This week we’ll take a look at another type of audio amplifier, the class AB amp. As we saw in Hack #19 the class A amp has a transistor that is biased with a voltage that makes it essentially stay turned on all the time. This single transistor handles the entire waveform both positive and negative.
The class B amplifier uses two transistors. One handles the positive side of the waveform and the other handles the negative side of the waveform. Each transistor is in an off state until the waveform crosses over the zero point and then it must turn on. This takes about 0.7 volts so there is a small amount of time that the transistor is not amplifying the signal. This lag time at the crossover point can cause distortion. There is a way around this however, the class AB amplifier.
In the class AB amp the transistors are biased in such a way so as to never fully turn off. They are kept on by two biasing diodes which allow a small amount of collector current to flow even when there is no signal present. This means then that the transistor will be “ON” for more than half a cycle of the waveform but much less than a full cycle giving a conduction angle of between 180 to 360o or 50 to 100% of the input signal depending upon the amount of additional biasing used. The amount of diode biasing voltage present at the base terminal of the transistor can be increased in multiples by adding additional diodes in series.
There’s a great tutorial on class AB amps on Electronics-Tutorials.ws. I encourage you to visit this site where you’ll find a wealth of educational information on electronics.
This project is fairly easy to build and all the parts are readily available. You can use just about any NPN and PNP transistors, just make sure they’re within similar specs to each other.
Keep on hackin!
2 – 2N3904 NPN transistors
1 – 2N3906 PNP transistor
1 – 47 uF electrolytic capacitor
1 – 470 uF electrolytic capacitor
1 – 100K ohm resistor
1 – 1K ohm resistor
2 – 1N4148 diodes