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Power Amp

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There are a few things worth noting about power amplifers. They use high current transistors and here are some characteristics. As it turns out, the high current leads to large voltage drops across the resistors unless you use low value resistors. Power amps drive loads that require high current. It seems like you could still use a low current transistor. But here is the problem. Since the load is a low value resistor, and you still need voltage gain, the emitter resistor must be a low value. If you use a low value emitter resistor under low current, there isn't much voltage drop. Since the low current transistor can't drop a lot of voltage because of it's low power rating, you are stuck with a collector voltage that is very close to ground. And in order to get linear amplification, you need a collector voltage that is somewhere close to half the supply. Maybe your thinking that you could operate the low current transistor at a very low current, okay. Also you may think that since the load is being supplied power by the supply, you might not need a large signal, okay. Well operating the transistor under low current might get you the collector voltage, but the change in current is low in small transistors and therefore you will not get much gain. If you don't need a large signal then yes, you can use a small transistor and let the supply power the load.

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A power amplifier with toobs! He, he. ;D ;D

Kevin,
What are you talking about?
Power amplifiers must provide a fairly high voltage swing with a fairly high current.
Just use a low-current transistor or a few (or an opamp) for the voltage gain and a pair of complementary power transistors as emitter followers to provide the current.

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You can do it that way, but isn't the main reason for the emitter followers the current gain. I know that opamps have emitter followers, but I doubt they were ever intended to deliver signals of large amplitude. This is because of nonlinearity produced by the extreme change in VCE of the transistors. Not only is beta sacrificed but the change in current used to produce the large signal is excessive.

But what I was refering to was a just a simple power transistor used as an amplifier. These are just considerations.

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Hi Kevin,
A single simple power transistor used as an amplifier driving a low impedance load is lousy.

Modern opamps can deliver at least 24Vp-p into a 10k load with non-linearity so low that you can't hear or see it and can hardly measure it. When you include power emitter-followers in the negative feedback loop of an opamp, then you have a really good power amplifier.
There are also many really good IC power amplifiers. ;D

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Okay. So why go to the trouble of even biasing the thing. Let's say that we can use virtually any supply voltage, any gain, any load, any input, and at any frequency so long as it's a modern opamp. And we still don't have to worry about the integrity of the signal.

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Okay. So why go to the trouble of even biasing the thing.

Are you talking about using a 0V reference bias voltage for the input of a capacitively coupled opamp, or biasing idle current for the power emitter followers to avoid crossover distortion?

Let's say that we can use virtually any supply voltage

Most opamps have an absolute max supply voltage of plus and minus 18V. An OPA445AP has plus and minus 45V.

any gain

You need enough extra gain for plenty of negative feedback at high frequencies.

any load

Determined by the output power emitter followers, their heatsinks and the power supply current.

any input

Within reason.

and at any frequency so long as it's a modern opamp.

Opamps introduced in the last 20 years have a much wider bandwidth and lower noise than older ones like a 741 or LM324.

And we still don't have to worry about the integrity of the signal.

That's right. A power amp made with an opamp driving power emitter followers is nearly perfect, repeatable and very inexpensive. ;D

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