Push-Pull Amps

[SECONDandBOWERY]

Is it possible to arrange op-amps in a push-pull configuration?  Obviously there'd have to be a phase splitter but apart from that, how would this be done?  I understand about how tubes work in push-pull, but I've never seen op-amps like that.  I imagine it wouldn't be too different, but I'm just curious.

I think the op amps would have to be biased differently.  70% idle current, not 100%, right?  That would put it into class AB instead of class A, which means it could be configured in push-pull.

Am I looking at this the right way?

Rock'n'Roll,
--Andy

[teemuk]

The circuit configuration defines whether you need an output transformer or a phase splitter. It is possible to build a push-pull circuit without either one too - just two opamp stages working identically but biased differently. What you're suggesting about class B/AB is possible by seriously altering the bias points of the opamps. i.e. bias voltage of Vcc to one opamp and bias voltage of Vee to the other. This allows more swing on either positive or negative side of the wave but clips the other halfwave completely. Finally the two signals would be summed (i.e. using output transformer or mixer resistors). To lower crossover distortion the bias point could be altered. This works at least in theory but I don't know how well real-life devices could cope with such an arrangement - Probably not very well and there seems to be so many things that could go wrong. Frankly, I see no point in the whole thing: Why beg for more crossover distortion and instability when you can gain more power by using parallel or bridged configuration.

[joecool85]

I always thought that bridged was in essence a push pull setup.  I guess I was wrong...

[teemuk]

I think it actually is. I consider the bridge circuit analogous to a PP output transformer coupled amplifier, in which the load connects between the two output nodes . However, one can not make a bridged amplifier by grounding the other side of the load and using only one node for load input: The opposite phase signals would cancel each other out. With PP transformer coupling you don't have to care about this as long as you retain the right winding polarity in the primary side. Note that tubes have no negative polarity so the amplified signal has to be phase splitted to acquire push-pull effect - then the OT's primary acts as a phase inverter for the other amp stage's output. As a result the signals are either "summed up correctly" or an (opposite) phase difference affects between the load nodes. This allows either amplifying both halfwaves (class A) or using each stage to amplify it's own portion of halfwave (class B). I hope this made some sense.

[SECONDandBOWERY]

I kind of grasp what you just said, but not all of it.  I don't exactly understand what bridged circuits are.  Can you please try to explain them to me?  Maybe bridging two op-amps would be better than trying to fiddle with all that push-pull nastiness.

Rock'n'Roll,
--Andy

[el mo]

maybe: http://en.wikipedia.org/wiki/Bridged_amplifier

[SECONDandBOWERY]

Thanks!

Rock'n'Roll,
--Andy

[joecool85]

Bridged amps are my favorite    Nothing like a bridged LM3886 setup  :tu:

[SECONDandBOWERY]

If I wanted to bridge some LM3886's, would I just build them like regular circuits and then hook them up to the speaker as shown in the wikipedia article (the output of one circuit to the + of the speaker, the output of the other circuit to the - of the speaker) or do I have to change the biasing at all?

Rock'n'Roll,
--Andy

[teemuk]

No. You don't have to change bias. The idea is that bridged amps are as identical as they can be. They just have to work in an opposite phase.

[SECONDandBOWERY]

So do you need to make any modifications to the cuircuits or add any phase splitters/phase inverters?  Or can you just hook 'em up to the speaker as is?

Rock'n'Roll,
--Andy

[teemuk]

That depends of your circuit. Obviously you have to phase split the signal if you're planning to use two identical amp stages. If you're planning to bridge something it probably isn't a good idea to use one circuit as non-inverting and the other as inverting since the gains would have to be matched - though with this kind of circuit you would not need a phase splitter. I hope this answered your question.

Edit:
http://linkan.lewander.com/projects/amp/amp.htm
http://www.national.com/an/AN/AN-1192.pdf

[SECONDandBOWERY]

So if I'm understanding this correctly (and that's a big "if"), if I was running two LM3886 amplifiers (like the ones from http://www.chipamp.com) bridged, I could just stick a phase inverter in front of them and i should be good to go!

Rock'n'Roll,
--Andy

[el mo]

hi,
it's a bit of a joke, more a sort of thought-experiment, but it should work in theory, no?

[joecool85]

Thats basically it, but normally you have to split the signal before the amps (like shown), then just run a couple resistors with the speaker to allow for room for error.  There is a ton of good posts about it at diyaudio.com since a lot of those guys have bridged LM3886s for subwoofer usage.