Simple Class A Transistor Amplifier

Now on the Internet you can find a huge number of circuits of various amplifiers on the chips, mainly the TDA series. They have quite good characteristics, good efficiency and are not so expensive, in connection with this and are so popular. However, against their background, transistor amplifiers remain undeservingly forgotten, although they are difficult to configure, but no less interesting.

Amplifier Circuit

In this article we will look at the process of assembling a very unusual amplifier working in the classroom "A" and containing only 4 transistors. This scheme was developed in 1969 by English engineer John Linsley Hood, despite his old age, it still remains relevant.
A simple transistor amplifier class A
Unlike amplifiers on chips, transistor amplifiers require careful tuning and selection of transistors.This scheme is no exception, although it looks very simple. Transistor VT1 - input, PNP structure. You can experiment with various low-power PNP transistors, including germanium ones, for example, MP42. Well proven in this scheme as VT1 transistors, such as 2N3906, BC212, BC546, KT361. Transistor VT2 - structure NPN, medium or low power, here are suitable KT801, KT630, KT602, 2N697, BD139, 2SC5707, 2SD2165. Particular attention should be paid to the output transistors VT3 and VT4, or rather, their gain. This well fit KT805, 2SC5200, 2N3055, 2SC5198. It is necessary to select two identical transistors with the closest possible gain, thus it should be more than 120. If the amplification factor of the output transistors is less than 120, then a transistor with high gain (300 and more) must be placed in the driver cascade (VT2).

Amplifier selection

Some of the ratings on the circuit are selected based on the circuit power supply voltage and the load resistance, some possible options are shown in the table:
A simple class A transistor amplifier
It is not recommended to raise the supply voltage of more than 40 volts, output transistors can fail.The peculiarity of class A amplifiers is a high quiescent current, and, therefore, a strong heating of the transistors. With a supply voltage of, for example, 20 volts and a quiescent current of 1.5 amps, the amplifier consumes 30 watts, regardless of whether a signal is applied to its input or not. At each of the output transistors, 15 watts of heat will be dissipated, and this is the power of a small soldering iron! Therefore, the transistors VT3 and VT4 need to be installed on a large heatsink using thermal paste. This amplifier is prone to self-excitation, so the Zobel circuit is set at its output: a resistor of 10 Ohm and a 100 nF capacitor connected in series between the ground and the common point of the output transistors (in the diagram this circuit is shown by a dotted line). When the amplifier is first turned on, it is necessary to turn on an ammeter to break its supply line to control the rest current. While the output transistors have not warmed up to operating temperature, it can float a little, it is quite normal. Also when you first turn on you need to measure the voltage between the common point of the output transistors (collector VT4 and emitter VT3) and the ground, there must be half the supply voltage.If the voltage differs up or down, you need to twist trimmer R2.

Amplifier board:

The board is made by the LUT method.

The amplifier assembled by me

Simple Class A Transistor Amplifier
Simple Class A Transistor AmplifierSimple Class A Transistor AmplifierSimple Class A Transistor AmplifierSimple Class A transistor amplifier A few words about capacitors, input and output. The capacitance of the input capacitor in the diagram is designated 0.1 microfarad, but this capacity is not enough. A film capacitor with a capacitance of 0.68 - 1 μF should be used as an input, otherwise an undesirable cutoff of low frequencies is possible. The output capacitor C5 should be taken at a voltage not less than the supply voltage, you should not be greedy with the capacity. The advantage of the circuit of this amplifier is that it does not pose a danger to the speakers of the speaker system, because the speaker is connected through a separating capacitor (C5),This means that when a DC voltage appears at the output, for example, when the amplifier fails, the speaker will remain intact, because the capacitor will not miss the DC voltage.

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