Wideband RF signal amplifier. Cable TV Signal Amplifier Splitter or high frequency adjustable amplifier. Amplifier circuit diagram

A simple transistor amplifier can be a good tool for studying the properties of devices. The circuits and designs are quite simple; you can make the device yourself and check its operation, take measurements of all parameters. Thanks to modern field-effect transistors, it is possible to make a miniature microphone amplifier from literally three elements. And connect it to a personal computer to improve sound recording parameters. And the interlocutors during conversations will hear your speech much better and more clearly.

Frequency characteristics

Low (audio) frequency amplifiers are found in almost all household appliances - stereo systems, televisions, radios, tape recorders, and even personal computers. But there are also RF amplifiers based on transistors, lamps and microcircuits. The difference between them is that the ULF allows you to amplify the signal only at the audio frequency that is perceived by the human ear. Transistor audio amplifiers allow you to reproduce signals with frequencies in the range from 20 Hz to 20,000 Hz.

Consequently, even the simplest device can amplify the signal in this range. And it does this as evenly as possible. The gain depends directly on the frequency of the input signal. The graph of these quantities is almost a straight line. If a signal with a frequency outside the range is applied to the amplifier input, the quality of operation and efficiency of the device will quickly decrease. ULF cascades are assembled, as a rule, using transistors operating in the low and mid-frequency ranges.

Classes of operation of audio amplifiers

All amplifying devices are divided into several classes, depending on the degree of current flow through the cascade during the period of operation:

1. Class “A” - current flows non-stop during the entire period of operation of the amplifier stage.
2. In work class "B" current flows for half a period.
3. Class “AB” indicates that current flows through the amplifier stage for a time equal to 50-100% of the period.
4. In mode “C”, electric current flows for less than half the operating time.
5. ULF mode “D” has been used in amateur radio practice quite recently - a little over 50 years. In most cases, these devices are implemented on the basis of digital elements and have a very high efficiency - over 90%.

The presence of distortion in various classes of low-frequency amplifiers

The working area of ​​a class “A” transistor amplifier is characterized by fairly small nonlinear distortions. If the incoming signal spits out higher voltage pulses, this causes the transistors to become saturated. In the output signal, higher ones begin to appear near each harmonic (up to 10 or 11). Because of this, a metallic sound appears, characteristic only of transistor amplifiers.

If the power supply is unstable, the output signal will be modeled in amplitude near the network frequency. The sound will become harsher on the left side of the frequency response. But the better the stabilization of the amplifier's power supply, the more complex the design of the entire device becomes. ULFs operating in class “A” have a relatively low efficiency - less than 20%. The reason is that the transistor is constantly open and current flows through it constantly.

To increase (albeit slightly) efficiency, you can use push-pull circuits. One drawback is that the half-waves of the output signal become asymmetrical. If you transfer from class “A” to “AB”, nonlinear distortions will increase by 3-4 times. But the efficiency of the entire device circuit will still increase. ULF classes “AB” and “B” characterize the increase in distortion as the signal level at the input decreases. But even if you turn up the volume, this will not help completely get rid of the shortcomings.

Work in intermediate classes

Each class has several varieties. For example, there is a class of amplifiers “A+”. In it, the input transistors (low voltage) operate in mode “A”. But high-voltage ones installed in the output stages operate either in “B” or “AB”. Such amplifiers are much more economical than those operating in class “A”. There is a noticeably lower number of nonlinear distortions - no higher than 0.003%. Better results can be achieved using bipolar transistors. The operating principle of amplifiers based on these elements will be discussed below.

But there is still a large number of higher harmonics in the output signal, causing the sound to become characteristically metallic. There are also amplifier circuits operating in class “AA”. In them, nonlinear distortions are even less - up to 0.0005%. But the main drawback of transistor amplifiers still exists - the characteristic metallic sound.

"Alternative" designs

This is not to say that they are alternative, but some specialists involved in the design and assembly of amplifiers for high-quality sound reproduction increasingly prefer tube designs. Tube amplifiers have the following advantages:

1. Very low level of nonlinear distortion in the output signal.
2. There are fewer higher harmonics than in transistor designs.

