Class-A amplifier on oscilloscope?

[neazoi]

Hello,
I design an RF amplifier and I need it to operate in class-A
To ensure operation in class-A is it enough to see a clean amplifier sinewave on the oscilloscope, or do I need to make also other measurements or adjustments?

 

[biff44]

Class A amplifier is all about bias point. You put in the largest signal you hope to use with the amplifier, and look at the output for clipping. To a first order, you bias it so that it is not clipped, and you are done.

 

[FvM]

If you have particular requirements regarding linearity, intermodulation etc. you should check them explicitely. The load in measurement should correspondend to real operation conditions.

 

[neazoi]

Thank you
What is this signal represent? It is definitely not class-A but is it because of the overdrive of the amplifier or it is just another class of operation? (note the smaller glitches on the cucle ends and the asymmetrical clipping)
When I reduce the drive I have a clear sine wave. Does an overdriven amplifier appears the same as a class-B one?

 

[keith1200rs]

It could be overdrive or the bias point being in the wrong place maybe?

Keith.

 

[karesz]

Hi,
seems as some oscillating effect becouse overdriven is..
K.

 

[betwixt]

You could try checking linearity by connecting the input signal and output signal to the oscilloscope in X-Y mode. It will show a straight diagonal line if the circuit is linear and will easily indicate non-linearity or clipping by a curve or bend in the trace.

It doesn't actually prove you are in class A but I think linearity is what you are trying to achieve, not the method of getting it. Certainly, a class A amplifier with incorrect biasing will be shown straight away and it can be adjusted in real-time by watching the trace.

Brian.