If you were using mosfets, it would be more like 14 volts peak to peak into a decent load. Given that each BJT would "lose" 0.7 volts (because of the base emitter junction), the maximum output voltage will be about 20.6 volts peak to peak. You can only drive the output transistors with a signal that is restricted to the power rails and if this is (say) 24V dc - you should be able to drive a signal that is 22Vp-p to the power transistors. This is because, to turn-on a BJT, you only need about 0.6 to 0.7V whereas to get a MOSFET supplying several hundred milliamps you might need to drive its gate with 3 or 4 volts.Īgain, this will be a simple emitter-follower push-pull class AB output stage. I use the word effective to mean that your output voltage will swing higher/bigger for the same power supply with BJTs used in a simple push-pull circuit. You will probably make a more effective power output stage using BJTs for the same number of components compared to MOSFETs. Designing a amp with very low noise and very low distortion is not trivial. Since you will be going over BJT details more thoroughly, I'd stick to them for now. Those would require a different circuit topology to utilize properly, but can be used to make a good amp too. You can also use other types of transistors, like JFETs or MOSFETs. Those come from careful circuit design and attention to these parameters at every step along the way. In audio, overall noise and harmonic distortion are high priorities. You can certainly create a impressive audio amp with the transistors I mention, but you can also make a mess. If you are aiming for a few Watts, I'd probably go with basic parts like the TIP41 (NPN) and TIP42 (PNP).Īgain though, it's not the choice of transistor that will make or break this project.
There are lots of possible power transistors to use as the final output.
The ones I mentioned have reasonable gain and can handle up to 40 V, which should be plenty good enough to allow for a amp to impress your profesor with. If you have your own favorite jellybean small signal transistors, use them if you prefer. I'd pick jellybean parts like the 2N4401 (NPN) and 2N4403 (PNP) and stick with them for everything except for the final power output transistors. It will be the circuit, not the transistor, that makes the amp work well. You could successfully build a audio amp from many different types of BJTs.
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