How to reduce oscillation in a cascaded amplifier?

[J0hn]

Hi,

I am cascading two or more amplifiers in serie together. I am worry about oscillation. Is there any way avoid/reduce oscillation? And perhaps, someone can start by explaining, what is main thing that causes oscillation when cascading the amplifier?

Thanks and appreciate that if someone can advise me.

 

[biff44]

Re: Cascaded Amplifier

Well, what most often causes oscillation in cascaded amps if positive leakage feedback.

If you think of the standard gain stage, at low frequencies it looks like a classic transistor inverter stage. In other words the output is 180 degrees out of phase with the input. So if some of the output leaks back to the input, you are out of phase and get negative feedback--ie no oscillation.

Now think of putting two such stages in series. The phase shift from input to output now is 180 + 180 degrees or zero degrees. Now feedback from output to input is IN PHASE, and is all set up to cause an oscillation.

Obviously, at microwave frequencies you do not get exactly 180 degrees per stage, but the concept is the same. Feedback at some frequency where the round trip is n*360 degrees will try to make an oscillation.

So, in some cases, choosing the right number of stages to put in series can make it unconditionally stable.

In other cases, you really have to be careful of your packaging and connector integrity to reduce the amplitude of the feedback leakage to be less than the forward gain.

You also have to be carefull about RFI on the bias lines, etc.