The term “amplifier” covers a very wide range, from a one transistor preamp to an ultra sophisticated high power hi-fi system. There is little doubt that the circuit shown here is very simple. The output is in the order 250mW – quite sufficient for most purposes and comparable to that of the average transistor radio. The distortion level is rather high, being about 5%.

The amplifier is also reasonably sensitive and will give full output with an input of about 50mV. Input impedance is about 50kR.

The slider from the volume control is connected to the, base of Q1 via a DC blocking capacitor. Q1 is connected as a conventional common emitter amplifier with R2 provides the base bias and R3 acting as the collector load. This stage is directly connected to the second transistor which is a PNP type. In this way the current passing through Q1 provides the bias for the second transistor. The output of the second transistor is connected directly to the speech coil of the loudspeaker. This is not normally good practice since the standing current in the output transistor continually biases the coil either slightly in or out from its usual operating point. However if a large speaker is used, as it should be, this has very little effect and, since we are not aiming at hi-fi, it does not matter.

The tone control comprises C2 and RV2 which are connected between the collector and base of Q1 At high resistance settings RV2 has little effect but on minimum settings the 100nF feeds back the high frequencies out of phase, thus cancelling them.

For this circuit to work properly, R3 must be selected with great care. The value shown here of 39 ohms is a typical one and, although it may be used for initial setting up to ensure the circuit is operating, the value should be found by experiment. If it is too low there will be severe distortion at higher volume settings. If it is too high the current drain will be excessive even though the quality of reproduction will be good.

It is very important that Q2 is fitted with a heat sink as it will get very not.

The speaker impedance is not all that critical and in the prototype speakers with an impedance as low as 8 ohms and as high as 80 ohms all worked well, although changing the speaker impedance will also necessitate a change in the value of R3.

Tags: amplifier, amplifier circuit, simple amplifier, simple amplifier circuit