A simple amplifier on the transistors with their own hands. Amplifier on one transistor: circuit

The amplifier on transistors, despite its already long history, remains a favorite subject of research for both novice and venerable radio amateurs. And this is understandable. It is an indispensable component of the most popular radio amateur devices: radio receivers and amplifiers of low (sound) frequency. We will consider how to build the simplest low-frequency amplifiers on transistors.

Frequency response of the amplifier

In any TV or radio receiver, in each music center or audio amplifier you can find transistor amplifiers of sound (low frequency - LF). The difference between audio transistor amplifiers and other types is their frequency characteristics.

The sound amplifier on transistors has a uniform frequency response in the frequency band from 15 Hz to 20 kHz. This means that all input signals with a frequency within this range of the amplifier converts (amplifies) approximately the same. In the figure below, in the coordinates "amplification factor Ku - frequency of the input signal" shows the ideal frequency response curve for the sound amplifier.

This curve is practically flat from 15 Hz to 20 kHz. This means that such an amplifier should be used for input signals with frequencies between 15 Hz and 20 kHz. For input signals with frequencies above 20 kHz or below 15 Hz, the efficiency and quality of its operation rapidly decrease.

The type of frequency characteristic of the amplifier is determined by the electro-radioelements (ERE) of its circuit, and primarily by the transistors themselves. A transistor audio amplifier is usually assembled on so-called low- and mid-frequency transistors with a total bandwidth of input signals from tens and hundreds of Hz to 30 kHz.

Class of operation of the amplifier

As is known, depending on the degree of continuity of current flow during its period, the following classes of its work are distinguished through the transistor amplifying cascade (amplifier): "A", "B", "AB", "C", "D".

In the class of operation, the current "A" through the cascade flows for 100% of the period of the input signal. The work of the cascade in this class is illustrated by the following figure.

In the class of operation of the amplifier stage "AB" the current through it flows more than 50%, but less than 100% of the period of the input signal (see the figure below).

In the class of operation of the cascade "B", the current through it flows exactly 50% of the period of the input signal, as illustrated in the figure.

And finally, in the class of operation of the cascade "C" the current through it flows less than 50% of the period of the input signal.

Low-frequency amplifier on transistors: distortions in the main classes of work

In the working area, the transistor amplifier of class "A" has a low level of nonlinear distortion. But if the signal has impulse voltages that lead to saturation of transistors, higher harmonics (up to the 11th) appear around each "regular" harmonic of the output signal. This causes the phenomenon of the so-called transistor, or metallic, sound.

If the low-power transistor power amplifiers have unstabilized power, their output signals are modulated in amplitude near the network frequency. This leads to a hardness of sound on the left edge of the frequency response. Different ways of stabilizing the voltage make the design of the amplifier more complex.

The typical efficiency of a single-ended Class A amplifier does not exceed 20% due to the permanently open transistor and the continuous flow of a constant current component. It is possible to perform a class A amplifier in push-pull mode, the efficiency will increase somewhat, but the half-waves of the signal become more asymmetric. Translating the same cascade from the class of work "A" into the class of work "AB" increases four-fold nonlinear distortions, although the efficiency of its scheme increases.

In amplifiers of the same classes "AB" and "B", distortions increase as the level of the signal decreases. Involuntarily I want to cut this amplifier louder for the fullness of the sensations of the power and dynamics of music, but often this helps a little.

Intermediate work classes

The class of work "A" has a variety - class "A +". In this case, low-voltage input transistors of the amplifier of this class operate in class "A", and high-voltage output transistors of the amplifier go over to the "B" or "AB" classes when they are exceeded by input signals of a certain level. The economy of such cascades is better than in the pure class "A", and the nonlinear distortions are less (up to 0.003%). However, the sound is also "metallic" because of the presence of higher harmonics in the output signal.

At amplifiers of one more class - "AA" the degree of nonlinear distortion is even lower - about 0.0005%, but higher harmonics are also present.

Return to transistor amplifier class "A"?

Today, many experts in the field of quality sound reproduction advocate a return to tube amplifiers, since the level of nonlinear distortion and higher harmonics introduced by them into the output signal is certainly lower than that of transistors. However, these advantages are to no small extent offset by the need for a matching transformer between the high-resistance tube output stage and low-impedance audio columns. However, with a transformer output, a simple transistor amplifier can be made, as will be shown below.

