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Design >> RF Design >> Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
https://designers-guide.org/forum/YaBB.pl?num=1317786837

Message started by jianke on Oct 4^th, 2011, 8:53pm

Title: Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
Post by jianke on Oct 4^th, 2011, 8:53pm

Recently I met an interesting question for RF amplifier design. As we know, by adding a series inductor at Emitter, we can increase the stability of a BJT Common-Emitter (CE) amplifier. When it comes to a Common-Base (CB) or a Common-Collector (CC) amplifier, what should we do to increase its stability?

My objective is at 2.14GHz: (1) the CB amplifier works as a “current follower” (voltage amplification); (2) the CC amplifier works as a “voltage follower” (current amplification).

What I have tried for a CB amplifier is:
(1) Adding a series resistor at output, but the S parameters becomes really bad, typically S11 and S22. Also, the voltage gain decreases much.
(2) Using the ADS optimization tool to improve stability and S-parameters performance, but the results are not good. Typically the CB amplifier cannot work as a “current follower”. Also, the voltage gain decreases much.

The attachments are ADS simulations files and some simulations results. I hope they would be useful for your analysis. Thanks!

Title: Re: Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
Post by jianke on Oct 7^th, 2011, 7:10pm

Doesn't anyone have an idea or any comments?
My goal is to improve the stability to avoid oscillation at 2.14GHz.
So, what should I do for Common-Base (CB) or a Common-Collector (CC) configuration?
Waiting for your suggestion. Best Regards!

Title: Re: Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
Post by purplewolf on Oct 9^th, 2011, 3:28pm

for common drain ( or common collector) amplifier put a series RC circuit in parallel with the the gate of amplifier .

Title: Re: Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
Post by weber8722 on Nov 17^th, 2011, 8:33am

These things often help:

CC: series R at base or emitter
CE: series R at base or at collector
CB: series R at base

This is often bad:
CC: C at emitter

However, there is also some try-and-error. For instance, a cascode should be quite stable, but often its not due to large gain. Or in CE a L at emitter helps, because it reduces the gain, but also element self-resonances can destroy good performance and stability a lot. :-/

Title: Re: Stabilization Methods for Common-Base (CB) and Common-Collector (CC) at RF Fre
Post by rfcooltools.com on Nov 17^th, 2011, 12:04pm

jianke,

If you study some of the older VCO architectures you can get an insight into what to avoid.

Common base amplifier Colpits oscilator comes to mind.

Zbe is base emittor parasitic capacitor 1/(j w Cpi).
Ze is parasitic capacitance to ground for example 1/(j w Cpara).

Impedance looking into the base Zin= Zbe + Ze + Zbe*Ze*gm

Notice that Zbe*Ze*gm will be negative and real. This is why the colpits works, but not what you want . Positive resistance in series with the base, if greater than the negative should negate this oscillation mechanism.

http://rfcooltools.com