Single-stage and two-stage amplifiers. Simple two-stage amplifier Practical circuit of a four-stage transistor amplifier

A low-frequency amplifier is an integral part of any modern radio, TV, tape recorder and many other radio devices. Without low-frequency amplifiers, loud reception of programs from radio broadcasting stations, sound accompaniment of television programs, recording and playback of sound would be impossible.

There was also a single-stage low-frequency amplifier in your single-transistor receiver, but its amplification is not enough for loud-speaking radio reception. Therefore, it is necessary to increase the number of amplifier stages.

Try to mount a simple two-stage amplifier and conduct a series of experiments with it. Such an amplifier can, for example, be connected to a detector receiver, resulting in a 0-V-2 receiver. And with the 1-V-1 reflex receiver it forms a 1-V-3 receiver, providing reliable reception of not only local, but also powerful distant radio stations.

The amplifier will require low-power low-frequency transistors MP39...MP42 with a static current transfer coefficient of at least 30.

The schematic diagram of the first version of such a low-frequency amplifier is shown in Fig. 52. Its first stage is formed by a transistor V1, resistors R1, R2, capacitor C1. It should remind you of a single-stage low-frequency amplifier, familiar from the sixth workshop (see Fig. 29). The resistor became the only load of the transistor (instead of telephones) R2. The second stage of the amplifier on a transistor V2 similar to the first one, but its load is phones IN 1. Electrolytic capacitor C2(same as C1) is an element of interstage communication.

Fundamentally, the second amplifier stage works the same way as the first. The only difference is that the first stage amplifies the input low-frequency signal, and the second one amplifies the signal already amplified by the first stage. As a result, the amplifier's sensitivity increases and the sound will be louder.

You installed a single-stage amplifier at the fifth workshop. Now add a second cascade to it. The result is a two-stage low-frequency amplifier. To the collector; transistor circuit VI of the first stage, which has now become 1 stage of pre-amplification of the low-frequency signal, turn on the load resistor R2 resistance 4.7...5.6 kOhm, and the telephones into the collector circuit of the second stage transistor. To set the same quiescent current of the first stage transistor (1...1.2 mA), the resistance of the base resistor R1 needs to be reduced. The quiescent current of the collector of the second transistor is within 4..6 mA, corresponding to the operating mode of the output stage, set by selecting a resistor R3.

Do not make a mistake in the polarity of the electrolytic capacitor C2: the negative plate must be connected to the collector of the first transistor, and the positive plate to the base of the second transistor.

Connect a subscriber speaker to the amplifier input and, as during experiments with a single-stage amplifier, use it as an electrodynamic microphone. Now that the amplifier has become a two-stage amplifier, phones sound much louder.

The circuit of another version of a two-stage low-frequency amplifier is shown in Fig. 53. Here is a transistor VI connected according to a circuit with a common collector (emitter follower), and its load is the emitter R-n transistor transition V2, connected according to a common emitter circuit. Both transistors, whose currents are interconnected, form, as it were, a single amplification stage. Output transistor operating mode V2 determined by the emitter current of the input transistor, which is selected by a resistor R1.

The advantages of the amplifier of this option are simplicity and fewer parts. This amplifier, in addition, has a significantly higher input impedance than the amplifier of the first option, which allows you to connect a piezoelectric pickup to it and thus play a record. In general, it works the same as the amplifier of the first option.

It may happen that in this version of the amplifier the collector current of the transistor V2 will be large (more than 8...10 mA) and will not decrease with increasing resistor resistance R1. This happens if the reverse collector current Iko of the first transistor is greater than the same parameter of the second transistor, in this case you should try to swap the transistors or bypass the emitter junction of the second transistor with a resistor with a resistance of 100...200 Ohms (in Fig. 53 it is shown with dashed lines).

