On Mon, 11 Oct 2004 06:57:51 -0400, Dave Hall
wrote in :

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.


What I described is kind of a psuedo-Darlington connection. It has
less gain than the classic Darlington, but has a higher frequency
response and a lower VBEsat. It also has the Darlington's high
linearity. And while the final will still saturate, it will do so with
more difficulty. The problem of thermal runaway is avoided by mounting
both transistors on the same heatsink. The real problem comes from the
collector capacitances (the devices being RF power transistors), so
the pairs must be carefully chosen and matched for phase using a few
extra reactive components. Lot's of math. Needless to say, it's not a
popular design except in some very high power amplifiers where other
methods would be more expensive.


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.....


I'm sure they would find a way.


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.


The real expense is paid in the initial design of a custom component,
and that's just an one-time expense. After the design is finalized
even custom components are relatively inexpensive, especially when
ordered in large quantities. (For custom inductors, Micrometals has
been a real good company to work with over the years. They even have
composite cores that would be ideal for applications just like this.)


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


Well, many do. I'm sure you've looked at component lists and seen
inductors that are listed only as in-house numbers. Sometimes that
applies to semiconductors, too (such as the Peavey dual-diode I was
talking about in another thread). Sometimes capacitors are chosen not
for their linearity but for their -non-linearity. And everything I
said here about linearity also applies to other factors -- seemingly
run-of-the-mill components are sometimes carefully selected for
specific temperature coefficients, equivalent series resistances,
breakdown characteristics, etc. Even tubes and transistors are often
selected for gain figures that fall within a range smaller than the
defined tolerances of the part specification. What often appears to be
an off-the-shelf part may not have an off-the-shelf replacement. An
example is the use of sequential mic/keying relays used on many older
tranceivers -- the modification to the relay is rarely documented, and
replacement with a generic (synchronous) relay can quickly burn out
the screens in the finals. Been there, done that, and the T-shirt is
now a shop-rag.

A good design requires a LOT of engineering, not just big heatsinks
and pretty PCB artwork. The specs for Brian's amp don't demonstrate
much engineering at all. Heck, even a little negative feedback would
help that amp immensely but he won't even to -that- much. Oh well.

I didn't notice, but is he still using a single relay to switch both
the input and output?






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