But there is one huge disadvantage that outweighs all the advantages - you definitely need to install a device for coordination. The fact is that the tube stage has a very high resistance - several thousand Ohms. But the speaker winding resistance is 8 or 4 Ohms. To coordinate them, you need to install a transformer.

Of course, this is not a very big drawback - there are also transistor devices that use transformers to match the output stage and the speaker system. Some experts argue that the most effective circuit is a hybrid one - which uses single-ended amplifiers that are not affected by negative feedback. Moreover, all these cascades operate in ULF class “A” mode. In other words, a power amplifier on a transistor is used as a repeater.

Moreover, the efficiency of such devices is quite high - about 50%. But you should not focus only on efficiency and power indicators - they do not indicate the high quality of sound reproduction by the amplifier. The linearity of the characteristics and their quality are much more important. Therefore, you need to pay attention primarily to them, and not to power.

Single-ended ULF circuit on a transistor

The simplest amplifier, built according to a common emitter circuit, operates in class “A”. The circuit uses a semiconductor element with an n-p-n structure. A resistance R3 is installed in the collector circuit, limiting the flow of current. The collector circuit is connected to the positive power wire, and the emitter circuit is connected to the negative wire. If you use semiconductor transistors with a p-n-p structure, the circuit will be exactly the same, you just need to change the polarity.

Using a decoupling capacitor C1, it is possible to separate the alternating input signal from the direct current source. In this case, the capacitor is not an obstacle to the flow of alternating current along the base-emitter path. The internal resistance of the emitter-base junction together with resistors R1 and R2 represent the simplest supply voltage divider. Typically, resistor R2 has a resistance of 1-1.5 kOhm - the most typical values ​​for such circuits. In this case, the supply voltage is divided exactly in half. And if you power the circuit with a voltage of 20 Volts, you can see that the value of the current gain h21 will be 150. It should be noted that HF ​​amplifiers on transistors are made according to similar circuits, only they work a little differently.

In this case, the emitter voltage is 9 V and the drop in the “E-B” section of the circuit is 0.7 V (which is typical for transistors on silicon crystals). If we consider an amplifier based on germanium transistors, then in this case the voltage drop in the “E-B” section will be equal to 0.3 V. The current in the collector circuit will be equal to that flowing in the emitter. You can calculate it by dividing the emitter voltage by the resistance R2 - 9V/1 kOhm = 9 mA. To calculate the value of the base current, you need to divide 9 mA by the gain h21 - 9 mA/150 = 60 μA. ULF designs usually use bipolar transistors. Its operating principle is different from field ones.

On resistor R1, you can now calculate the drop value - this is the difference between the base and supply voltages. In this case, the base voltage can be found using the formula - the sum of the characteristics of the emitter and the “E-B” transition. When powered from a 20 Volt source: 20 - 9.7 = 10.3. From here you can calculate the resistance value R1 = 10.3 V/60 μA = 172 kOhm. The circuit contains capacitance C2, which is necessary to implement a circuit through which the alternating component of the emitter current can pass.

If you do not install capacitor C2, the variable component will be very limited. Because of this, such a transistor-based audio amplifier will have a very low current gain h21. It is necessary to pay attention to the fact that in the above calculations the base and collector currents were assumed to be equal. Moreover, the base current was taken to be the one that flows into the circuit from the emitter. It occurs only if a bias voltage is applied to the base output of the transistor.

But it must be taken into account that collector leakage current absolutely always flows through the base circuit, regardless of the presence of bias. In common emitter circuits, the leakage current is amplified by at least 150 times. But usually this value is taken into account only when calculating amplifiers based on germanium transistors. In the case of using silicon, in which the current of the “K-B” circuit is very small, this value is simply neglected.

Amplifiers based on MOS transistors

The field-effect transistor amplifier shown in the diagram has many analogues. Including using bipolar transistors. Therefore, we can consider, as a similar example, the design of an audio amplifier assembled according to a circuit with a common emitter. The photo shows a circuit made according to a common source circuit. R-C connections are assembled on the input and output circuits so that the device operates in class “A” amplifier mode.

