# Stability in Power Amplifier - RF Cafe Forums

The original RF Cafe Forums were shut down in late 2012 due to maintenance issues. Original posts:

Noise
Post subject: stability in Power amplifier Posted: Tue Feb 27, 2007 9:56 am

Captain

Joined: Tue May 02, 2006 4:59 am
Posts: 15
hello,

How can I simulate and calculate stability in power amplifier ?
In small signal i can use the s-parameters for that (K>1 and |delta|>0) but in power amplifier the s-parameter is not available because its define only for small siganl.

thanks

noise

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fred47
Post subject: Power amplifiers and S-ParametersPosted: Wed Feb 28, 2007 3:46 am

General

Joined: Wed Feb 22, 2006 3:51 pm
Posts: 104
Hi!
Let's phrase your objection more precisely:

Manufacturers do not provide s-parameter data for their transistors when used in anything other than Class A amplifiers.

Large signal amplifiers certainly reflect power. Measure the magnitude and the angle of the reflection, and you can calculate an s-parameter.

What gives?

S parameters vary unpredictably when a transistor is driven into nonlinear operation. By "unpredictably" I mean that they depend on parameters that vary from device to device, and which aren't usually measured or listed at all.

There are entire books written on the subject - Cripps has written two good ones (check www.artechhouse.com).

Good Luck!
Fred

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Jeanalmira
Post subject: Posted: Mon Apr 09, 2007 9:37 pm

General

Joined: Tue Mar 15, 2005 11:43 pm
Posts: 65
Location: Singapore
Hi :

I agree with fred47.
S-parameters are not limited to small-signal. Let's put it this way, S-parameters are basically port-to-port measurement, it can be small-signal and large-signal. It depends on the measurement (power) conditions.

According to experience, stability can be defined well by small-signal, by defining K and delta. Most of cases, it wont change too much in large-signal. Unless, there are some "unwanted" positive feedback happens or driven transistor to extreme bias point.

I hope it helps.

Cheers,
Jean

Posted  11/12/2012

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