There is also a point of view that the ultimate sound quality can only be provided by a hybrid tube-transistor amplifier, all cascades of which are single-ended, not covered by negative feedbacks and operate in class "A". That is, such a power repeater is an amplifier on one transistor. Its scheme can have the maximum achievable efficiency (in class "A") no more than 50%. But neither the power nor the efficiency of the amplifier is an indicator of the quality of sound reproduction. In this case, the quality and linearity of the characteristics of all EREs in the scheme acquire special significance.

Since single-cycle schemes receive such a perspective, we will consider below their possible options.

Single-ended amplifier on one transistor

Its circuit, made with a common emitter and RC-connections for input and output signals for operation in class "A", is shown in the figure below.

It shows the transistor Q1 of the npn structure. Its collector through the current-limiting resistor R3 is connected to the positive terminal + Vcc, and the emitter to -Vcc. The amplifier on the transistor of the pnp structure will have the same circuit, but the power supply terminals will be interchanged.

C1 is a separating capacitor by means of which the source of the variable input signal is separated from the constant voltage source Vcc. In this case, C1 does not prevent the passage of the alternating input current through the base-emitter transition of transistor Q1. Resistors R1 and R2 together with the resistance of the transition "EB" form a voltage divider Vcc for selecting the operating point of the transistor Q1 in the static mode. Typical for this scheme is the value R2 = 1 kΩ, and the position of the operating point is Vcc / 2. R3 is a pull-up resistor of the collector circuit and serves to create an output voltage signal on the collector.

Suppose that Vcc = 20 V, R2 = 1 kOhm, and the current gain is h = 150. We select voltage on the emitter Ve = 9 V, and assume that the voltage drop at the "EB" transition is equal to Vbe = 0,7 V. This value corresponds to the so-called silicon transistor. If we were considering an amplifier on germanium transistors, then the voltage drop at the open transition "EB" would be Vbe = 0.3 V.

The emitter current, approximately equal to the collector current

Ie = 9 B / 1 kΩ = 9 mA ≈ Ic.

The current of the base is Ib = Ic / h = 9 mA / 150 = 60 μA.

Voltage drop across the resistor R1

V (R1) = Vcc - Vb = Vcc - (Vbe + Ve) = 20 V - 9.7 V = 10.3 V,

R1 = V (R1) / Ib = 10.3 V / 60 μA = 172 kOhm.

C2 is needed to create a circuit for the passage of the variable component of the emitter current (actually the collector current). If it were not, the resistor R2 would severely restrict the variable component, so that the amplifier under consideration on the bipolar transistor would have a low current gain.

In our calculations, we assumed that Ic = Ib h, where Ib is the base current flowing into it from the emitter and arising when the bias voltage is applied to the base. However, the leakage current from the Icb0 collector always flows through the base (both in the presence of displacement and without it). Therefore, the actual collector current is Ic = Ib h + Icb0 h, i.e. The leakage current in the circuit with the MA is amplified 150 times. If we considered an amplifier using germanium transistors, this circumstance should be taken into account in the calculations. The fact is that germanium transistors have a substantial Icb0 of the order of several μA. In silicon, it is three orders of magnitude smaller (about a few nA), so they are usually neglected in calculations.

Single-ended amplifier with MIS transistor

Like any field-effect amplifier, the circuit under consideration has its analogue among amplifiers on bipolar transistors. Therefore, let us consider the analog of the previous scheme with a common emitter. It is implemented with a common source and RC-connections for input and output signals for operation in class "A" and is shown in the figure below.

Here, C1 is the same separation capacitor by which the source of the variable input signal is separated from the DC voltage source Vdd. As you know, any FET amplifier should have the gate potential of its MIS transistors below the potentials of their sources. In this circuit, the gate is grounded by a resistor R1, which usually has a large resistance (from 100 kΩ to 1 MΩ) so that it does not shunt the input signal. The current through R1 does not practically pass, so the gate potential in the absence of an input signal is equal to the potential of the earth. The potential of the source is higher than the ground potential due to the voltage drop across the resistor R2. Thus, the gate potential is below the source potential, which is necessary for normal operation of Q1. The capacitor C2 and the resistor R3 have the same function as in the previous scheme. Since this circuit has a common source, the input and output signals are phase-shifted by 180 °.