Now, continuing the experiments, connect it with a single-transistor reflex receiver (assembled earlier according to the circuit in Fig. 50) to turn them into a single 1-U-W receiver. Do it this way. into the collector circuit of the transistor V1 receiver 1-V-1, instead of telephones and a blocking capacitor, turn on a load resistor with a resistance of 2.7 ... 3.3 kOhm (in Fig. 54 R4) and to the connection point of the loads of this transistor (high-frequency choke L3 and resistor R4) Connect the bass amplifier. Now the input electrolytic capacitor C1 of the two-stage amplifier will be a capacitor €4, transistor VI first stage transistor V4, and the transistor V2 second stage transistor V5 combined receiver 1-K-3. Of course, the numbering and some other details will change. Draw a diagram of such a receiver yourself, connecting, of course, the negative and positive conductors of the reflex receiver and the two-stage low-frequency amplifier, since their power source is common.

What should be the polarity of the input electrolytic capacitor now? C4 connected amplifier? The same as the polarity of the similar interstage capacitor of the amplifier of the first option (see. C2 in Fig. 52). This means that when connecting the amplifier to the receiver, do not forget to change the polarity of this capacitor.

To set the collector current of the transistor VI within t…t.2 mA, include a resistor in its base circuit (R1 in Fig. 50 and 54) higher resistance 220…470 kOhm,

Connect an outdoor antenna and ground to the receiver, turn on the power and tune it to the local broadcast station.” Telephones should sound very loud. Disconnect the ground and adjust the input circuit to the same station. The telephones began to sound weaker, but still loud. Replace the external antenna with a piece of wire K..1.5 m long and adjust the input circuit again. The receiver continues to work.

Now turn off the epa aftfetmy sch By turning the receiver in a horizontal plane and at the same time adjusting the input circuit with a variable capacitor, achieve reception of signals from the same station. You have a receiver with a magnetic antenna formed by a ferrite rod with an input circuit coil located on it.

Is it possible to include a dynamic head of direct radiation at the output of such a receiver? It is possible, but only through a low-frequency step-down transformer, with which you can match the relatively high resistance of the amplifier's output circuit with the low resistance of the voice coil of the dynamic head. The role of such a transformer, called a matching transformer, or more often an output transformer, can be performed by a subscriber loudspeaker transformer without any modifications. Connect it to the collector circuit of the output transistor instead of telephones (in Fig. "55 t|" Ng former T L). The loudspeaker will sound louder if you connect an external antenna to the receiver and ground it.

The output stages of transistor LF amplifiers are made push-pull, which significantly increases their output power. A special workshop will be devoted to an amplifier with such a cascade. And in the next workshop we will talk about a high-frequency oscillation amplifier.

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

A two-stage amplifier with RC coupling between stages is shown in Fig. 11. Resistor-capacitance coupling is the most common type in AC amplifiers. Its disadvantage is the limitation of low frequencies. If the amplifier must enhance low frequencies, the capacitance of the coupling capacitors is large. Diagram of a two-stage amplifier with RC coupling between stages. Transistors Q1 and Q2 operate in class A mode, specified by bias circuits R1-R9 and R2-R7, respectively. These two stages are isolated from each other using a decoupling capacitor

Rice. eleven. Two stage amplifier

The overall gain of an amplifier is approximately equal to the product of the gains of each stage multiplied by the gain of the adjacent stage. In our case, the device contains two stages assembled according to a common emitter (CE) circuit, and each of them provides amplification in power, voltage and current.

On the oscillogram (Fig. 10), taken while the amplifier was operating in the electronic laboratory on an IBM PC in the automated environment N1.Multisim 10.1.1. you can see that the alternating input and output voltage pulses are in phase. This is explained simply, the second stage rotates the voltage pulse of the first stage in phase by 180 degrees.

Thus, in a two-stage amplifier we obtained phase coincidence of the input and output voltage pulses. Amplifier modeling performed in the automated program Multisim 10.1.1 is presented in the oscillogram in Fig. 12. The results of the experiment completely coincide with the theoretical premises; here we observe an amplification of the input signal in voltage and phase coincidence after operation of the second amplifier stage.

Rice. 12. Voltage oscillogram

Two-stage amplifier based on field-effect transistors

Rice. 13. Two-stage amplifier based on field-effect transistors

The overall transmission coefficient of the amplifier shown in Fig. 13, as in the previous case, is equal to the product of the gain factors of each stage multiplied by the coefficient of the adjacent stage. In our case, the device also contains two stages. The amplifier simulation, performed in the automated program Multisim 10.1.1, is presented in the oscillogram in Fig. 14. It is noted that the gain is slightly lower than in an amplifier based on bipolar transistors, but with all this, the use of a field-effect transistor has its advantages, such as a significantly higher input impedance, which is an important condition when cascading electronic devices.

