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Oscillation and Circuit Mismatch - RF Cafe Forums

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

Amateur Radio | Antennas | Circuits & Components | Systems | Test & Measurement

Simon Liu
Post subject: Oscillation
Unread postPosted: Wed Apr 06, 2005 5:44 am
User avatar

Joined: Wed Jan 05, 2005 2:20 am
Posts: 4
Location: BeiJing,China
Is there any guys will share some documents about the Oscillation due to circuit mismatch. When we did some phone board testing ,from PA output, the VSWR around 5:1, I suspected there would be a high oscillation power which might blow up the PA due to mismatch between PA output and Antenna. I am not very clear with the Oscillation, need some material for explanation !

Thank you! :idea:


Post subject:
Unread postPosted: Wed Apr 06, 2005 6:35 am

Hello Simon,

In general oscillation might occure when your PA has a reflection coefficient greater than 1 at its output or input (Gamma in, Gamma out) , this leads to a situation of return gain instead of return loss.

You can measure your S11, S22 paramaeters (be very careful when measuring S22 with Netowkr Analyzer, use pad and add its impact on the retuen loss to the result), then calculate the Rollett factor, if this factor is less than 1 you are bound to oscillation.

There are several ways to prevent oscillation:

1) To reduce the gain of the amplifier, you can do that by using a shunt or series resistor eithr in the input or output. If this is PA it is better to put the resistor at the input to avoid the use of high power resistor.
2) Uniliterlization - to make your amplifier unilateral. Meaning to decreas the value of S12. If your application is broadband use a resistor between the base and collector of the transistor to reduce the reactance of the junction capacitance.
3) Matching network at the output that will make your PA become stable. However, there are trade-offs in the case of a PA between your design parameters as linearity and efficiency to stability, so you have to be careful with the design of such matching network that will satisfy all these trade-offs.

Hope this helps,


Post subject:
Unread postPosted: Thu Apr 07, 2005 12:56 am


High VSWR can be predicted causing oscillation, due to the mismatch. Therefore, RF PA design always try to avoid having VSWR >2.

I agree with another fellow about improving the VSWR.

First, you may try putting resistive load transistor, in the gate side of the device. It will help to reduce the oscillation, and sometimes also improved VSWR. But some trade-off maybe caused by putting higher resistor values, the gain and bandwidth might be worse.

the other popular technique is negative feedback technique. By using this technique it'll reduce the return loss as well as S12, therefore it'll improve the PA stability.

I hope it helps :)

Jean :-)

Posted  11/12/2012

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