I was reading Audio Power Amplifier Design Handbook, Fifth Edition, from Douglas Self. I did not see any MOSFET circuits in that book.
Is there any reason why BJTs are used mostly in audio amplifiers?
Will MOSFETs give the same or better performance as BJTs in audio applications?
When MOSFETs were first introduced, there was a flurry of excitement amongst audio engineers. Class AB BJT output stages tended to have bad crossover distortion, which was ascribed to their sharpish turn on characteristic. FETs were more like valves, with their gentler turn on, and voltage rather than current drive, and so many class AB designs were done with them.
Unfortunately, MOSFETs have a tempco that changes sign with Vgs. When using them as switching devices with high Vgs, their positive tempco means they not only share current nicely between devices, but distribute it nicely across the die. When used in the linear region with low Vgs however, they are inclined to thermally run away. If you try to use a standard MOSFET in the linear region, you must derate it very heavily. Very few FETs have a 'DC' curve in their second breakdown graph. There are 'designed for linear' MOSFETs, but they are very expensive, and not very available. BJT's are just more robust, especially when you push your amplifier to '11'.
Crossover distortion has less to do with the turn on characteristic, and more to do with how it's controlled. Once amplifier design caught up with using BJT's properly, FETs offered no advantages, and designers stopped trying to build linear amplifiers with them.
Class D is of course quite recent history, and there fast switching MOSFETs are a perfect fit.
As well as more predictable biasing and lower cost as discrete components, BJTs generally introduce less voltage noise than MOSFETs, both 1/f noise and white noise. Older MOSFETs were particularly bad for noise and drift.
From Trade-offs Between CMOS, JFET, and Bipolar Input Stage Technology is a comparison of modern relatively high-end integrated circuit op-amps with BJT, JFET and MOSFET front ends:
One of the original MOSFET-input op-amps, the CA3140, has a typical noise density of 40nV/√Hz at 1kHz vs 5nV/√Hz for a bipolar NE5532. It's not too hard to get voltage noise density in the 1nV/√Hz range with bipolar or discrete JFET transistors.
Current noise is less for MOSFETs and JFETs, of course, but audio circuits tend to be low impedance, so voltage noise dominates.
You need to put into context why there are no FET circuits in the book.
This is basically a history question.
That 5th edition is 17 years old book from 2009.
The 1st edition came out 30 years ago in 1996.
The author started to work on audio amplifiers about 50 years ago approximately in 1975.
The only sensible components to do audio amplifiers back then were BJTs. When suitable FETs started to be available, they would be more expensive, more sensitive to their operating conditions, required more complex and more expensive circuitry around them, and simply put the whole FET circuit would look very different from a BJT circuit.
If you have already years of experience how to do good enough amplifiers that are simple, easy and robust for the required purpose with BJTs, there are not much reasons to use FETs.
So if you rarely use FETs, you might not have the same experience using them, might not have suitable well working circuits or building blocks that use them, so you also don't write about FET circuits in your book.
May I know is there any reason why BJT is used mostly in Audio Amplifiers.
To firmly activate a BJT, the required base-emitter voltage is about 0.8 volts. Compare this to a MOSFET where it might be a few volts between gate and source.
Then consider that the power driver for an audio amplifier is dominated by a "voltage follower" push-pull stage. This means that the BJT output can swing to within a volt of either power rail whereas for a MOSFET stage (source follower), the equivalent is 2 or 3 volts and, back in the 1980s or 1990s it might have been 5 to 8 volts.
So, the MOSFET is/was not favoured compared to the BJT because it couldn't deliver the power on a simple power rail.
Will MOSFET also give same or better performance as BJT in Audio applications?
Well, you would have to fix the problem mentioned above first. This can be done with an auxiliary supply used for driving the MOSFETs that might be 5 volts higher (negatively as well as positively) beyond the main power rails.
Once this is done I see no reasons why equivalent performance couldn't be achieved.