The alternating current from the signal source is separated from the direct supply voltage by capacitor C1. The field-effect transistor amplifier must necessarily have a gate potential that will be lower than the same source characteristic. In the diagram shown, the gate is connected to the common wire via resistor R1. Its resistance is very high - resistors of 100-1000 kOhm are usually used in designs. Such a large resistance is chosen so that the input signal is not shunted.

This resistance almost does not allow electric current to pass through, as a result of which the gate potential (in the absence of a signal at the input) is the same as that of the ground. At the source, the potential turns out to be higher than that of the ground, only due to the voltage drop across resistance R2. From this it is clear that the gate has a lower potential than the source. And this is exactly what is required for the normal functioning of the transistor. It is necessary to pay attention to the fact that C2 and R3 in this amplifier circuit have the same purpose as in the design discussed above. And the input signal is shifted relative to the output signal by 180 degrees.

ULF with transformer at the output

You can make such an amplifier with your own hands for home use. It is carried out according to the scheme that works in class “A”. The design is the same as those discussed above - with a common emitter. One feature is that you need to use a transformer for matching. This is a disadvantage of such a transistor-based audio amplifier.

The collector circuit of the transistor is loaded by the primary winding, which develops an output signal transmitted through the secondary to the speakers. A voltage divider is assembled on resistors R1 and R3, which allows you to select the operating point of the transistor. This circuit supplies bias voltage to the base. All other components have the same purpose as the circuits discussed above.

Push-pull audio amplifier

It cannot be said that this is a simple transistor amplifier, since its operation is a little more complicated than those discussed earlier. In push-pull ULFs, the input signal is split into two half-waves, different in phase. And each of these half-waves is amplified by its own cascade, made on a transistor. After each half-wave has been amplified, both signals are combined and sent to the speakers. Such complex transformations can cause signal distortion, since the dynamic and frequency properties of two transistors, even of the same type, will be different.

As a result, the sound quality at the amplifier output is significantly reduced. When a push-pull amplifier operates in class “A”, it is not possible to reproduce a complex signal with high quality. The reason is that increased current constantly flows through the amplifier's shoulders, the half-waves are asymmetrical, and phase distortions occur. The sound becomes less intelligible, and when heated, signal distortion increases even more, especially at low and ultra-low frequencies.

Transformerless ULF

A transistor-based bass amplifier made using a transformer, despite the fact that the design may have small dimensions, is still imperfect. Transformers are still heavy and bulky, so it's better to get rid of them. A circuit made on complementary semiconductor elements with different types of conductivity turns out to be much more effective. Most modern ULFs are made precisely according to such schemes and operate in class “B”.

The two powerful transistors used in the design operate according to an emitter follower circuit (common collector). In this case, the input voltage is transmitted to the output without loss or gain. If there is no signal at the input, then the transistors are on the verge of turning on, but are still turned off. When a harmonic signal is applied to the input, the first transistor opens with a positive half-wave, and the second one is in cutoff mode at this time.

Consequently, only positive half-waves can pass through the load. But the negative ones open the second transistor and completely turn off the first. In this case, only negative half-waves appear in the load. As a result, the signal amplified in power appears at the output of the device. Such an amplifier circuit using transistors is quite effective and can provide stable operation and high-quality sound reproduction.

ULF circuit on one transistor

Having studied all the features described above, you can assemble the amplifier with your own hands using a simple element base. The transistor can be used domestic KT315 or any of its foreign analogues - for example BC107. As a load, you need to use headphones with a resistance of 2000-3000 Ohms. A bias voltage must be applied to the base of the transistor through a 1 MΩ resistor and a 10 μF decoupling capacitor. The circuit can be powered from a source with a voltage of 4.5-9 Volts, a current of 0.3-0.5 A.

If resistance R1 is not connected, then there will be no current in the base and collector. But when connected, the voltage reaches a level of 0.7 V and allows a current of about 4 μA to flow. In this case, the current gain will be about 250. From here you can make a simple calculation of the amplifier using transistors and find out the collector current - it turns out to be equal to 1 mA. Having assembled this transistor amplifier circuit, you can test it. Connect a load to the output - headphones.

