On Fri, 08 Oct 2004 17:26:07 -0700, Frank Gilliland
wrote:

On Fri, 08 Oct 2004 11:54:22 -0400, Dave Hall
wrote in :

On Fri, 08 Oct 2004 07:22:54 -0700, Frank Gilliland
wrote:
snip
If Brian appears and brings an open mind, maybe I'll show him how to
use predistortion in the bias regulator to linearize the output. But
that's a big 'if'.

Can you do that for RF amps?


Absolutely. One method is to tap the input signal to a small RF power
transistor with similar nonlinear characteristics and use it to shunt
the input to the final. That method also eliminates the need for a
base resistor and improves bias regulation.

That's sounds more like adaptive active bias. It's a bit tough to find
a biasing device that tracks the precise non-linear characteristics of
the amplification device, and you have to be especially watchful of
thermal runaway.



Overall, the amp requires
a little more drive power, but the benefits are well worth the costs.

This would not be a problem in the CB area, where the tendency is to
overdrive them anyway.....


There are other methods that work even better such as high-frequency
OP-amps with nonlinear feedback, inductors with nonlinear saturation
characteristics... just about any part of the circuit can be tailored
to compensate for a nonlinear power device.

I guess it all boils down to how much you want to invest in a good
design. At some point, you reach that magical point of diminishing
returns.

I wonder why more ham and commercial two-way radio amps aren't using
better designs than the simple basic stuff.

Dave
"Sandbagger"