Class AB Amplifier using only one transistor

[Twisha]

How to explain a Class AB amplifier using only one transistor (and not push pull)?

 

[Audioguru]

Don't be silly. A class-AB circuit has two transistors conducting a small idle current then they go push-pull when they have an input signal.
A single transistor can be class-A or class-C.

 

[FvM]

Single transistor class AB makes no sense. Either A or C.

 

[Audioguru]

The internet has all kinds of mistakes on it. There are examples of a single class-C transistor and they wrongly say it is operating in class-AB. Here is one of them:

 

[betwixt]

Practically it makes no sense but strictly speaking, I suppose a NON-linear amplifier could be made, one with a current-limited bias so it could cope with tiny signals (in Class A) but most of the bias was derived directly from the input signal (and therefore rectified by the B-E conduction). Class C would probably be static biased out of conduction and rely on the drive to bias it, rather than slightly in to conduction.

I can't think of a practical application for such an amplifier except to deliberately introduce distortion, maybe as a harmonic generator?

Brian.

 

[mtwieg]

A class AB amp is literally defined as having a conduction angle greater than 90 and less than 180. There's no reason a class AB can't be made with a single transistor. Take any class A amp and overdrive its input into cutoff and you will get class AB operation.

The internet has all kinds of mistakes on it. There are examples of a single class-C transistor and they wrongly say it is operating in class-AB. Here is one of them:

According to textbook definitions, that is a class AB amplifier. Class C has the bias point below cutoff.

 

[FvM]

A class AB amp is literally defined as having a conduction angle greater than 90 and less than 180. There's no reason a class AB can't be made with a single transistor. Take any class A amp and overdrive its input into cutoff and you will get class AB operation.

I see that people tried to make different formal definition to differentiate the class-X types.

Apart from the quoted conduction angle specification, I see that most authors (probably not all) presume that class-AB and class-B make only sense for push-pull circuits while class-A and class-C are assumed to be single transistor circuits. The consideration seems plausible from a practical viewpoint.

I agree that we could call the overloaded class-A operation "class-AB". But is there any practical use of it?

 

[mtwieg]

Of course? Classes A to C are a continuum of bias points which gradually trade efficiency for linearity. If both extremes are useful, I don't see why the cases in between wouldn't be. I've frequently seen class AB used for RFPAs, sometimes with envelope tracking and/or predistortion.

 

[Audioguru]

An old rock and roll circuit called "Fuzz Face" made as much distortion as possible with two transistors biased wrongly.

 

[vfone]

Most of the integrated RF Power Amplifiers used in mobile phones are two or three stage PAs, using a single transistor in the last stage (with some exceptions in CMOS PAs).

This last stage transistor always operate (more or less) in Class-AB, due to a compromise that is needed between linearity, efficiency, out-of-band noise power, etc.

Even PAs used in Envelope Tracking approach (which theoretically needs a Class-C bias), they bias the last stage transistor slightly in AB class.

And the same, the old standard GSM/GMSK PAs, which theoretically they can use only Class-C stages (due to constant envelope modulation), all of them use the last stage transistor biased in AB class, otherwise they never pass Burst Mask and Spectrum due to Switching test.

In Audio never can use a single stage transistor biased in Class-AB.

 

[Twisha]

Thanks everyone!
So what I gather is that a Class AB amplifier designed using a single transistor can be used in applications where linearity is not a very important parameter.

 

[vfone]

Yes you can say like this or, Class-AB is used when you want to improve a bit the strong non-linearity of a Class-C stage.

 

[mtwieg]

Thanks everyone!
So what I gather is that a Class AB amplifier designed using a single transistor can be used in applications where linearity is not a very important parameter.

Or when you have feedback or predistortion to correct for nonlinearity, but need better efficiency.

 

[BigBoss]

A transistor can of course be biased as AB-Class ( so bias point can be between somewhere A-Class and B-Class ) but it doesn't make sense to bias a single transistor so that it will create so much distortion because of output waveform.But yet, it's theoretically possible..

 

[Audioguru]

Acid rock (music?) has a lot of distortion.