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Rice. 14. Voltage oscillogram

Common-source field-effect transistor amplifier

Rice. 15. Common-source field-effect transistor amplifier

An amplifier cascade assembled on a field-effect transistor using a common-source (CS) circuit. The operation of the circuit is similar to the operation of an amplifier with an OE and can provide a high power gain, but in contrast, the field-effect transistor has a significantly higher input resistance compared to a bipolar one. The features of the circuit are as follows: through the leakage resistor R2, a very small gate leakage current is diverted to the chassis. Resistor R3 provides the necessary reverse bias, raising the source potential above the gate potential. In addition, this resistor also ensures the stability of the amplifier's DC mode. The load resistor is R3. It can have very high resistance (more than 1.5 MOhm). Source decoupling capacitor C2 eliminates negative AC feedback through resistor R1. When a signal is applied to the input of the amplifier, the drain current changes, in turn causing a change in the output voltage at the drain of the transistor. During the positive half cycle of the input signal, the gate voltage increases in the positive direction, the reverse bias voltage of the gate-source junction decreases and hence the I-drain current of the FET increases. An increase in the I-drain leads to a decrease in the output (drain) voltage, and the negative half-cycle of the amplified signal is reproduced at the output. Conversely, a negative half-cycle of the input signal corresponds to a positive half-cycle of the output signal.

Output stages based on "twos"

As a signal source we will use an alternating current generator with a tunable output resistance (from 100 Ohms to 10.1 kOhms) in steps of 2 kOhms (Fig. 3). Thus, when testing the VC at the maximum output resistance of the generator (10.1 kOhm), we will to some extent bring the operating mode of the tested VC closer to a circuit with an open feedback loop, and in another (100 Ohm) - to a circuit with a closed feedback loop.

The main types of composite bipolar transistors (BTs) are shown in Fig. 4. Most often in VC, a composite Darlington transistor is used (Fig. 4a) based on two transistors of the same conductivity (Darlington “double”), less often - a composite Szyklai transistor (Fig. 4b) of two transistors of different conductivity with a current negative OS, and even less often - a composite Bryston transistor (Bryston, Fig. 4 c).
The "diamond" transistor, a type of Sziklai compound transistor, is shown in Fig. 4 g. Unlike the Szyklai transistor, in this transistor, thanks to the “current mirror”, the collector current of both transistors VT 2 and VT 3 is almost the same. Sometimes the Shiklai transistor is used with a transmission coefficient greater than 1 (Fig. 4 d). In this case, K P =1+ R 2/ R 1. Similar circuits can be obtained using field-effect transistors (FETs).

1.1. Output stages based on "twos". "Deuka" is a push-pull output stage with transistors connected according to a Darlington, Szyklai circuit or a combination of them (quasi-complementary stage, Bryston, etc.). A typical push-pull output stage based on a Darlington deuce is shown in Fig. 5. If emitter resistors R3, R4 (Fig. 10) of input transistors VT 1, VT 2 are connected to opposite power buses, then these transistors will operate without current cut-off, i.e. in class A mode.

Let's see what pairing the output transistors will give for the two "Darlingt she" (Fig. 13).

In Fig. Figure 15 shows a VK circuit used in one of the professional and onal amplifiers.

The Siklai scheme is less popular in VK (Fig. 18). At the early stages of the development of circuit design for transistor UMZCHs, quasi-complementary output stages were popular, when the upper arm was performed according to the Darlington circuit, and the lower one according to the Sziklai circuit. However, in the original version, the input impedance of the VC arms is asymmetrical, which leads to additional distortion. A modified version of such a VC with a Baxandall diode, which uses the base-emitter junction of the VT 3 transistor, is shown in Fig. 20.

In addition to the considered “twos,” there is a modification of the Bryston VC, in which the input transistors control transistors of one conductivity with the emitter current, and the collector current controls transistors of a different conductivity (Fig. 22). A similar cascade can be implemented on field-effect transistors, for example, Lateral MOSFET (Fig. 24).

