If I were to design a Class AB amplifier, which transistors would I use?

Question

I am designing a simple amplifier, specifically a class AB amp, and want my load resistance to be 8 ohms. I have two small diodes in my circuit, and am contemplating using transistors 2N3904 (npn) and 2N3906 (pnp). Am I on the right track?

Answer 1

In a class AB design a negative voltage is applied to the grid which controls the flow of electrons.

You should now be aware of of the basic building blocks that appear in Power Amplifiers. That Class A amplifiers employ a high Quiescent Current (sometimes called bias current or standing current) large enough that the transistor currents are large even when the output signal level is small. It should be clear that the power efficiency of Class A amplifiers is therefore poor, but they can offer good signal performance due to avoiding problems with effects due to low-current level nonlinearities causing Distortion. You should now also see why a Double Ended output is more efficient than a Single Ended design. You should also know that Class B has a very low (perhaps zero) Quiescent Current, and hence low standing power dissipation and optimum power efficiency. However it should be clear that in practice Class B may suffer from problems when handling low-level signals and hence Class AB, with its small but controlled Quiescent Current is often the preferred solution in practice. It should also be clear how we can employ something like a Rubber Zener to set the optimum Quiescent Current and power dissipation for a given task.

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Answer 2

Yes, you are. This would be a push-pull emitter follower, AB driver. NPN collector to +Vs, PNP collector to -Vs. The bases biased from the two diodes string, with the diodes biased with resistors to each power rail. You can connect the center of the diode string as the input.

As a emitter follower type amplifier, you will have current gain, and less than unity voltage gain. But this is what you need to drive a low resistance like 8 ohms.

Answer 3

Those transistors will work, but only if you want the power output of the amplifier to be on the order of 1 Watt.

If this is your first time constructing an amplifier, then you should probably stick with low power (1 Watt or less) and learn from that -- then you can graduate to designing a higher power level amplifier.

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Answer 4

use whichever transistor that best meets the need of your amplifier, is it for audio, for signal generator, RF. For small signal amplification BJT will work or for some higher voltages MOS

Answer 5

I personally would use MOS. I would do this because of the fact MOS is more linear than BiPolar. With MOS you do not need all of the DC biasing resistors which you need for BJT. BJT is also less efficient.