Hi All,
I have just built my first generic mono 3 stage amp on a couple of bread boards and was wondering why I am getting approx 5Vdc on my outputs (measured with respect to mains cap ground). A quick description of the amp:
Input: differential pair (bc377's) with simple current source and resistors on the tail.
Vas: Just one TIP42C with 33p Cdom, standard Vbe and current source.
Output: driver & output emitter followers(TIP41C's & TIP42C's).
VCC: +-24.5V
The main thing I don't understand is how the DC conditions are analysed on the outputs and Vas. I understand that adjusting my Vbe from 1-3V sets the quiesant current and I have seen the voltage accross my emitter resistors when I do this indicating that at least that part is working. But why would I still have 5Vdc on the outputs? Am I supposed to be setting something somewhere else for DC offset? Can maybe someone give me a link to the theory I'm talking about? There is probably something quite trival or perhaps fundmental that I am missing here......
Oh well at least I'm not getting smoke!
I have just built my first generic mono 3 stage amp on a couple of bread boards and was wondering why I am getting approx 5Vdc on my outputs (measured with respect to mains cap ground). A quick description of the amp:
Input: differential pair (bc377's) with simple current source and resistors on the tail.
Vas: Just one TIP42C with 33p Cdom, standard Vbe and current source.
Output: driver & output emitter followers(TIP41C's & TIP42C's).
VCC: +-24.5V
The main thing I don't understand is how the DC conditions are analysed on the outputs and Vas. I understand that adjusting my Vbe from 1-3V sets the quiesant current and I have seen the voltage accross my emitter resistors when I do this indicating that at least that part is working. But why would I still have 5Vdc on the outputs? Am I supposed to be setting something somewhere else for DC offset? Can maybe someone give me a link to the theory I'm talking about? There is probably something quite trival or perhaps fundmental that I am missing here......
Oh well at least I'm not getting smoke!
Well the obvious thing is that one rail is conducting more than the other... thus DC offset.
There must be a means of adjusting or setting the current through the output stage, not just the bias, for each rail - of course one can assume that the negative feedback is going to try set the centerpoint... so maybe you should check the *wiring*? Most problems with a circuit that is known to work (is it known to work) are due to wiring errors, including miswires, and components that look like the right one, but put in the wrong place...
_-_-bear
There must be a means of adjusting or setting the current through the output stage, not just the bias, for each rail - of course one can assume that the negative feedback is going to try set the centerpoint... so maybe you should check the *wiring*? Most problems with a circuit that is known to work (is it known to work) are due to wiring errors, including miswires, and components that look like the right one, but put in the wrong place...
_-_-bear
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