The hybrid output stage according to the Sziklai circuit with field-effect transistors as outputs is shown in Fig. 28. Let's consider the circuit of a parallel amplifier using field-effect transistors (Fig. 30).

As an effective way to increase and stabilize the input resistance of a “two”, it is proposed to use a buffer at its input, for example, an emitter follower with a current generator in the emitter circuit (Fig. 32).

Of the “twos” considered, the worst in terms of phase deviation and bandwidth was the Szyklai VK. Let's see what using a buffer can do for such a cascade. If instead of one buffer you use two on transistors of different conductivities connected in parallel (Fig. 35), then you can expect further improvement in parameters and an increase in input resistance. Of all the considered two-stage circuits, the Szyklai circuit with field-effect transistors showed itself to be the best in terms of nonlinear distortions. Let's see what installing a parallel buffer at its input will do (Fig. 37).

The parameters of the studied output stages are summarized in Table. 1 .

Analysis of the table allows us to draw the following conclusions:
- any VC from the “twos” on the BT as a UN load is poorly suited for working in a high-fidelity UMZCH;
- the characteristics of a VC with a DC at the output depend little on the resistance of the signal source;
- a buffer stage at the input of any of the “twos” on the BT increases the input impedance, reduces the inductive component of the output, expands the bandwidth and makes the parameters independent of the output impedance of the signal source;
- VK Siklai with a DC output and a parallel buffer at the input (Fig. 37) has the highest characteristics (minimum distortion, maximum bandwidth, zero phase deviation in the audio range).

Output stages based on "triples"

In high-quality UMZCHs, three-stage structures are more often used: Darlington triplets, Shiklai with Darlington output transistors, Shiklai with Bryston output transistors and other combinations. One of the most popular output stages at present is a VC based on a composite Darlington transistor of three transistors (Fig. 39). In Fig. Figure 41 shows a VC with cascade branching: the input repeaters simultaneously operate on two stages, which, in turn, also operate on two stages each, and the third stage is connected to the common output. As a result, quad transistors operate at the output of such a VC.

The VC circuit, in which composite Darlington transistors are used as output transistors, is shown in Fig. 43. The parameters of the VC in Fig. 43 can be significantly improved if you include at its input a parallel buffer cascade that has proven itself well with “twos” (Fig. 44).

Variant of VK Siklai according to the diagram in Fig. 4 g using composite Bryston transistors is shown in Fig. 46. In Fig. Figure 48 shows a variant of the VK on Sziklai transistors (Fig. 4e) with a transmission coefficient of about 5, in which the input transistors operate in class A (thermostat circuits are not shown).

In Fig. Figure 51 shows the VC according to the structure of the previous circuit with only a unit transmission coefficient. The review will be incomplete if we do not dwell on the output stage circuit with Hawksford nonlinearity correction, shown in Fig. 53. Transistors VT 5 and VT 6 are composite Darlington transistors.

Let's replace the output transistors with field-effect transistors of the Lateral type (Fig. 57

Anti-saturation circuits of output transistors contribute to increasing the reliability of amplifiers by eliminating through currents, which are especially dangerous when clipping high-frequency signals. Variants of such solutions are shown in Fig. 58. Through the upper diodes, excess base current is discharged into the collector of the transistor when approaching the saturation voltage. The saturation voltage of power transistors is usually in the range of 0.5...1.5 V, which approximately coincides with the voltage drop across the base-emitter junction. In the first option (Fig. 58 a), due to the additional diode in the base circuit, the emitter-collector voltage does not reach the saturation voltage by approximately 0.6 V (voltage drop across the diode). The second circuit (Fig. 58b) requires the selection of resistors R 1 and R 2. The lower diodes in the circuits are designed to quickly turn off the transistors during pulse signals. Similar solutions are used in power switches.