Amplifier with transformer output

The third single-stage simple transistor amplifier, shown in the figure below, is also made with a common emitter for operation in class "A", but with a low-impedance speaker it is connected via a matching transformer.

The primary winding of the transformer T1 is the load of the collector circuit of the transistor Q1 and develops an output signal. T1 transmits the output signal to the speaker and provides matching of the output impedance of the transistor with a low (of the order of several Ohm) speaker impedance.

The voltage divider of the collector power supply Vcc, assembled on the resistors R1 and R3, provides the choice of the operating point of the transistor Q1 (supply of bias voltage to its base). Assignment of the remaining elements of the amplifier is the same as in the previous schemes.

Two-stroke audio amplifier

The two-stroke low-pass amplifier on the two transistors splits the audio signal input into two opposite-phase half-waves, each amplified by its own transistor cascade. After performing this amplification, the half-waves are combined into an integral harmonic signal, which is transmitted to the acoustic system. Such a conversion of the low-frequency signal (splitting and re-fusion), naturally, causes irreversible distortions in it, due to the difference in the frequency and dynamic properties of the two transistors of the circuit. These distortions reduce the sound quality at the amplifier output.

Push-pull amplifiers operating in class "A" do not sufficiently reproduce complex audio signals, since a constant current of increased magnitude continuously flows in their shoulders. This leads to asymmetry of the half-waves of the signal, phase distortions and, ultimately, loss of intelligibility of sound. When heated, two powerful transistors increase the signal distortion in the low and infra-low frequencies by half. But still, the main advantage of the push-pull circuit is its acceptable efficiency and increased output power.

The push-pull circuit of the power amplifier on transistors is shown in the figure.

This is an amplifier for working in class "A", but can be used and class "AB", and even "B".

Transformerless Transistor Power Amplifier

Transformers, despite the successes in their miniaturization, still remain the most cumbersome, heavy and expensive ERE. Therefore, a way was found to eliminate the transformer from the two-stroke circuit by performing it on two powerful complementary transistors of different types (npn and pnp). Most modern power amplifiers use this principle and are designed to work in the "B" class. The circuit of such a power amplifier is shown in the figure below.

Both of its transistors are included in the scheme with a common collector (emitter follower). Therefore, the circuit transmits the input voltage to the output without amplification. If there is no input signal, both transistors are on the on-state border, but they are off.

When a harmonic signal is applied to the input, its positive half-wave opens TR1, but turns pnp transistor TR2 completely into cut-off mode. Thus, only a positive half-wave of the amplified current flows through the load. The negative half-wave of the input signal opens only TR2 and locks TR1, so that a negative half-wave of amplified current is fed into the load. As a result, a full sinusoidal signal is amplified on the load (due to current amplification).

Amplifier on one transistor

To assimilate the foregoing, we collect a simple amplifier on transistors with our own hands and understand how it works.

As a load of a low-power transistor T of type BC107, we'll turn on headphones with a resistance of 2-3 kOhm, shift the bias voltage to the base from a high-resistance resistor R * of 1 MΩ, decoupling the electrolytic capacitor C from 10 μF to 100 μF into the base circuit of T. Feed the circuit We will be 4.5 V / 0.3 A.

If the resistor R * is not connected, then there is neither a base current Ib nor a collector current Ic. If the resistor is connected, the voltage on the base rises to 0.7 V and the current Ib = 4 μA flows through it. The current gain of the transistor is 250, which gives Ic = 250Ib = 1 mA.

Having assembled a simple amplifier on transistors with our own hands, we can now test it. Connect the headphones and put your finger on point 1 of the circuit. You will hear a noise. Your body perceives the radiation from the mains at 50 Hz. The noise heard by you from the headphones, and is this radiation, only an amplified transistor. Let us explain this process in more detail. The AC voltage with a frequency of 50 Hz is connected to the base of the transistor through the capacitor C. The voltage on the base is now equal to the sum of the DC bias voltage (approximately 0.7 V) coming from the resistor R * and the AC voltage from the finger. As a result, the collector current receives an alternating component with a frequency of 50 Hz. This alternating current is used to shift the membrane of the speakers back and forth with the same frequency, which means that we can hear a 50 Hz tone at the output.

Listening to the 50 Hz noise level is not very interesting, so you can connect low-frequency sources (CD player or microphone) to points 1 and 2 and hear amplified speech or music.