Touch the amplifier input with your finger - a characteristic noise should appear. If it is not there, then most likely the structure was assembled incorrectly. Double-check all connections and element ratings. To make the demonstration more clear, connect a sound source to the ULF input - the output from the player or phone. Listen to music and evaluate the sound quality.

Broadband amplifiers are an integral part of many radio systems and devices. In some cases, among others, they are subject to matching requirements with a standard 50- or 75-ohm path. One of the most successful circuit solutions for constructing such

amplifiers is the use of cross-feedback connections (L1, L2, L3), ensuring input and output matching, a constant value of the upper limit frequency as the number of amplifier stages increases, and high repeatability of their characteristics. In addition, cross-feedback amplifiers require virtually no setup.

Amplifier specifications:

1. Operating frequency band.. 0.5-70 MHz.
2. Output voltage, not less than... 1 V.
3. Gain.....20±1 dB.
4. Input/output impedance.. 50 Ohm.
5. Current consumption....... 120mA.
6. Supply voltage.........12V.
7. Input VSWR, no more than......1.5.
8. Output VSWR, no more.........3.
9. Overall dimensions..... 70x45 mm.

Schematic diagram

In Fig. Figure 1 shows a schematic diagram of an amplifier with cross-feedback, in which the output stage is implemented according to the Darlington circuit, that is, a series-parallel connection of transistors is used, which makes it possible to increase the output voltage level (L.4). In Fig.

Figure 2 shows a drawing of the printed circuit board.

The amplifier contains two preliminary stages on transistors ME1 and ME2 and an output stage on transistors MEZ and ME4 connected according to a Darlington circuit.

All amplifier stages operate in class A mode with consumption currents of 27 mA, which are set by selecting the values ​​of resistors R1, R5, R9, R13. Resistors R3, R7, R10, R14 are local feedback resistors. Resistors R4, R8, R12 are general feedback resistors.

Rice. 1. Schematic diagram of a wideband RF amplifier.

The printed circuit board (Fig. 2) measuring 70x45 mm is made of fiberglass foil on both sides with a thickness of 2...3 mm. Dotted lines in Fig.

2 indicates the places where the ends are metalized, which can be done using metal foil, which is soldered to the bottom and top of the board.

Fig.2. RF amplifier printed circuit board.

Setting up the amplifier consists of the following steps. First, using resistors R1, R5, R9, R13, the quiescent currents of the amplifier transistors are set. Then, varying the value of resistor R4 within small limits, the voltage standing wave ratio at the amplifier input is minimized.

The voltage standing wave ratio at the amplifier output is minimized using resistor R12. By changing the value of resistor R8, the bandwidth and gain of the amplifier are adjusted.

If necessary, the upper limit frequency of the amplifier can be increased. To do this, replace the KT315G transistors with higher-frequency ones. In this case, for the circuit shown in Fig.

1, the upper limit frequency will be of the order of 0.25...0.3 Ft, where Ft is the cut-off frequency of the transistor base current transfer coefficient (L.5). The use of the circuit design under consideration allows the creation of amplifiers with an upper limit frequency of up to 2 GHz (L.2). When constructing them, it should be taken into account that the general feedback circuits, consisting of elements C4, R4; C6, R8; C7, R12 should be as short as possible.

This is explained by the need to eliminate excessive phase delay of the signal in these circuits. Otherwise, the amplitude-frequency response of the amplifier in the high-frequency region appears to rise. With a significant lengthening of these circuits, self-excitation of the amplifier is possible.

Titov A. Rk2005, 1.

Literature:

1. Titov A. A. Simplified calculation of a broadband amplifier. Radio engineering, 1979, No. 6, p. 88-90.
2. Avdochenko B.I., Dyachko A.N. and others. Ultra-wideband amplifiers based on bipolar transistors. Communication technology. Ser. Radio measuring equipment, 1985, Vyl. 3, p. 57-60.
3. Abramov F.G., Volkov Yu.A. etc. Matched broadband amplifier. Instruments and experimental techniques. 1984. No. 2, p. 111-112.
4. Titov A.A., Ilyushchenko V.N. Broadband amplifier. Utility model patent No. 35491 Ros. patent and trademark agencies. Publ. 01/10/2004 Bulletin. 1.
5. Petukhov V.M. Transistors and their foreign analogues: A reference book in 4 volumes.