Often, to improve the quality, UMZCHs are equipped with separate power supply, increased by 10...15 V for the input stage and voltage amplifier and decreased for the output stage. In this case, in order to avoid failure of the output transistors and reduce the overload of the pre-output transistors, it is necessary to use protective diodes. Let's consider this option using the example of modification of the circuit in Fig. 39. If the input voltage increases above the supply voltage of the output transistors, additional diodes VD 1, VD 2 open (Fig. 59), and the excess base current of transistors VT 1, VT 2 is dumped onto the power buses of the final transistors. In this case, the input voltage is not allowed to increase above the supply levels for the output stage of the VC and the collector current of transistors VT 1, VT 2 is reduced.

Bias circuits

Previously, for the purpose of simplicity, instead of a bias circuit in the UMZCH, a separate voltage source was used. Many of the considered circuits, in particular, output stages with a parallel follower at the input, do not require bias circuits, which is their additional advantage. Now let's look at typical displacement schemes, which are shown in Fig. 60, 61.

Stable current generators. A number of standard circuits are widely used in modern UMZCHs: a differential cascade (DC), a current reflector ("current mirror"), a level shift circuit, a cascode (with serial and parallel power supply, the latter is also called a "broken cascode"), a stable generator current (GST), etc. Their correct use can significantly improve the technical characteristics of UMZCH. We will estimate the parameters of the main GTS circuits (Fig. 62 - 6 6) using modeling. We will assume that the GTS is a load of the UN and is connected in parallel with the VC. We study its properties using a technique similar to the study of VC.

Current reflectors

The considered GTS circuits are a variant of a dynamic load for a single-cycle UN. In an UMZCH with one differential cascade (DC), to organize a counter dynamic load in the UN, they use the structure of a “current mirror” or, as it is also called, a “current reflector” (OT). This structure of the UMZCH was characteristic of the amplifiers of Holton, Hafler, and others. The main circuits of the current reflectors are shown in Fig. 67. They can be either with a unity transmission coefficient (more precisely, close to 1), or with a greater or lesser unit (scale current reflectors). In a voltage amplifier, the OT current is in the range of 3...20 mA: Therefore, we will test all OTs at a current of, for example, about 10 mA according to the diagram in Fig. 68.

The test results are given in table. 3.

As an example of a real amplifier, the S. BOCK power amplifier circuit, published in the journal Radiomir, 201 1, No. 1, p. 5 - 7; No. 2, p. 5 - 7 Radiotechnika No. 11, 12/06

The author's goal was to build a power amplifier suitable for both sounding "space" during festive events and for discos. Of course, I wanted it to fit in a relatively small-sized case and be easily transported. Another requirement for it is the easy availability of components. In an effort to achieve Hi-Fi quality, I chose a complementary-symmetrical output stage circuit. The maximum output power of the amplifier was set at 300 W (into a 4 ohm load). With this power, the output voltage is approximately 35 V. Therefore, the UMZCH requires a bipolar supply voltage within 2x60 V. The amplifier circuit is shown in Fig. 1 . The UMZCH has an asymmetrical input. The input stage is formed by two differential amplifiers.

A. PETROV, Radiomir, 201 1, No. 4 - 12

Rice. 1 Two-stage transistor amplifier.

