[cfgeeks] Does this schematic pass the sniff test?

[Steve Litt]

Hi all,

I just found out my customer expects me to create all the exercise diagrams. 
Most customers want to supply the diagrams so their technologists can practice 
on things they regularly work on.

So I have to supply several simple diagrams to troubleshoot (one team 
conceptually "breaks" something, and the other team asks measurable questions 
(what's the voltage on the collector of the Class A transistor?

One of my diagrams will be the power stage of an audio amplifier (bipolar 
transistors). I've created and attached such a diagram. The purpose is a 
teaching exercise, and I've found from past experience that you need to go 
simple on these exercises, so I can't just copy a schematic out of a service 
manual or something.

But it has to at least work to some degree. It might be moderately distorted, 
it might be a little oscillatory at times, it might have a slight DC offset at 
the output, but it's got to function as a DC coupled audio amplifier.

Obviously I'm concerned with the feedback circuit, which almost has to 
introduce a slight DC offset on the output at 0volt input. I just didn't have 
time to mess with zeners to bring the DC of the feedback significantly 
negative of ground.

What I want to avoid in class is someone correctly saying "hey, the way this 
thing is designed, it will rail up to +28 or "hey, the way this is designed, 
it will continuously conduct 10amps through the outputs" or "hey, the way this 
is designed, it will clip the negative half of the wave."

By the way, resistances are purposely not specified. Obviously the output 
transistor emitter resistors would be about a half an ohm power resistors, the 
driver emitter resistors would be around 100 or so, and the ratio of the two 
resistors in the feedback circuit on either side of the base of the negative 
transistor of the diff amp would define the circuit's voltage amplification, 
but that's all implementation details -- I'm concerned about the basic design.

So tell me, do you think this is a reasonable excuse of an amp that will serve 
the purpose of troubleshooting exercises? It won't have to be built, people 
will just theorize a defect and the other team will ask pertinent questions.

Thanks

SteveT

Steve Litt
Recession Relief Package
http://www.recession-relief.US
Twitter: http://www.twitter.com/stevelitt

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