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. Second, the emitters you are feeding with the DC
bias are very low impedance. 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. It's not the first time that
circuits shown in a reference manual are not production ready. 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.

www.telstar-electronics.com

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

Frank Gilliland wrote:
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

Yes Frank... look for an ally. Lord knows you need one... LOL

www.telstar-electronics.com

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

Thanks Frank for posting this link. It supports what I've been saying
all along about the plain-old diode method working like crap. So why
don't you go ahead and use that method on your new amp... LOL

www.telstar-electronics.com

On 27 Sep 2006 03:05:16 -0700, "Telstar Electronics"
wrote in
om:

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

Thanks Frank for posting this link.


No problem.


It supports what I've been saying
all along about the plain-old diode method working like crap.


The diode method works, it's just not idiot-proof so it's no good for
a production CB amp. So why didn't you incorporate the concept in your
earlier amps instead of vehemently defending your no-bias Class C POS
by claiming that "linearity is not required for SSB"?


So why
don't you go ahead and use that method on your new amp... LOL


Naw, I have a better system.

Frank Gilliland wrote:
The diode method works, it's just not idiot-proof so it's no good for
a production CB amp.

Well, I claim it doesn't work... unless you want to tell me how you can
parallel two diodes together... without any ballasting... and have both
of them turned on. That's essentially what that circuit is doing.
Unlike you... I have certainly tried it before. It doesn't work... for
the reason I stated.

http://search.ebay.com/_W0QQfgtpZ1QQ...Q2delectronics

On 27 Sep 2006 04:42:20 -0700, "Telstar Electronics"
wrote in
om:


Frank Gilliland wrote:
The diode method works, it's just not idiot-proof so it's no good for
a production CB amp.

Well, I claim it doesn't work... unless you want to tell me how you can
parallel two diodes together... without any ballasting... and have both
of them turned on. That's essentially what that circuit is doing.


No, it's not. I explained this before but apparently it flew over your
head. So I'll try to make it a bit simpler for you: Instead of having
a single diode checking the heat of two transistors, this guy used one
diode on -each- transistor. They may be wired in parallel but they are
-not- expected to work at the same time. Why? Because the bias will be
fixed by the diode with the lowest Vf, which will be the hotter of the
two diodes (Vf decreases as temperature increases).


Unlike you... I have certainly tried it before. It doesn't work... for
the reason I stated.


Well Brian, I -have- used the circuit before, in several different
variations, and yes it -does- work.

Frank Gilliland wrote:
Well Brian, I -have- used the circuit before, in several different
variations, and yes it -does- work.

Then show me the circuit that worked.

http://search.ebay.com/_W0QQfgtpZ1QQ...Q2delectronics

On Wed, 27 Sep 2006 05:10:56 -0700, Frank Gilliland
wrote in
:

On 27 Sep 2006 04:42:20 -0700, "Telstar Electronics"
wrote in
. com:


Frank Gilliland wrote:
The diode method works, it's just not idiot-proof so it's no good for
a production CB amp.

Well, I claim it doesn't work... unless you want to tell me how you can
parallel two diodes together...


Guess what, Brian? I took a closer look at the pic and it turns out
the diodes are IN SERIES! Doesn't matter tho, because as I was
browsing the other photos it appears that the amp has a seperate bias
regulator circuit that was added and is controlled by the Vf of the
two diodes in series, which is an even better method.