The effect of the amplifier as a whole is as follows. The electrical signal supplied through capacitor C1 to the input of the first stage and amplified by transistor V1, from the load resistor R2 through the separating capacitor C2 is supplied to the input of the second stage. Here it is amplified by transistor V2 and telephones B1, connected to the collector circuit of the transistor, and is converted into sound. What is the role of capacitor C1 at the amplifier input? It performs two tasks: it freely passes alternating signal voltage to the transistor and prevents the base from being shorted to the emitter through the signal source. Imagine that this capacitor is not in the input circuit, and the source of the amplified signal is an electrodynamic microphone with low internal resistance. What will happen? Through the low resistance of the microphone, the base of the transistor will be connected to the emitter. The transistor will turn off as it will operate without the initial bias voltage. It will open only with negative half-cycles of the signal voltage. And the positive half-cycles, which further close the transistor, will be “cut off” by it. As a result, the transistor will distort the amplified signal. Capacitor C2 connects the amplifier stages via alternating current. It should pass well the variable component of the amplified signal and delay the constant component of the collector circuit of the first stage transistor. If, along with the variable component, the capacitor also conducts direct current, the operating mode of the output stage transistor will be disrupted and the sound will become distorted or disappear completely. Capacitors that perform such functions are called coupling capacitors, transition or isolation capacitors . Input and transition capacitors must pass well the entire frequency band of the amplified signal - from the lowest to the highest. This requirement is met by capacitors with a capacity of at least 5 µF. The use of large capacitance coupling capacitors in transistor amplifiers is explained by the relatively low input resistances of the transistors. The coupling capacitor provides capacitive resistance to alternating current, which will be smaller the greater its capacitance. And if it turns out to be greater than the input resistance of the transistor, a portion of the AC voltage will drop across it, greater than at the input resistance of the transistor, which will result in a loss in gain. The capacitance of the coupling capacitor must be at least 3 to 5 times less than the input resistance of the transistor. Therefore, large capacitors are placed at the input, as well as for communication between transistor stages. Here, small-sized electrolytic capacitors are usually used with mandatory observance of the polarity of their connection. These are the most characteristic features of the elements of a two-stage transistor low-frequency amplifier. To consolidate in memory the principle of operation of a transistor two-stage low-frequency amplifier, I propose to assemble, set up and test in action the simplest versions of amplifier circuits below. (At the end of the article, options for practical work will be proposed; now you need to assemble a prototype of a simple two-stage amplifier so that you can quickly monitor theoretical statements in practice).

Simple, two-stage amplifiers

Schematic diagrams of two versions of such an amplifier are shown in (Fig. 2). They are essentially a repetition of the circuit of the now disassembled transistor amplifier. Only on them the details of the parts are indicated and three additional elements are introduced: R1, SZ and S1. Resistor R1 - load of the source of audio frequency oscillations (detector receiver or pickup); SZ - capacitor that blocks loudspeaker head B1 at higher sound frequencies; S1 - power switch. In the amplifier in (Fig. 2, a) transistors of the p - n - p structure operate, in the amplifier in (Fig. 2, b) - in the n - p - n structure. In this regard, the polarity of switching on the batteries powering them is different: a negative voltage is supplied to the collectors of the transistors of the first version of the amplifier, and a positive voltage is supplied to the collectors of the transistors of the second version. The polarity of switching on electrolytic capacitors is also different. Otherwise the amplifiers are exactly the same.

Rice. 2 Two-stage low-frequency amplifiers on transistors of the p - n - p structure (a) and on transistors of the n - p - n structure (b).

