT341 or similar.

Capacitors such as KM, KD, etc. The frequency band of this antenna amplifier can be expanded in the low-frequency region by a corresponding increase in the capacitances that make up the circuit.

Radio elements for the antenna amplifier option for the range 50-210 MHz:

R1=47k, R2=470, R3=110, R4=47k, R5=470, R6=110. R7=47k, R8=470. R9=110, R10=75;
C1=47, C2=1n, C3=47, C4=68, C5=47, C6=68, C7=47, C8=68;
T1, T2, TZ - GT311A, GT341 or similar.

Capacitors such as KM, KD, etc. When repeating this device, all requirements must be met. required for installation of high-frequency structures: minimum lengths of connecting conductors, shielding, etc.

An antenna amplifier designed for use in the ranges of television signals (and higher frequencies) can be overloaded with signals from powerful MW, HF, VHF radio stations. Therefore, a wide bandwidth may not be optimal, since this may interfere with the normal operation of the amplifier. This is especially true in the lower region of the operating range of the amplifier.

For the circuit of the reduced antenna amplifier, this can be significant, because the slope of the gain decay in the lower part of the range is relatively low.

You can increase the steepness of the amplitude-frequency characteristic (AFC) of this antenna amplifier by using 3rd order high pass filter. To do this, an additional LC circuit can be used at the input of this amplifier.

The diagram for connecting an additional LC high-pass filter to the antenna amplifier is shown in fig. 2(b).

Additional filter parameters (indicative):

C=5-10;
L - 3-5 turns of PEV-2 0.6. winding diameter 4 mm.

It is advisable to adjust the frequency band and shape of the frequency response using appropriate measuring instruments (sweep frequency generator, etc.). The shape of the frequency response can be adjusted by changing the values of capacitances C, C1, changing the pitch between turns L1 and the number of turns.

Using the described circuit solutions and modern high-frequency transistors (microwave transistors - microwave transistors), you can build an antenna amplifier for the UHF range. This amplifier can be used both with a UHF radio receiver, for example, part of a VHF radio station, or in conjunction with a TV.

Figure 3 shows UHF antenna amplifier circuit.

Fig.3. UHF antenna amplifier circuit and connection diagram.

The main parameters of the UHF range amplifier:

Frequency band 470-790 MHz,
Gain - 30 dB,
Noise figure -3 dB,
Input and output resistance - 75 Ohm,
Consumption current - 12 mA.

One of the features of this circuit is the supply voltage to the antenna amplifier circuit through the output cable, through which the output signal is supplied from the antenna amplifier to the radio signal receiver - a VHF radio receiver, for example, a VHF radio receiver or TV.

The antenna amplifier consists of two transistor stages connected according to a common emitter circuit. At the input of the antenna amplifier, a 3rd order high-pass filter is provided, which limits the operating frequency range from below. This increases the noise immunity of the antenna amplifier.

Radioelements:

R1=150k, R2=1k, R3=75k, R4=680;
C1=3.3, C10=10, C3=100, C4=6800, C5=100;
T1, T2 - KT3101A-2, KT3115A-2, KT3132A-2.
Capacitors C1, C2 type KD-1, the rest - KM-5 or K10-17v.
L1 - PEV-2 0.8 mm, 2.5 turns, winding diameter 4 mm.
L2 - RF choke, 25 µH.

Figure 3 (b) shows the connection diagram of the antenna amplifier to the antenna jack of the TV receiver (to the UHF band selector) and to the remote 12 V power supply. In this case, as can be seen from the diagram, power is supplied to the circuit through the coaxial cable used and for transmitting an amplified UHF radio signal from an antenna amplifier to a receiver - a VHF radio or a TV.

Connection radio elements, Fig. 3 (b):

C5=100;
L3 - RF choke, 100 uH.

The installation is carried out on a double-sided fiberglass SF-2 by a hinged method, the length of the conductors and the area of the contact pads are minimal, it is necessary to provide for a thorough shielding of the device.

Establishing an amplifier is reduced to setting the collector currents of transistors and are regulated using R1 and R3, T1 - 3.5 mA, T2 - 8 mA; the shape of the frequency response can be adjusted by selecting C2 within 3-10 pF and changing the pitch between the turns of L1.

Literature: Rudomedov E.A., Rudometov V.E. - Electronics and espionage passions-3.

URC are active frequency-selective cascades of receivers operating at a fixed frequency or in a frequency range. They are used to ensure high sensitivity of radio receivers by pre-amplifying the signal and its frequency selection.

Basic requirements and quality indicators

1. Resonant voltage gain

Or in terms of power

where G in, G n - active components of the amplifier input and load conductances.

2. Frequency selectivity- mainly through the mirror channel of superheterodyne receivers (
).

3. URF noise figure, which largely determines the ability of the receiver to reproduce useful information at low levels of the received signal. From the point of view of the minimum noise level, it is sufficient that the power gain of the URF be at the level of 10-100, so the required number of stages usually does not exceed two.

