On 26 Sep 2006 16:10:26 -0700, "Telstar Electronics"
wrote in
.com:

LeIand C. Scot wrote:
http://perso.orange.fr/f6itv/p2032001.htm (look at location of diodes in
photo at right side)

Yes, this method is similar to a few "reference" designs shown in the
Motorola RF Data Manual . This is a poor method for two resaons. The
first is what I explained before... trying to get two diodes in
parallel to turn on together is very difficult... if not impossible on
a repeatable basis.


In the link above the diodes are not in parallel at all. Look at the
schematic.

In the link you initially provided they are indeed in parallel, but
that doesn't mean they are intended to conduct equal currents. If that
were the case then they would have current equalizing resistors in
series with each diode. From what I can see in the picture, it's clear
that only one diode will be working at any given time: the hot one.
And I'm sure that's exactly what was intended.


Second, the emitters you are feeding with the DC
bias are very low impedance.


Very low voltage, too. Some might even call it a "ground potential".
The bases, on the other hand......


This creates its own problem when
attempting to control the base current. This biasing scheme with temp
compensation is a "brute force" method that dissipates large amounts of
power... and plain doesn't work well.


Apparently it works well enough for a homebrew amp.


It's not the first time that
circuits shown in a reference manual are not production ready.


Hey, look what I found on a quick google search:

================
On 31 Jul 2005 15:58:33 -0700, "Professor"
wrote in
.com:


Frank... I wish I was like you... never wrong... and never had to be
corrected.


Yes, I already knew that, Brian. I tried to turn you onto the right
path years ago when you were hacking basic Motorola AN circuits that
were intended to be starting designs for engineers, not finished
products to be built by CB ampheads. But you were just too impatient
to get your "product" sold and get your share of that illegal market.
After damage control backfired in your face (because of your lack of
education) you vanished. Now you pop back up to spam the group every
time you think you have an improvement. You haven't made very many
changes, but look at the ones you -did- make -- all were suggestions
that I made when I told you all the reasons your amp sucked. Maybe you
learned those things from me or maybe somewhere else, but I was right
and you know it......
================

So contrary to recent opinion polls, it's clear that you -can- learn
things. You just can't admit that you learned anything from someone
else because that might be bad for business.


Motorola
also shows a much better method of temp comp bias in that same data
manual. It uses an op amp and sink mounted thermistor. I'm also using
an "active" approach to the temp comp bias in my amplifier... but a
totally different circuit using a bipolar transistor as the sense and
gain mechanism. I can hold the 500mA bias to 10% from -30 to +85C.


Hey Jan, wanna know why Brian won't cut loose his schematic? Because
Brian is a hack and his special bias circuit was most likely ripped
from this link, which was posted in this group a couple years ago:

http://www.ifwtech.co.uk/g3sek/tr-bias/tr-bias1.htm