Hi everyone,
I'm wondering if it is desirable (or at least possible) to configure output transistors to supply current to the load, but not amplify the voltage. Is this referred to as a Common Collector configuration?
My thought process is, there are so many terrific opamps out there - and also lots of really good lower power amplifier circuits out there. And things start getting pretty complicated (for a newbie like myself) when I try and pick apart descrete amplifier circuits.
So does it make sense to persue a design that has all the voltage amplification done by a good opamp, and then have say a large transistor provide the current/impedence matching for the loudspeakers?
I'm wondering if it is desirable (or at least possible) to configure output transistors to supply current to the load, but not amplify the voltage. Is this referred to as a Common Collector configuration?
My thought process is, there are so many terrific opamps out there - and also lots of really good lower power amplifier circuits out there. And things start getting pretty complicated (for a newbie like myself) when I try and pick apart descrete amplifier circuits.
So does it make sense to persue a design that has all the voltage amplification done by a good opamp, and then have say a large transistor provide the current/impedence matching for the loudspeakers?
It can be done and there are many such designs using just that idea. The problem is that the transistors (you need at least one NPN/PNP pair) still need a bias arrangement to overcome the dead crossover region and so minimise distortion.
Circuits such as these are excellent for learning though, and given enough design input can produce excellent results.
Circuits such as these are excellent for learning though, and given enough design input can produce excellent results.
If it makes you feel better, tons (98%?) of circuits do exactly that: output transistors supply current to the load, but do not amplify the voltage.
Some use an Op Amp as basic gain stage, but 2 problems appear:
1) most can´t stand more than +/-15V rails, which severely limit output.
2) even worse, most can´t swing rail to rail so peak to peak excursion is even lower.
3) not surprisingly, some Op Amp input amplifiers require gain in the Power Transistors (usually around 4X) so *they* can swing Rail to Rail and reach significative power ... sadly the opposite of what you want.
4) there are some transistor boosted chipamps (which are basically "Power Op Amps") so by definition are what you seek.
5) even better and simpler: you can straight use a chipamp and nothing else to get up to 80W which may be more than enough.
And is exactly as simple as using "just an Op Amp": a couple capacitors, 3 or 4 resistors, no need to "design" since datasheet examples work very well.
Some use an Op Amp as basic gain stage, but 2 problems appear:
1) most can´t stand more than +/-15V rails, which severely limit output.
2) even worse, most can´t swing rail to rail so peak to peak excursion is even lower.
3) not surprisingly, some Op Amp input amplifiers require gain in the Power Transistors (usually around 4X) so *they* can swing Rail to Rail and reach significative power ... sadly the opposite of what you want.
4) there are some transistor boosted chipamps (which are basically "Power Op Amps") so by definition are what you seek.
5) even better and simpler: you can straight use a chipamp and nothing else to get up to 80W which may be more than enough.
And is exactly as simple as using "just an Op Amp": a couple capacitors, 3 or 4 resistors, no need to "design" since datasheet examples work very well.
No voltage gain and only current gain is almost always the case in discrete output stages. It's called emitter follower or indeed common collector.
Edit: I see JMFahey made similar points at the same time. However, there are opamps that can handle much higher voltages, up to +/-70V, and thus you can still have an output stage with gain = 1.
Jan
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Wonderful! This kind of "compartmentalisation" is exactly what I wanted to explore, thank you!
High voltage opamps are not great performers, its much better to do opamp -> VAS -> OS and use a high performance opamp.
After an opamp stage you typically only need a small factor of voltage amplification, 5 or 6 sort of range, allowing lots of negative feedback in the VAS - one idea I've looked at is using a CFP output stage topology with gain but much higher resistor values as only a few tens of mA is needed to drive the OS transistors. This can take +/- 10V upto full rail swing reasonably well.
After an opamp stage you typically only need a small factor of voltage amplification, 5 or 6 sort of range, allowing lots of negative feedback in the VAS - one idea I've looked at is using a CFP output stage topology with gain but much higher resistor values as only a few tens of mA is needed to drive the OS transistors. This can take +/- 10V upto full rail swing reasonably well.
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