Here is a Class A & AB amplifier made from individual components. It will deliver several watts of power into an ordinary 8 ohm speaker.
It provides hands-on learning about audio topics such as cross-over distortion, bootstrapping, complementary pairs, and push-pull.
The power module has been designed for a maximum input signal of about 50mV. With a signal larger than this the 10K volume control acts as a potential divider to reduce the signal and prevent distortion.
For smaller signals you will need to use a pre-amplifier. Or you could increase the value of the 10K feedback resistor R5 and so increase the amplifier gain above 11 (the ratio of (R3+R5) to R3.) Do not reduce R3 which will also affect the frequency response of the amplifier.
There are various classes of amplifiers which can be used to drive a loud speaker. The text books list them: class A, B, AB & C. The classes are defined in terms of the amount of bias which is applied to the amplifier input. Class A is permanently and fully biased on, while Class C only conducts on the peaks of the signals. In this design we have used Classes A and AB.
Q1 & Q2 are arranged as a complementary pair in a common emitter mode. This gives a high voltage amplification. However, it has a high output impedence which means it is not suitable for driving an 8 ohm speaker directly. R1 & R2 apply sufficient bias to keep the transistor pair permanently turned on. The amount of bias places them in the middle of their conduction range so that they react to both positive and negative swings of the input signal. There is continuous current flowing whether or not a signal is actually present and being amplified. To reduce the wastage of quiescent current another class of amplifier, Class B, uses a pair of complementary transistors which are not quite biased on. The signal is amplified by using the NPN transistor to react to the positive voltage side of the input signal while the PNP reacts to the negative side. At any instant only one transistor is turned on. This is called push-pull; when the NPN transistor (like out Q5) turns on the output voltage is pulled up towards the positive supply rail, while turning on the PNP (Q6) pushes it towards ground. Class B solves the quiescent current problem of Class A amplifiers, but introduces another -crossover distortion. Because the first part of the signal is used to complete the turn-on bias for each transistor you introduce distortion in the output whenever the input signal swings between positive and negative, or negative to positive. Enter Class AB Amplifiers.
Class AB also uses a pair of complementary transistors acting in the same push-pull arrangement, but it adds sufficient bias between the base-emitter junctions to just turn on both transistors when there is no signal flowing. When a signal is applied one of the transistors will turn on more and the other turned off. A diode in series with an adjustable resistance (as in our circuit), or the controlled output voltage of another transistor is normally used to apply precise bias. If the bias is too small then there will still be some distortion; but if it is too large an increased quiescent current will flow which will waste power. In the schematic you can see that the second stage of our amplifier is set up as an emitter follower with the load connected to the emitter of the transistor rather than the collector as in the first stage. The voltage at the output 'follows' the voltage at the input. They are the same value except from a 0.65V drop across the base-emitter junction. The advantage of the arrangement is that it produces a large output current at a low output impedence. This is ideal for driving a speaker. Now let us look at the bias needed to remove the crossover distortion which is the special feature of Class AB amplifiers. Because our circuit uses two Darlington pairs in the final amplifier stage, the bias needed to just turn on Q3 - Q6 should be about 2.6V (4 x 0.65V.) This is supplied by the voltage drop across the green LED and the trimpot. After adjusting the pot to remove crossover distortion you you should have the following circuit values: the bias voltage is about 2.05V - 1.93V across the LED and 0.12V across the pot; a collector current of 5.44 mA and a pot resistance of 22 ohm.
POWER SUPPLY: 9 - 18vDC.