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Design >> RF Design >> Power Amplifier Stability
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Message started by aaron_do on Jan 13^th, 2013, 5:35pm

Title: Power Amplifier Stability
Post by aaron_do on Jan 13^th, 2013, 5:35pm

Hi all,

for a PA, the closer it is to instability, the higher the gain you can get. Just wondering, how close to instability do we normally design a PA for a high-end application? I guess you can look at the k-factor right?

thanks,
Aaron

Title: Re: Power Amplifier Stability
Post by RFICDUDE on Jan 13^th, 2013, 6:00pm

Gain isn't always the primary consideration for a PA. It is more common for power added efficiency (PAE) and possibly linearity to be primary design specifications. It is quite possible to achieve high/acceptable PAE without high gain.

That said, if you need a stable high gain power amplifier, then the first question is how power efficient does it need to be. If the answer is "efficiency doesn't matter, but linearity does" then using the Rollet stability factor (k and delta) is an ok approach. But if efficiency matters, then the amplifier is operating in some nonlinear fashion (Class AB, B, C, D, E). In these cases, the K and delta factors change as a function of power level, so you need to measure them with power sweeps using large signal S parameter analysis. Of course the trick is to keep the real part of the total impedance positive over all load conditions; otherwise, the amplifier may oscillate.

I hope other's chime in with their experience and approaches to PA stability.

Title: Re: Power Amplifier Stability
Post by aaron_do on Jan 14^th, 2013, 12:46am

OK I suppose my question was a little too general. Suppose we are talking about a class AB PA. Although k>1 doesn't guarantee stability, I think most designers still check k before taping out. Just wondering if there are any guidelines on an acceptable k-factor. Generally speaking, the lower the k-factor, the more you're getting out of your device.

Aaron