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Measurements >> RF Measurements >> Phase Margin for RF amp
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Message started by darin on Jan 14^th, 2007, 12:56pm

Title: Phase Margin for RF amp
Post by darin on Jan 14^th, 2007, 12:56pm

Hi,

I am working on a high-gain transimpedance amplifier that has a closed-loop bandwidth of about 70MHz. I am wondering what would be the overall best way to measure (on the bench) the phase margin in this system. So far I have 2 methods that I've tried in the past (with much lower frequency systems)

1) Apply small square-wave input and estimate phase margin from overshoot
2) Insert small resistor in FB loop, apply AC signal to this via transformer. Sweep input signal
up in frequency until voltage on one side of resistor is the same as voltae on the other (Av=1)
Phase difference between input and output signals is phase margin.

Are there any oher methods I am missing? These are common tests for control systems, but this is my first analog design with BW > 3MHz .

Thanks for any help offered.

-darin

Title: Re: Phase Margin for RF amp
Post by Ken Kundert on Jan 14^th, 2007, 11:36pm

You might look at the work of Middlebrook on this topic (http://www.ardem.com/). He was really into developing ways of measuring loop gain on the bench.

-Ken

Title: Re: Phase Margin for RF amp
Post by mg777 on Jan 15^th, 2007, 11:49am

Note that the phase margin will be sensitive to the termination at the input i.e; whether you're driving with a voltage source (low impedance) or a current source (high impedance). You can easily figure out that the current source case is the one that will cause you pain.

Power supply bypass is another point often overlooked by TIA designers. Make sure you model your off-chip caps correctly. Another recommendation is to use a small amount of local feedback on the 'open loop' amplifier - like current series.

Whatever method you use, ensure you run your sims with layout pex (parasitic extraction). TIA's are particularly sensitive to the feedback capacitance Cf.

M.G.Rajan
www.eecalc.com