I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

"Telstar Electronics" wrote in message
oups.com...
I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

No, someone has just put them in as an attempt to protect against reverse
polarity!
Slightly mad.

Paul wrote:
No, someone has just put them in as an attempt to protect against reverse
polarity!
Slightly mad.

Is that what you think that is???? Why have the diodes right on top of
the transistors? No, I think that was an effort to temp track the bias.
Well, at least they were trying. Unfortunately, don't think that's
going to work that way... LOL

www.telstar-electronics.com

On 24 Sep 2006 08:53:06 -0700, "Telstar Electronics"
wrote:

I came across this site... and picture. Are these diodes an attempt at
thermal tracking for the bias?
http://www.worldwidedx.com/gallery/a...pad_and_diodes

www.telstar-electronics.com

It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

www.telstar-electronics.com

Tacking the diodes on to the case of the transistor works better than
attaching them to the heat sink due to the relatively slow thermal response,
the heat sink changes temperature slower that the cap on the transistor. If
you want as fast a response as possible then the diodes are placed about as
well as you can get. I've also seen small diodes clipped on to the flange of
TO-3 style cases for the same reason. I've seen some high power transistors
used for motor drive and high frequency inverter use that has the
temperature sensing diode, or thermistor, built inside of the package.

http://pdf1.alldatasheet.co.kr/datas...50HM75STG.html

As the temperature of the base-emitter junction goes up the required forward
voltage for a given base current decreases. If nothing was done, the
base-emitter bias voltage held fixed, the base current for increases and
thus the collector current too. Of course increased collector current
results in more power dissipation, thus more heat and higher resulting
temperature, i.e. thermal runaway.

The diode used for bias temperature compensation has to very closely match
the Ib-Vbe curve of the transistor to work effectively . You can't use just
any diode and expect it to work very well. The problem with mounting the
diode on the heat sink is the large thermal mass. If the transistor starts
to go in to thermal runaway the heat sink will take a significant time to
heat up, thus reducing the bias to the base-emitter junction, long after the
runaway has started. You want to get the compensation right as soon as
possible, reduce the base-emitter bias, before the junction gets too hot.
The small cap on the transistor will heat up much faster that anything else
giving the designer a chance to design a fast responding bias circuit which
could avoid transistor destruction.

Diode bias circuits are on their way out in favor of more advance bias
circuits. The chief problem with diode bias compensation is either too much,
the transistor gets starved for needed bias current, or not enough, tendency
to go in to thermal runaway.

The reason the transistor can get starved for bias current is simple. The
Beta, current gain, of a bipolar transistor is not constant over the
operating range, it varies based on the instantaneous operating point. At
some point the gain is so low that the current flowing through the bias
compensation diode may drop to a low value, or go to zero, limiting the bias
current forcing the operation to go non-linear over part of the cycle.
That's why some of the simple diode bias circuits draw so much standing
current. It has to supply the worst case bias current. Newer active bias
circuits use a voltage source type design. The bias voltage tracks the
transistor temperature and since it is a "voltage source" there is no
practical limitation on the bias current. The transistor will draw what it
needs without being limited by the current limiting resistors in the diode
bias type of circuit.

I'm sure somebody is going to nit-pick this post. They're welcomed.

--
Regards,
Leland C. Scott
KC8LDO

"Telstar Electronics" wrote in message
ups.com...
wrote:
It provides negative thermal feedback.

The voltage across the transistor will decrease as the transistor
temperature increases. This prevents thermal runaway.

That's what it's supposed to do... I claim it can't work as shown. It's
not quite that simple as tacking a few diodes on the bias circuit and
laying them on top of the transistors... LOL

www.telstar-electronics.com

On Sun, 24 Sep 2006 23:30:24 -0400, "LeIand C. Scot"
wrote in :

snip
I'm sure somebody is going to nit-pick this post. They're welcomed.


Not picking nits, just taking a different approach.....

Thermal bias compensation works to a point but neglects one important
issue: it takes time for heat to travel from the junction to the
outside of the package, and thermal runaway can happen faster than a
-thermal- compensation circuit can respond to it. Since the heat
buildup is due to excessive EC current, it makes more sense to control
the EC current directly.

There are two solutions that use this approach. One is to include a
resistor on the emitter with a TC opposite of the transistor. Not
precision but much faster response. The other is to monitor the EC
voltage and the base current; send both measurements to a differential
OP amp and use the output as feedback for the bias regulator.

I tend to favor the first choice because it has the added benefit of
improving linearity. I would only use the second choice in a high
profit, 'stick-it-to-the-consumer' product.

On Sun, 24 Sep 2006 21:59:32 -0700, Frank Gilliland wrote:

On Sun, 24 Sep 2006 23:30:24 -0400, "LeIand C. Scot"
wrote in :

snip
I'm sure somebody is going to nit-pick this post. They're welcomed.


Not picking nits, just taking a different approach.....

Thermal bias compensation works to a point but neglects one important
issue: it takes time for heat to travel from the junction to the
outside of the package, and thermal runaway can happen faster than a
-thermal- compensation circuit can respond to it.

Exactly. That's why those diodes are place on the ceramic cap of the
device and not on the heat sink.

Since the heat
buildup is due to excessive EC current, it makes more sense to control
the EC current directly.

There are two solutions that use this approach. One is to include a
resistor on the emitter with a TC opposite of the transistor. Not
precision but much faster response.

This is done in many audio amps. The chief problem is the negative
feedback introduced by the emitter resistor. At auto frequencies this
resistor is bypassed by a rather large electrolytic capacitor sized such
that at the lowest frequency of interest the reactance is much smaller
that the emitter resistor value. Thus the "AC" gain isn't affected much by
the emitter resistor.

Believe it or not I've seen many of the old Motorola RF devices use
internal emitter resistors. Those took the form of many small tungsten
bonding wires from different areas of the emitter structure to the emitter
terminal. The main idea there was the many wires, resistors, in parallel
resulted in a very small overall emitter resistor. Also they found that a
problem called "second break down" would occur if they didn't do this.
What it amounted too was local hot spots, thermal runaway, in tiny areas
of the transistor's emitter structure. I think the term they used for RF
devices built this way was "emitter ballasting".

The other is to monitor the EC
voltage and the base current; send both measurements to a differential
OP amp and use the output as feedback for the bias regulator.

You would have to look at the "DC" emitter current minus the "AC"
component, which I don't think is going to be so easy to do.

Regards,
Leland C. Scott
KC8LDO

On a sunny day (Sun, 24 Sep 2006 23:30:24 -0400) it happened "LeIand C. Scot"
wrote in :


I'm sure somebody is going to nit-pick this post. They're welcomed.

No it is a nice post, I appreciate it, we should have more like this.

LeIand C. Scot wrote:
Tacking the diodes on to the case of the transistor works better than
attaching them to the heat sink due to the relatively slow thermal response,
the heat sink changes temperature slower than the cap on the transistor.

That's simply not true... while the heat sinks response might be a bit
slower... it's a much better indicator of the temp of the transistor
die. The ceramic cap on those transistors will overshoot the die temp
in both directions because of it's poor thermal contact with the die.

www.telstar-electronics.com