We continue the conversation about the direct amplification transistor receiver, which began at the seventh workshop. By then connecting the detector receiver to a single-stage low-frequency amplifier, you thereby turned them into a 0-V-1 receiver. Then I assembled a single-transistor reflex receiver, and at the previous workshop I added a two-stage low-frequency amplifier to it - the result was a 1-V-3 receiver. Now try adding a high frequency (HF) modulated preamp stage to it to make it a 2-V-3 receiver. The sensitivity in this case will be sufficient to receive not only local, but also distant broadcasting stations on the magnetic antenna.

What is required for such a single-stage RF amplifier? Basically - a low-power high-frequency transistor of any of the P401...P403, P416, P422, GT308 series, as long as it is in good working order, several capacitors, a resistor and a ferrite ring of grade 600NN with an outer diameter of 8...10 mm. The coefficient h21E of the transistor can be in the range of 50...100. You should not use a transistor with a large static current transfer coefficient - an experienced amplifier will be prone to self-excitation.

The circuit diagram of the amplifier is shown in Fig. 56. The amplifier itself is formed only by a transistor V1 and resistors R1, R2. Resistor R2 acts as a load, and the base resistor R1 determines the operating mode of the transistor. The collector load of the transistor can be a high-frequency choke - the same as in a reflex receiver.

Custom contour L1 C1 and communication coil L2 refer to the input circuit, capacitor C2- dividing. This part is an exact repetition of the input part of the receiver you have already tested. Capacitor Immediately, resistor R, diode V2, phones B1 s The blocking capacitor Sbl forms a detector circuit necessary for testing the amplifier.

How does such an amplifier work? Fundamentally the same as a single-stage low-frequency amplifier. It only amplifies not audio frequency oscillations, like that amplifier, but modulated high frequency oscillations coming to it from the coupling coil L2. The high-frequency signal, amplified by the transistor, is allocated to the load resistor R2 (or other collector load) and can be fed to the input of a second stage for additional amplification or to a detector to convert it into a low-frequency signal.

Mount the amplifier parts on a temporary (cardboard) board, as shown on the right in Fig. 56. Move here and connect the parts of the input circuit (L1C1) and the communication coil (L2) of the receiver to the amplifier. Don't forget to include an isolating capacitor in the coupling coil circuit C2. Connect the battery voltage 9 V and, choosing a base resistor R1, set the collector current of the transistor within 0.8...1.2 mA. Don’t forget: the resistance of the base resistor should be greater, the greater the static current transfer coefficient of the transistor (the value of this resistor indicated in the diagram corresponds to the coefficient h21E transistor about 50).

Now mount a detector circuit on a separate small cardboard, connecting in series the phones B1 with a blocking capacitor Sbl with a capacity of 2200..3300 pF, a point diode V2 any series and separator nyu capacitor Immediately with a capacity of 3300...6800 pF, Resistor resistance R maybe 4.7...6.8 kOhm. Connect this circuit between the collector and emitter of the transistor, that is, to the output of the amplifier, and connect an outdoor or indoor antenna and, of course, grounding to the input circuit L1C1. When tuning the input circuit to the wave of a local radio station, its high-frequency signal will be amplified by the transistor VI, detected by diode V2 and converted by phones IN 1 into sound. Resistor R in this circuit is necessary for normal operation of the detector. Without it, phones will sound quieter and distorted.

For the next experiment with an RF amplifier, a high-frequency step-down transformer is needed (Fig. 57). Wind it on a ring of 600NN grade ferrite (the same as the core of the high-frequency choke of the reflex stage of the receiver). Its primary winding L3 should contain 180..200 turns of wire PEV or PEL 0.1...0.12, and the secondary L 4 60...80 turns of the same wire.

