2
\$\begingroup\$

I have been going through the push pull stage as power amplifier. Even though the gain in this topology is less than 1, still people use it. I am assuming this is because in emitter-follower OR source-follower topology the input impedance is high and output impedance is low, that is why we are using these stages as power amplifiers.

BUT, operational amplifiers also depict the same characteristics, and hence I believe can be used as power amplifiers.

So why the book does not discuss op-amp as power amplifiers? Does op-amp lack something to act as power amplifier?

Can anyone be kind enough to throw some light in the above thinking.

\$\endgroup\$
8
\$\begingroup\$

The main power contribution of an emitter follower push-pull amplifier is from its current gain. This can be quite substantial.

Op amps generally have low output current, on the order of 20 mA or so. That most common type are much better suited to voltage amplification.

If you put the two together you can produce large amplification of small signals. The op amp can be used as a preamplifier, to bring a signal of say millivolts up to volts (but milliamps of current), followed by the push/pull stage that keeps the voltage about the same but amplifies the current to produce watts of output.

There is such thing as power op amps intended for use without a separate power stage following them. The LM675 (datasheet here) is one example, but a search on the term "power op amp" will bring up many others.

The closed-loop output impedance of an op amp is generally already quite low, so it's not ruled out strictly for that reason. Rather it's the fact that it "saturates" (i.e. cannot deliver any more current) at a milliamp level and would generally start behaving badly (e.g. increased distortion) if used to drive a low-impedance load.

\$\endgroup\$
0
\$\begingroup\$

Indeed, most op amps are designed for voltage gain. The output current which is available is usually a less important parameter.

If the required current is not too high, you could indeed use an op amp as a power stage. However, most are actually designed for a small current, since the current in the output stage is usually the biggest contributor to the power consumption of the device.

A very important characteristic of a power amplifier is its quiescent current consumption, meaning the current consumption when no output current is delivered.

The op amp normally has a class A type output, meaning it always consumes the same amount of current, regardless of load it sees. Moreover, it's theoretical efficiency is only 25%. If a high output power is required, this means at least 3 times this power is burned in the op amp itself.

A push-pull topology is class AB and therefore much more efficient compared to an op amp. Also, the quiescent current can be an order of magnitude lower.

If you'd like to know more, this wikipedia page might be a good starting point: https://en.wikipedia.org/wiki/Amplifier#Power_amplifier_classes

\$\endgroup\$
  • 2
    \$\begingroup\$ Do you have any evidence to support the assertion that "The op amp normally has a class A type output"? Most of the ones that I've seen have totem-pole (push-pull) output buffers. \$\endgroup\$ – Dave Tweed Aug 16 '16 at 13:08
  • \$\begingroup\$ @Dave, I'm by no means an expert on op amps, but the ones I encounter usually don't include a power stage. However, you're right that this is not always the case. If your op amp includes a power stage, there is of course no reason not to use it as such, except when running into current limitations. \$\endgroup\$ – Kristof Aug 16 '16 at 14:22
  • 1
    \$\begingroup\$ The class of an output and how much power it can handle are two entirely different things. Can you provide a single example of an IC opamp with a class-A output stage? \$\endgroup\$ – Dave Tweed Aug 16 '16 at 14:39
  • \$\begingroup\$ @DaveTweed Wouldn't proper totem-pole be better seen as push-push ? :-). ) Not what you asked for but interesting - push-pull per side with followers following. You could use each output side separately if desired if perverse enough. | 40A 100V ! :-) - apexanalog.com/resources/products/pa50u.pdf \$\endgroup\$ – Russell McMahon Aug 16 '16 at 15:12

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service, privacy policy and cookie policy

Not the answer you're looking for? Browse other questions tagged or ask your own question.