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Design >> RF Design >> Load line equation
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Message started by Venture on Apr 20^th, 2014, 3:39am

Title: Load line equation
Post by Venture on Apr 20^th, 2014, 3:39am

Hi there,
New to the forum so go easy on me!

I'm trying to find the most accurate load line equation for designing an RF power amplifier using a mosfet transistor. The device itself is MRF173 and I need 80W from 28v. So far I have seen three variations on the same theme, all which give different results.

What do you guys think?
Here are the 3 equations I've seen...

Vcc 2 / 2 x Po

Vcc - Vsat 2 / 2 x Po

Vcc x 0.85 2 / 2 x Po

I thought the middle Vsat one was from Steve Cripps and more applicable to bipolar but someone has said you can use it for a mosfet and assume 2V for Vsat.
Sorry about the presentation of the equations, the first two 2s are supposed to be 'sqaured'. I will figure out how to do it properly soon!

Thanks, Venture

Title: Re: Load line equation
Post by aaron_do on Apr 20^th, 2014, 6:14pm

Hi,

if I had to pick one of the equations, it would be the second one. The third looks like the 0.85 was found by experience, and the first is missing the effect of Vsat. However, you can't just assume Vsat is 2 V, since some PAs are designed with a 1.8-V supply. In fact, I think the value of Vsat is going to depend on the linearity required by your PA since there isn't exactly a well defined clipping of the signal due to Vsat...

For a first-pass design equation, equation 2 is fine...

Aaron

Title: Re: Load line equation
Post by Venture on Apr 24^th, 2014, 4:40am

Hi Aaron,

Thanks for the info. I've been digging a bit since posting and it seems that the mosfet equivalent of Vsat is VGth, so can vary like you say. For the kind of FETs i'm using it seems to vary from about 2-4V.

Many thanks.