4. Sustainability, characterizes the absence of self-excitation of the amplifier.

In addition, in terms of their performance, the RF amplifier should provide amplification of signals in a certain dynamic range with distortions not exceeding a given level.

Taking into account that the URC operates in the mode of amplifying weak signals, we will consider the amplifying device as a linear active 4-pole.

Resonant amplifier stage of moderately high frequencies

In the range of moderately high frequencies ( f < 300 MHz) to describe the properties of amplifying stages, it is convenient to use the system Y-parameters, in which the equation of a linear 4-terminal network is written in the form (5.1)

(5.1)

where , and ,- voltages and currents at the input and output of the 4-terminal network, respectively,

- parameters in the short circuit mode at the input and output of the 4-terminal network.

The most general scheme of the resonant cascade can be represented as (Fig. 5.1).

The figure shows a diagram of a resonant amplifier, in which the circuit LC partially connected as transistor output VT 1, so the input of the next stage on the transistor VT 2 . In both cases, autotransformer coupling is used. However, in such an amplifier, these connections can be implemented in another known way, for example, by a transformer.

Elements R 1 , R 2 , ,are used to set the mode of operation of the active element VT 1 by direct current. Necessary filtering by nutrition is carried out by a filter R f , C f . The calculation of these elements is carried out in the same way as it is done for aperiodic amplifiers. Therefore, the issues of setting the operating point of resonant amplifiers are not considered here.

Regardless of the type of connection of the amplifying device with the resonant circuit, the resonant amplifier can be represented as the following equivalent circuit (Fig. 5.2).

It follows from the presented equivalent circuit that

(5.2)

When using a double autotransformer coupling, the load conductance can be represented as

, (5.3)

where,
.

The voltage gain can be obtained using expressions (5.1) and (5.2). Given these expressions, one can obtain

(5.4)

From the last expression, one can get

(5.5)

Where do we get

, (5.6)

where is the total equivalent conductivity of the circuit.

The resonant properties of the cascade are determined by the frequency response of the conduction
, and the latter corresponds to the resonant characteristic of the oscillatory circuit LC. The equivalent resistance of the oscillatory circuit included in the collector circuit of the transistor can be represented as follows

Total equivalent loop resistance
can be imagined

, (5.8)

where
-generalized detuning of the contour.

The stage gain at the resonant frequency can be represented as

, (5.9)

where
.

- transformation ratio from the output of the first active element to the input of the next one.

With this in mind, for the resonant cascade, we obtain the following expression for the gain

(5.10)

In terms of structure, the resulting formula corresponds to the formula for determining the gain of an aperiodic cascade, only the resonant circuit is used as a load in the latter.

Aperiodic URF increase only the signal-to-noise ratio and the sensitivity of the receiver. Most often they are used in direct amplification transistor receivers in the LW and MW bands; As a load of aperiodic RFP can

Fig.9. Schemes of aperiodic cascades of radio frequency amplifiers:

a) - resistor; b) - transformer.

serve as a choke, resistor or transformer. Resistor cascade URF (Fig. 9. a ) is easy to implement and configure. In transformer URCh (Fig. 9.b ) makes it easier to match the output of one stage with the input of the next. In addition, the URF transformer stage can be easily converted into a reflex one.

Resonant RF amplifiers provide signal amplification and increase not only real sensitivity, but also selectivity in the image channel. Transistor resonant URF in the ranges of DV, SV and KB are assembled according to the scheme with OE (Fig. 10 ), and in the VHF band - according to the scheme with OB.

URF cascades may contain one or two resonant circuits. A single loop RF amplifier gives less gain but is easier to manufacture and tune. Circuits with inductively coupled loops allow you to change the coupling and get the most gain or better selectivity. By changing the connection over the range, it is possible to somewhat compensate for the uneven transmission coefficient of the input circuits.

Amplifiers of the radio frequency of the VHF band are performed according to cascade schemes. They have better characteristics than conventional URF.

In terms of gain, a cascode amplifier is equivalent to a single-stage amplifier with the forward conductance of the first transistor and the load of the second. The cascode circuit is used in meter waveband amplifiers. It is advantageous to perform the first stage of the circuit on a field-effect transistor, which has a low noise level and low active input conductivity, while the selective receiver system connected at the input of the cascode amplifier will be shunted less. In the second stage, a drift transistor is preferred, which is switched on according to the scheme with ABOUT and providing the highest stable gain.

Fig.10. RF amplifier cascade.

With this implementation of the cascode amplifier circuit, its stable gain coefficient increases, the noise level is significantly reduced, and the selectivity of the radio signal path of the receiver is increased, which is their advantage. Cascode circuits (low noise and high stable gain) on electronic tubes, usually triodes, connected according to the common cathode - common grid scheme, have similar advantages.

The principle of superheterodyne reception.
Detection and amplification of low frequency signals.