Connect winding L3 of the high-frequency transformer to the collector circuit of the transistor instead of the load resistor, and to its winding L4 connect the same detector circuit as in the previous experiment, but without the coupling capacitor and resistor, which are not needed now. How does it sound now? phones? Louder. This is explained by better matching of the output impedance of the amplifier and the input impedance of the detector target than in the first experiment.

And now, using the diagram shown in Fig. 58, connect this single-stage amplifier to the input of the 1-V-3 reflex receiver transistor. The RF receiver amplifier became two-stage. The connecting element between the cascades was the coil L4 high-frequency transformer included in the base circuit of transistor V 2 (in the receiver 1-V-Z the transistor W1 was used) instead of the communication coil (there was L2) with the former input configurable circuit. Now an external antenna and grounding are not needed - reception is carried out using the magnetic antenna W1. whose role: is performed by a ferrite rod with a coil located on it L1 input configurable circuit L1 C1.

So, together with a two-stage low-frequency amplifier, a four-transistor direct amplification receiver 2-U-W was trained. The receiver may be self-exciting. This is because, firstly, it is reflexive, and reflexive receivers are generally prone to self-excitation, and secondly, the conductors connecting the experimental amplifier cascade with the reflex cascade are long. If the new stage, together with the magnetic antenna, is mounted compactly on the same receiver board, making the circuits as short as possible, there will be fewer reasons for self-excitation. This is also facilitated by the decoupling filter cell. R2 C3 in the negative power circuit of the first transistor of the RF amplifier, which eliminates the connection between the stages through a common lithium source and thereby prevents self-excitation of the high-frequency path of the receiver.

But the second stage of the RF amplifier may be the same as the first, that is, not reflexive, and the connection between them may not be a transformer. The diagram of a possible amplifier version is shown in Fig. 59. Here the load of the transistor V1 the first stage, as in the first experiment of this workshop (see Fig. 56), is resistor R2; The high-frequency signal voltage created across it through a capacitor NW supplied to the base of the transistor V2 the second cascade, exactly the same as the first. The signal, additionally amplified by the transistor of the second stage, is removed from its load resistor R4 ( the same; like R 2) and through capacitor C 4 (such as NW) goes to the detector on diode V 3, is detected by it, and the low frequency oscillations created across its load resistor R5, are fed to the input of the bass amplifier.

In this version, the second cascade and detector are like an unfolded reflex cascade of the previous version. But the transistor only amplifies high-frequency oscillations. And if you connect it to a two-stage low-frequency amplifier, you get a direct amplification receiver 2- V-2. The amplification of the low-frequency signal will decrease somewhat, telephones or the loudspeaker head at the output of such a receiver will sound a little quieter, but the danger of self-excitation of its high-frequency path will be reduced. This loss can be partially compensated by increasing the voltage of the low-frequency signal at the output of the detector by including a second diode in the detector cascade (shown in dashed lines in Fig. 59 V4), as you did in one of the experiments in the seventh workshop (see Fig. 50), or use a transistor in the detector cascade.

Try to experiment with low-frequency amplifier options, compare the quality of their work and draw appropriate conclusions for the future.

One more tip. When experimenting with one or another version of the receiver, draw and remember its complete circuit diagram. For what? A radio amateur, even a beginner, must draw diagrams of such devices from memory. The circuit diagram will also help you better understand the operation of the receiver as a whole and its parts, and will make it easier to find faults in it.

Literature: Borisov V.G. Workshop for a beginner radio amateur. 2nd ed., revised. and additional - M.: DOSAAF, 1984. 144 p., ill. 55k.

This RF transmitter amplifier circuit (at 50 MHz) has 100 W of output power. I used this UHF with my FT-736R for DX SSB. It amplifies the signal exactly 10 times. The device is perfect for taxi drivers' car radios operating in the 50 and 27 MHz bands (with contour tuning).

If you want to build this RF amplifier, build it on a double-sided PCB to increase the ground area. Transistor 2SC2782 needs a decent radiator. Maximum output power is 120W.

RF power amplifier circuit

PCB drawing

Amplifier specifications:

• Input Power: 10W
• Output Power: 100W
• Working Frequency: 50-52MHz
• Operating mode: FM - SSB
• Operating Voltage: DC 10-16V
• Working Current: 10 amps.

