Simple transistor amplifier class "A"

Now on the Internet you can find a huge number of circuits of various amplifiers on microcircuits, mainly the TDA series. They have quite good characteristics, good efficiency and are not that expensive, which is why they are so popular. However, against their background, transistor amplifiers, which, although difficult to set up, are no less interesting, remain undeservedly forgotten.

Amplifier circuit

In this article we will look at the process of assembling a very unusual amplifier, operating in class “A” and containing only 4 transistors. This scheme was developed back in 1969 by the English engineer John Linsley Hood; despite its old age, it remains relevant to this day.

Unlike amplifiers on microcircuits, transistor amplifiers require careful tuning and selection of transistors. This scheme is no exception, although it looks extremely simple. Transistor VT1 – input, PNP structure. You can experiment with various low-power PNP transistors, including germanium ones, for example, MP42.Transistors such as 2N3906, BC212, BC546, KT361 have proven themselves well in this circuit as VT1. Transistor VT2 - NPN structures, medium or low power, KT801, KT630, KT602, 2N697, BD139, 2SC5707, 2SD2165 are suitable here. Particular attention should be paid to the output transistors VT3 and VT4, or rather, their gain. KT805, 2SC5200, 2N3055, 2SC5198 are well suited here. You need to select two identical transistors with the gain as close as possible, and it should be more than 120. If the gain of the output transistors is less than 120, then you need to put a transistor with a high gain (300 or more) in the driver stage (VT2).

Selection of amplifier ratings

Some ratings in the diagram are selected based on the circuit supply voltage and load resistance; some possible options are shown in the table:

It is not recommended to increase the supply voltage above 40 volts; the output transistors may fail. A feature of class A amplifiers is a large quiescent current, and, consequently, strong heating of the transistors. With a supply voltage of, for example, 20 volts and a quiescent current of 1.5 amperes, the amplifier consumes 30 watts, regardless of whether a signal is supplied to its input or not. At the same time, 15 watts of heat will be dissipated on each of the output transistors, and this is the power of a small soldering iron! Therefore, transistors VT3 and VT4 need to be installed on a large radiator using thermal paste.

This amplifier is prone to self-excitation, so a Zobel circuit is installed at its output: a 10 Ohm resistor and a 100 nF capacitor connected in series between ground and the common point of the output transistors (this circuit is shown as a dotted line in the diagram).

When you first turn on the amplifier, you need to turn on an ammeter to monitor the quiescent current. Until the output transistors warm up to operating temperature, it may float a little, this is quite normal. Also, when you turn it on for the first time, you need to measure the voltage between the common point of the output transistors (collector VT4 and emitter VT3) and ground, there should be half the supply voltage there. If the voltage differs up or down, you need to twist the trimming resistor R2.

Amplifier board:

The board is made using the LUT method.

Amplifier I built

A few words about capacitors, input and output. The capacitance of the input capacitor in the diagram is indicated as 0.1 µF, but such a capacitance is not enough. A film capacitor with a capacity of 0.68 - 1 μF should be used as the input, otherwise an unwanted cutoff of low frequencies is possible. The output capacitor C5 should be set to a voltage no less than the supply voltage; you should also not be greedy with the capacitance.

The advantage of the circuit of this amplifier is that it does not pose a danger to the speakers of the acoustic system, because the speaker is connected through a coupling capacitor (C5), this means that if a constant voltage appears at the output, for example, when the amplifier fails, the speaker will remain intact, After all, the capacitor will not allow DC voltage to pass through.