Since the radio frequency amplifier is located at the input of the radio receiver, its noise characteristics mainly determine the characteristics of the entire device as a whole. It is the noise figure of the RF amplifier that determines . The non-linear properties of the amplifier are evaluated by the characteristics IP2 and IP3. To ensure high linearity in all stages of the receiver are used. The dot is a very important parameter.

In connection with the microminiaturization of the modern element base and the associated miniaturization of the nodes of the radio receiver, it is now possible to use circuit solutions on the microwave that were previously used at much lower frequencies. This is due to the fact that the dimensions of the block relative to the wavelength of the working oscillation become less than one tenth of the wavelength, and as a result, when developing this block, wave effects during the propagation of oscillations can be neglected.

An additional increase in the stability of the circuit is achieved by including low-pass filters at the input and output of the transistor stage. These filters are calculated for the entire frequency band in which the transistor retains amplifying properties. As a result, phase balance is not maintained over the entire frequency range and self-excitation becomes impossible. The same filter converts the input and output resistance of the transistor to a standard resistance of 50 ohms. The input and output capacitance is included in the filter. RF amplifier with matching circuits at the input and output is shown in Figure 1.

Figure 1. Schematic diagram of an RF amplifier with an input and output impedance of 50 ohms on a common base transistor

In this circuit, R1 ... R3 is implemented in direct current. Capacitor C2 provides grounding of the base of the transistor at high frequency, and capacitor C3 filters the power circuits from interference. Inductor L2 is the collector load of transistor VT1. It passes the supply current into the collector circuit VT1, but at the same time decouples the power supply for alternating current of the radio frequency. Low-frequency filters L1, C1 and C4, L3 provide a transformation of the input and output resistance of the transistor to 50 ohms. The applied low-frequency filter circuit allows you to include in its composition the input or output capacitance of the transistor. The input capacitance of the transistor VT1, together with the capacitance C1, forms the input filter of the amplifier, and the output capacitance of the same transistor, together with the capacitance C4, forms the output low-frequency filter.

Another common RF amplifier circuit is the cascode amplifier circuit. In this scheme, two are connected in series - and with a common base. Such a solution makes it possible to further reduce the value of the pass capacitance of the amplifier. The most common cascode amplifier circuit is a circuit with galvanic coupling between transistor stages. An example of a cascode RF amplifier circuit assembled on bipolar transistors is shown in Figure 2.

Figure 2. Schematic diagram of a cascode RF amplifier

In this circuit, just like in the circuit shown in Figure 1, an emitter stabilization circuit for the operating point of the transistor VT2 is used. Capacitor C6 provides for the elimination of negative feedback at the frequency of the received signal. In some cases, this capacitor is not installed to increase the linearity of the amplifier and in order to reduce the gain of the radio frequency amplifier.

Capacitor C2 provides grounding of the base of the transistor VT1 for alternating current. Capacitor C4 filters the AC power supply. Resistors R1, R2, R3 determine the operating points of transistors VT1 and VT2. Capacitor C3 decouples the base circuit of the transistor VT2 for direct current from the previous stage (input bandpass filter). AC collector circuit load is inductor L2. As in the RF amplifier circuit with a common base, low-pass filters are applied at the input and output of the cascode amplifier. Their main purpose is to ensure the transformation of the input and output resistance to a value of 50 ohms.

Please note that three outputs of the circuit are enough to supply the input voltage and supply voltage, as well as remove the output amplified voltage. This allows you to make an amplifier in the form of a microcircuit with literally three leads. Such cases have minimal dimensions, and this makes it possible to avoid wave effects even at sufficiently high frequencies of the working signal.

Currently, radio frequency amplifier circuits are produced by a number of companies in the form of ready-made microcircuits. For example, we can name such microcircuits as RF3827, RF2360 from RFMD, ADL5521 from Analog Devises, MAALSS0038, AM50-0015 from M / A-COM. These microcircuits use gallium arsenide field-effect transistors. The upper amplified frequency can reach 3GHz. In this case, the noise figure ranges from 1.2 to 1.5 dB. An example of a circuit diagram of a radio frequency amplifier using an integrated circuit MAALSS0038 from M / A-COM is shown in Figure 3.

Figure 3. Schematic diagram of an RF amplifier using the integrated circuit MAALSS0038

RF signals in the range from hundreds of megahertz to units of gigahertz can only be amplified under the condition of very small dimensions of the microcircuits and careful study of the design of the printed circuit board. That is why all manufacturers of RF amplifiers give examples of printed circuit boards. An example of the design of a radio frequency amplifier printed circuit board assembled on a MAALSS0038 chip from M / A-COM is shown in Figure 4.

Figure 4. RF Amplifier PCB Design

It should be noted that often a filter similar to the input filter is often placed between the output of the RF amplifier and the input of the frequency converter, as shown in Figure 2. It allows you to increase the suppression of side channels generated in the frequency converter. Since the input impedance of the filter and the output impedance of the RF amplifier are 50 ohms, their coupling usually does not cause problems.

Literature:

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