The diagram was taken from one Chinese site and successfully repeated, only the elements of the automatic reception-transmission switching detector (crossed out in the diagram) were not used. To create UHF frequencies from 100 megahertz, use.

High frequency amplifiers (UHF) are used to increase the sensitivity of radio receiving equipment - radios, televisions, radio transmitters. Placed between the receiving antenna and the input of the radio or television receiver, such UHF circuits increase 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 (receive-transmit devices - 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 circuits often used to increase radio sensitivity. The values ​​of the elements used depend on specific conditions: on the frequencies (lower and upper) of the radio range, on the antenna, on the parameters of the subsequent stage, on the supply voltage, etc.

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

It is necessary to recall that the reference data for transistors provides maximum frequency parameters. 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 specified in the passport. However, for an RF amplifier connected according to the OE circuit, the maximum nameplate frequency must be reduced by at least an order of magnitude or more.

Fig.1. Examples of circuits of simple high-frequency (UHF) amplifiers using 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 in a common emitter (CE) circuit, provide relatively high gain, but their frequency properties are relatively low.

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

Figure 1 (b) shows wideband high frequency amplifier circuit (UHF) on one transistor turned on according to a common base scheme. The LC circuit is included in the collector circuit (load). 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, 1/3 of the turns are drained.

Figure 1 (c) shows another broadband circuit UHF on one transistor, included according to a common base scheme. An RF choke 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.

The values ​​of capacitors and circuit are given for frequencies of the MF and HF ranges. For higher frequencies, for example, for the VHF range, the capacitance values ​​should be reduced. In this case, D01 chokes can be used.

Capacitors such as KLS, KM, KD, etc.

L1 coils are chokes; for the CB range these can be coils on rings 600NN-8-K7x4x2, 300 turns of PEL 0.1 wire.

Higher gain value can be obtained by using multi-transistor circuits. These can be various circuits, for example, made on the basis of an OK-OB cascode amplifier using transistors of different structures with serial power supply. One of the variants of such a UHF scheme is shown in Fig. 1 (d).

This UHF circuit has significant gain (tens or even hundreds of times), but cascode amplifiers cannot provide significant gain at high frequencies. Such schemes are usually used at frequencies in the LW and SV ranges. However, with the use of ultra-high frequency transistors and careful design, such circuits can be successfully applied 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.

The capacitor and circuit values ​​are given for frequencies in the CB range. For higher frequencies, such as the HF band, capacitance values ​​and loop inductance (number of turns) must be reduced accordingly.

Capacitors such as KLS, KM, KD, etc. Coil L1 - for the CB 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 transistor UHF makes it possible to amplify radio signals. coming from antennas, in television bands - 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 is shown in Fig. 2 (a).

Fig.2.2. MV antenna amplifier circuit.

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 included in 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;
• C 1=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. requirements for installation of HF structures: minimum lengths of connecting conductors, shielding, etc.

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

For the circuit of the given 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 response (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 the specified amplifier.

The connection diagram for 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 PEV-2 0.6. winding diameter 4 mm.

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

Using the described circuit solutions and modern high-frequency transistors (ultra-high-frequency transistors - microwave transistors), you can build an antenna amplifier for the UHF range. This amplifier can be used either 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.

Main parameters of the UHF range amplifier:

• Frequency band 470-790 MHz,
• Gain - 30 dB,
• Noise figure -3 dB,
• Input and output impedance - 75 Ohm,
• Current consumption - 12 mA.

One of the features of this circuit is the supply of 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 in a circuit with a common emitter. A 3rd order high-pass filter is provided at the input of the antenna amplifier, limiting the range of operating frequencies 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 are type KD-1, the rest are 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 a diagram of connecting the antenna amplifier to the antenna socket of the TV receiver (to the UHF selector) and to a remote 12 V power source. 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 to a TV.

Radio connection elements, Fig. 3 (b):

• C5=100;
• L3 - RF choke, 100 µH.

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

Setting up the amplifier comes down to setting the collector currents of the transistors and are regulated using R1 and RЗ, 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 turns of L1.

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