afremont:
Oops, you're right the minimum hfe is around 40. But average current isn't the point, this is about idiot-proof non-self-destructive design.If the average is 200mA then the max is 400mA which will melt things if it gets stuck on.
Well, I'm not sure that would be self-destructive. In any case, as I said before, just rate the collector resistor at 3w. Problem solved. But read on...
afremont:
I don't see any need for a class A amp, common emitter class B should be plenty good enough and use less power. Sound quality certainly isn't the issue. An emitter follower would be good enough I think.
Class 'A' because I'm sure someone will likely find fault with any other solution. Since they have not seen it before, they will assume it just doesn't work. Not that they won't find fault with a class 'A'. Also, class 'A' will only require a couple of extra parts. Since the very simple was apparently too simple, why not go the extra step and make it something everyone can look up and wrap their heads around. BTW, I can't seem to get my head around an emitter follower solution. Not that it won't work.
So, below is the class 'A' I came up with. Draws 210mA @ 5V and delivers 125mw into 8ohms from 20Hz to 6KHz. Has typical single stage class 'A' distortion on sine a triangle waveforms and is biased to start clipping more or less symmetrically at Vin = about 4.5V. Square waves produce an expected over-shoot spike which is controlled by C3. If you don't care about the over-shoot, you can leave C3 out. The over-shoot on square waves causes and appreciable increase in gain at frequencies above 6KHz. Increasing C3 can help with this.
It works as described and does not get hot, explode, glow, kick the dog or beat the wife despite what you may believe. You can leave the drive pin high or low as you wish. If you want to prove it is does anything other than what I've stated, build one and show me actual results. So, all you 'Bash Street Kids', have at it.