In any of these amplifier options, transistors with a static current transfer coefficient h21e of 20 - 30 or more can operate. A transistor with a large coefficient h21e must be installed in the pre-amplification stage (first) - The role of load B1 of the output stage can be performed by headphones, a DEM-4m telephone capsule. To power the amplifier, use a 3336L battery (popularly called a square battery) or network power supply(which was proposed to be made in the 9th lesson). Pre-amplifier assemble on breadboard , and then transfer its parts to the printed circuit board, if such a desire arises. First, mount only the parts of the first stage and capacitor C2 on the breadboard. Between the right (according to the diagram) terminal of this capacitor and the grounded conductor of the power source, turn on the headphones. If you now connect the input of the amplifier to the output jacks of, for example, a detector receiver tuned to some radio station, or connect any other source of a weak signal to it, the sound of a radio broadcast or a signal from the connected source will appear in the phones. By selecting the resistance of resistor R2 (the same as when adjusting the operating mode of a single-transistor amplifier, what I talked about in lesson 8 ), achieve the highest volume. In this case, a milliammeter connected to the collector circuit of the transistor should show a current equal to 0.4 - 0.6 mA. With a power supply voltage of 4.5 V, this is the most advantageous operating mode for this transistor. Then mount the parts of the second (output) stage of the amplifier, and connect the telephones to the collector circuit of its transistor. Phones should now sound significantly louder. Perhaps they will sound even louder after the collector current of the transistor is set to 0.4 - 0.6 mA by selecting resistor R4. You can do it differently: mount all the parts of the amplifier, select resistors R2 and R4 to set the recommended transistor modes (based on the currents of the collector circuits or the voltages on the collectors of the transistors) and only then check its operation for sound reproduction. This way is more technical. And for a more complex amplifier, and you will have to deal mainly with such amplifiers, this is the only correct one. I hope you understand that my advice on setting up a two-stage amplifier applies equally to both options. And if the current transfer coefficients of their transistors are approximately the same, then the sound volume of telephones and amplifier loads should be the same. With a DEM-4m capsule, the resistance of which is 60 Ohms, the quiescent current of the cascade transistor must be increased (by decreasing the resistance of resistor R4) to 4 - 6 mA. The schematic diagram of the third version of a two-stage amplifier is shown in (Fig. 3). The peculiarity of this amplifier is that in its first stage a transistor of the p - n - p structure operates, and in the second - a n - p - n structure. Moreover, the base of the second transistor is connected to the collector of the first not through a transition capacitor, as in the amplifier of the first two options, but directly or, as they also say, galvanically. With such a connection, the range of frequencies of amplified oscillations expands, and the operating mode of the second transistor is determined mainly by the operating mode of the first, which is set by selecting resistor R2. In such an amplifier, the load of the transistor of the first stage is not the resistor R3, but the emitter p-n junction of the second transistor. The resistor is needed only as a bias element: the voltage drop created across it opens the second transistor. If this transistor is germanium (MP35 - MP38), the resistance of resistor R3 can be 680 - 750 Ohms, and if it is silicon (MP111 - MP116, KT315, KT3102) - about 3 kOhms. Unfortunately, the stability of such an amplifier when the supply voltage or temperature changes is low. Otherwise, everything that is said in relation to the amplifiers of the first two options applies to this amplifier. Can amplifiers be powered from a 9 V DC source, for example from two 3336L or Krona batteries, or, conversely, from a 1.5 - 3 V source - from one or two 332 or 316 cells? Of course, it is possible: at a higher voltage of the power supply, the load of the amplifier - the loudspeaker head - should sound louder, at a lower voltage - quieter. But at the same time, the operating modes of the transistors should be somewhat different. In addition, with a power supply voltage of 9 V, the rated voltages of electrolytic capacitors C2 of the first two amplifier options must be at least 10 V. As long as the amplifier parts are mounted on a breadboard, all this can be easily verified experimentally and the appropriate conclusions can be drawn.

Rice. 3 Amplifier with transistors of different structures.

Mounting the parts of an established amplifier on a permanent board is not a difficult task. For example, (Fig. 4) shows the circuit board of the amplifier of the first option (according to the diagram in Fig. 2, a). Cut the board out of sheet getinax or fiberglass with a thickness of 1.5 - 2 mm. Its dimensions shown in the figure are approximate and depend on the dimensions of the parts you have. For example, in the diagram the power of the resistors is indicated as 0.125 W, the capacitance of the electrolytic capacitors is indicated as 10 μF. But this does not mean that only such parts should be installed in the amplifier. The power dissipation of resistors can be any. Instead of electrolytic capacitors K5O - 3 or K52 - 1, shown on the circuit board, there may be capacitors K50 - 6 or imported analogues, also for higher rated voltages. Depending on the parts you have, the amplifier's PCB may also change. You can read about techniques for installing radio elements, including printed circuit installation, in the section "ham radio technology".

Rice. 4 Circuit board of a two-stage low-frequency amplifier.

Any of the amplifiers that I talked about in this article will be useful to you in the future, for example for a portable transistor receiver. Similar amplifiers can be used for wired telephone communication with a friend living nearby.

Schematic diagrams of two versions of such an amplifier are shown in Figure 2.7. They are essentially a repetition of the circuit of the now disassembled transistor amplifier. Only on them the details of the parts are indicated and three additional elements are introduced: R1, SZ and S1. Resistor R1 - load of the source of audio frequency oscillations (detector receiver or pickup); SZ - capacitor that blocks loudspeaker head B1 at higher sound frequencies; S1 - power switch. In the amplifier in (Fig. 2.7, a) transistors of the p - n - p structure operate, in the amplifier in (Fig. 2.7, b) - in the n - p - n structure. In this regard, the switching polarity of the batteries feeding them is different: a negative voltage is supplied to the transistor collectors of the first version of the amplifier, and a positive voltage is supplied to the transistor collectors of the second version. The polarity of switching on electrolytic capacitors is also different. Otherwise the amplifiers are exactly the same.

