This amplifier can be built into any low-power equipment with low-voltage power supply: receivers, walkie-talkies, hearing aids and other similar equipment.

Specifications:
Maximum output power (Load 8 Ohm, 1 kHz) = 0.3 W
Nominal supply voltage (0.3W, 8 Ohm) = 3V
THD + N (at maximum output power, 1kHz) = 1 - 1.5%

Amplifier schematic diagram:

Device and principle of operation

The amplifier consists of two nodes: the input stage on the T1 transistor and the output push-pull on the T2 - T5 transistors. The signal amplified by transistor T1 goes to the load R1 and the output stage. The transistors of the output stage form two so-called "arms" of the output stage. The transistors in these "arms" are of different structure, which is a prerequisite for this amplifier. Since the KT315 transistor opens with a positive, and KT361 with a negative voltage, then the "shoulders" of the output stage formed by them amplify only that half-wave of the signal coming from the transistor T1, which "opens" the transistors that form them. It turns out like this: T3 and T4 amplify the positive half-waves of the signal, T2 and T5 are negative. At the junction point of the emitters of transistors T4 and T5, the signal is combined and fed to the load. Since this amplifier is characterized by step-type distortions that will inevitably appear during the operation of this amplifier, resistor R2 is turned on to attenuate them. This resistor creates a small bias voltage at the base of the transistors and reduces signal distortion.

This amplifier requires careful tuning, namely:
By selecting the resistor R1, the initial quiescent current of the transistors is set (the current flowing through the transistors in the absence of a signal). By selecting this resistor, it is necessary to set the quiescent current at the level of 5 - 7 mA.
By selecting the resistance of the resistor R5, it is necessary to set the voltage at the junction point of the output stage transistors equal to half the supply voltage, that is, 1.5 V.

Possible additions

If the device to which the amplifier is connected does not have a tone control or the signal taken from it is weak, you can assemble a pre-amplifier.

If the tone control is not needed, then it can be excluded from the circuit.
The resistor R4 is assembled passive regulator timbre HF - LF with one resistor. Resistor R3 is a volume control. All amplification of the signal falls on the transistor. Don't be confused by the lack of a capacitor between R3 and the collector of the transistor. Everything works like that.
Parts used and possible replacement.

This amplifier was assembled by surface mounting, so there is no printed circuit board file. Although drawing a signet for this amplifier is not at all difficult.

An easy-to-use amplifier is made on transistors of different structures and has a voltage gain of about 10. The maximum input voltage can be about 0.1 V.

The principle of operation of a push-pull amplifier

The first stage is assembled on a transistor VT1, the second - on VT2 and VT3 of different structure. The first stage produces voltage amplification of the audio frequency signal, with both half-waves being the same. The second - amplifies the current signal, however, the cascade on the VT2 transistor works with positive half-waves, and on the VT3 transistor - with negative ones.

The DC mode is chosen such that the voltage at the junction point of the emitters of the second stage transistors is about half the voltage of the power supply. This mode is achieved by switching on the resistor R2 back connection. The collector current of the input transistor, passing through the diode VD1, leads to a voltage drop across it, which is the bias voltage at the bases of the input transistors relative to their emitters - it allows you to reduce the distortion of the amplified signal.

The load is connected to the amplifier through an electrolytic capacitor C2. When the amplifier operates on a dynamic head with an impedance of 8 ohms to 10 ohms, the capacitance of this capacitor should be at least twice as large.

Photo of the circuit assembly

Take a look at the load connection of the first amplifier stage, which is the resistor R4. Its upper terminal is connected to the lower terminal of the load. This is the so-called "voltage boost" circuit, due to which a small value of the audio frequency of positive is supplied to the base circuit of the output transistors. feedback leveling the operating conditions of transistors.

List of used parts

The purpose of this article is to pay tribute to one of the most popular transistors of the 70s - 90s - KT315. Availability, small size and pretty good parameters allowed radio amateurs to use the KT315 transistor in different schemes, from simple to microcomputers. The tables below show the main parameters of the KT315 line.

Limiting parameters of transistors KT315 at T = 25 ° C

Parameters of KT315 transistors at T = 25 ° C

A little background: - the first planar - epitaxial transistor of the late 60s, that is, when in the manufacturing process the emitter, collector and base are made sequentially on one silicon wafer. To do this, it is necessary to dop a silicon wafer doped into type n (collector) to a certain depth into type p (base), and from above dope once more to a smaller depth into type n (emitter). Next, using a scriber, the plate must be cut into parts, and each part must be packed in a plastic case.
Such a manufacturing process was much cheaper than alloy technology, and made it possible to obtain previously unthinkable parameters of a transistor (in particular, an operating frequency of up to 300 MHz).
And of course the installation of the crystal is not in metal case, and on a metal tape with leads led to a reduction in the cost of production - a crystal, on the lower side of which the collector was soldered to the central lead, and the base and emitter were connected with a welded wire, filled with plastic, the excess parts of the tape were cut off - and KT315 was obtained like this.

Let's give a couple of examples of circuits on the KT315 transistor.

1. Headphone amplifier.

While the loop is intact, the base of the transistor is connected to ground and the transistor is off. When entering the protected area, the intruder breaks the wire, a positive bias enters the base of the transistor and the transistor opens, which ultimately leads to the operation of the electromagnetic relay. The relay contact circuit may contain a siren, radio transmitter, or other.

3. VLF output power indicator.

C1, C2 - 10 microfarads x 16B

D11 - KD510A

Rx - 300 Ohm - 100 Kom (for each stage it is necessary to select.)

D1 - D10 - LEDs of different colors.

Portable speaker for player or phone on tda2822m chip Amplifier check photo:

The following figure provides a list of required parts:

Almost any of the medium and high power bipolar transistors can be used in the circuit. n - p - n structures, for example KT 817. It is desirable to put a film capacitor at the input with a capacity of 0.22 - 1 MkF. An example of film capacitors in the following photo:

I give a drawing of a printed circuit board from the program Sprint-Layout:


The signal is taken from the output of an mp3 player or telephone, ground and one of the channels are used. In the following figure, you can see the wiring diagram of the Jack 3.5 plug for connecting to a signal source:


If desired, this amplifier, like any other, can be equipped with a volume control by connecting a 50K ohm potentiometer according to the standard scheme, 1 channel is used:


In parallel with the power supply, if there is no large electrolytic capacitor in the power supply after the diode bridge, you need to supply the electrolyte for 1000 - 2200 MkF, with an operating voltage greater than the supply voltage of the circuit.
An example of such a capacitor:

Download printed circuit board amplifier on one transistor for the sprint - layout program can be found in the site section My files.

You can evaluate the sound quality of this amplifier by watching the video of its work on our channel.

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