Oscillation and circuit mismatch - RF Cafe Forums
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Post subject: Oscillation
Unread postPosted: Wed Apr 06, 2005 5:44 am
Joined: Wed Jan 05, 2005 2:20 am
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:
Unread postPosted: Wed Apr 06, 2005 6:35 am
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
Hope this helps,
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 :)