Figure 2.7 - Two-stage low-frequency amplifiers on transistors of the p - n - p structure (a) and on transistors of the n - p - n structure (b).

In any of these amplifier options, transistors with a static current transfer coefficient h21e of 20 - 30 or more can operate. A transistor with a large coefficient h21e must be installed in the pre-amplification stage (first) - The role of load B1 of the output stage can be performed by headphones, a DEM-4m telephone capsule.

To power the amplifier, a 3336L battery (popularly called a square battery) or an AC power supply is used. Pre-assemble the amplifier on a breadboard, and then transfer its parts to the printed circuit board, if such a desire arises. First, mount only the parts of the first stage and capacitor C2 on the breadboard. Between the right (according to the diagram) terminal of this capacitor and the grounded conductor of the power source, turn on the headphones. If you now connect the input of the amplifier to the output jacks of, for example, a detector receiver tuned to some radio station, or connect any other source of a weak signal to it, the sound of a radio broadcast or a signal from the connected source will appear in the phones.

Selecting the resistance of resistor R2 (the same as when adjusting the operating mode of a single-transistor amplifier. In this case, the milliammeter connected to the collector circuit of the transistor should show a current equal to 0.4 - 0.6 mA. With a power source voltage of 4.5 V, this is the most advantageous operating mode for this transistor. Then the parts of the second (output) stage of the amplifier are mounted, the telephones are connected to the collector circuit of its transistor. Now the telephones should sound much louder. Perhaps they will sound even louder after the collector current is set by selecting resistor R4 transistor 0.4 - 0.6 mA. You can do it differently: mount all the parts of the amplifier, select resistors R2 and R4 to set the recommended modes of the transistors (based on the currents of the collector circuits or the voltages on the collectors of the transistors) and only then check its operation for sound reproduction. This way is more technical. And for a more complex amplifier, it is the only correct one. And if the current transfer coefficients of their transistors are approximately the same, then the sound volume of the telephones - amplifier loads should be the same. With a DEM-4m capsule, the resistance of which is 60 Ohms, the quiescent current of the cascade transistor must be increased (by decreasing the resistance of resistor R4) to 4 - 6 mA.

The schematic diagram of the third version of a two-stage amplifier is shown in (Fig. 2.8). The peculiarity of this amplifier is that in its first stage a transistor of the p - n - p structure operates, and in the second - a n - p - n structure. Moreover, the base of the second transistor is connected to the collector of the first not through a transition capacitor, as in the amplifier of the first two options, but directly or, as they also say, galvanically. With such a connection, the range of frequencies of amplified oscillations expands, and the operating mode of the second transistor is determined mainly by the operating mode of the first, which is set by selecting resistor R2. In such an amplifier, the load of the transistor of the first stage is not the resistor R3, but the emitter p-n junction of the second transistor. The resistor is needed only as a bias element: the voltage drop created across it opens the second transistor. If this transistor is germanium (MP35 - MP38), the resistance of resistor R3 can be 680 - 750 Ohms, and if it is silicon (MP111 - MP116, KT315, KT3102) - about 3 kOhms.

Unfortunately, the stability of such an amplifier when the supply voltage or temperature changes is low. Otherwise, everything that is said in relation to the amplifiers of the first two options applies to this amplifier. Can amplifiers be powered from a 9 V DC source, for example from two 3336L or Krona batteries, or, conversely, from a 1.5 - 3 V source - from one or two 332 or 316 cells? Of course, it is possible: at a higher voltage of the power supply, the load of the amplifier - the loudspeaker head - should sound louder, at a lower voltage - quieter. But at the same time, the operating modes of the transistors should be somewhat different. In addition, with a power supply voltage of 9 V, the rated voltages of electrolytic capacitors C2 of the first two amplifier options must be at least 10 V. As long as the amplifier parts are mounted on a breadboard, all this can be easily verified experimentally and the appropriate conclusions can be drawn.

Figure 2.8 - Amplifier using transistors of different structures.

Mounting the parts of an established amplifier on a permanent board is not a difficult task.