Oscillation and Circuit Mismatch - RF Cafe Forums
Post subject: Oscillation
Unread postPosted: Wed
Apr 06, 2005 5:44 am
Joined: Wed Jan 05, 2005 2:20 am
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:
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
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:
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
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,
postPosted: Thu Apr 07, 2005 12:56 am
can be predicted causing oscillation, due to the mismatch. Therefore,
RF PA design always try to avoid having VSWR >2.
with another fellow about improving the VSWR.
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.
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 :)