Push-pull amp with OP

piney · 27th March 2005, 08:30 PM

Hi, I have a question regarding this simple common design:

http://www.tallstrom.com/amp.gif

To keep a long story short, I have some already fixed power supplies: + 24V and +-40V

The OP can´t handle +-40 so I figured I could use the 24V for it, and "lift" the input signal to 12V.
However, how does this work with the +-40V for the output transistors?
I guess I have to use a number of DC coupling capacitors..? And some gain thrown in in the feedback loop.

Do I lift the inverting input of the OP to 12V also?

Would this work at all?
Thankful for any help.
Cheers
Martin

cunningham · 28th March 2005, 02:34 AM

All you are doing in this circuit with the 2 transistors is amplifying the current, i.e. lowering the output impeadence of the op-amp. You can't get voltage gain from an emitter follower circuit and therefore limited by the maximum voltage the op-amp will handle and will not get enough voltage swing for +/- 40V. Also you have to bias the two transistors so that they are just barely on and not operating in cut-off, not with the bases connected. Because the Vbe turn-on voltage takes a small amount of time to achieve, you can't amplify full 360 degrees signal and you are left with crossover distortion unless the transistor is already biased on above the cut-off level.

27th March 2005, 08:30 PM	#1
piney diyAudio Member Join Date: Feb 2005 Location: Stockholm, Sweden	Push-pull amp with OP Hi, I have a question regarding this simple common design: http://www.tallstrom.com/amp.gif To keep a long story short, I have some already fixed power supplies: + 24V and +-40V The OP can´t handle +-40 so I figured I could use the 24V for it, and "lift" the input signal to 12V. However, how does this work with the +-40V for the output transistors? I guess I have to use a number of DC coupling capacitors..? And some gain thrown in in the feedback loop. Do I lift the inverting input of the OP to 12V also? Would this work at all? Thankful for any help. Cheers Martin

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28th March 2005, 02:34 AM	#2
cunningham diyAudio Member Join Date: Jun 2004 Location: Knoxville	All you are doing in this circuit with the 2 transistors is amplifying the current, i.e. lowering the output impeadence of the op-amp. You can't get voltage gain from an emitter follower circuit and therefore limited by the maximum voltage the op-amp will handle and will not get enough voltage swing for +/- 40V. Also you have to bias the two transistors so that they are just barely on and not operating in cut-off, not with the bases connected. Because the Vbe turn-on voltage takes a small amount of time to achieve, you can't amplify full 360 degrees signal and you are left with crossover distortion unless the transistor is already biased on above the cut-off level.

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