Why don't many high power RF transistor amplifiers use emitter stabilizing resistor and AC by-pass capacitors?
 

I have been looking at a variety of RF transistor amplifier
schematics including a 50 watt amp shown in the ARRL handbook (1996 -
page 17.96) , and they do not have emitter stabilizing resistors and
ac by-pass capacitors on the emitter leg of the final output
transistors. Is there a good reason for not using emitter stabilizing
resistors and ac by-pass capacitors in 100 watt output or more RF
transistor amplifiers?

Here are some on-line schematics I found of high power RF power
transistor amplifiers, and in all of them the emitter goes straight to
ground.

http://www.ameritron.com/man/pdf/ALS-500M.pdf

http://www.cbtricks.com/Amp/txstar/2...350hdv_sch.gif

http://www.cbtricks.com/Amp/palomar/...r_elite500.pdf

Just Cuz wrote:

Is there a good reason for not using emitter stabilizing
resistors and ac by-pass capacitors in 100 watt output or more RF
transistor amplifiers?

My hunch would be that most of those amps probably use ballasted emitter transistors
which mitagates the need for an emitter resistor. That, plus the fact that any impedance
in the emitter lead is an invitation to oscillation, and obtaining a good low Z bypass is
tough at HF especially at the current levels involved.

Bob WB0POQ

Just Cuz wrote:


I have been looking at a variety of RF transistor amplifier
schematics including a 50 watt amp shown in the ARRL handbook (1996 -
page 17.96) , and they do not have emitter stabilizing resistors and
ac by-pass capacitors on the emitter leg of the final output
transistors. Is there a good reason for not using emitter stabilizing
resistors and ac by-pass capacitors in 100 watt output or more RF
transistor amplifiers?

Here are some on-line schematics I found of high power RF power
transistor amplifiers, and in all of them the emitter goes straight to
ground.

http://www.ameritron.com/man/pdf/ALS-500M.pdf

http://www.cbtricks.com/Amp/txstar/2...350hdv_sch.gif

http://www.cbtricks.com/Amp/palomar/...r_elite500.pdf

Power burnt in the emitter resistor is power that isn't going out the
antenna. So it makes sense to use a fancier bias network and no emitter
resistor.

--
-------------------------------------------
Tim Wescott
Wescott Design Services
http://www.wescottdesign.com

I have been looking at a variety of RF transistor amplifier
schematics including a 50 watt amp shown in the ARRL handbook
(1996 -
page 17.96) , and they do not have emitter stabilizing resistors and
ac by-pass capacitors on the emitter leg of the final output
transistors. Is there a good reason for not using emitter
stabilizing
resistors and ac by-pass capacitors in 100 watt output or more RF
transistor amplifiers?

=================================

Most RF power transistors in the 50-100 watt range already have an
internal emitter resistor. It's usually less than a tenth of an ohm,
perhaps only a few hundreths of an ohm. Its purpose is to prevent
thermal runaway and make the device more tolerant of overload due to
momentary impedance mismatches. Internal resistors have an extemely
low inductance. They are NOT wirewound. They are an integral part of
the transistor construction and material. The manufacturer may not
mention their existence in his catalogue or data book.

RF power transistors are usually multi-emitter devices - all in
parallel. Each emitter has its own individual series resistor.

Such low-value external resistors are not available as discrete
wire-ended circuit components. Inductance would be too high at HF and
an invitation to self oscillation. Don't attempt to use normal circuit
components in emitter leads unless a transistor manufacturer
specifically recommends them.

HF and VHF power transistors are very fast fuses. They are worked very
close to their maximum voltage and current ratings.
----
Reg, G4FGQ

Most RF power transistors in the 50-100 watt range already have an
internal emitter resistor. It's usually less than a tenth of an
ohm,
perhaps only a few hundreths of an ohm. Its purpose is to prevent
thermal runaway and make the device more tolerant of overload due
to
momentary impedance mismatches. Internal resistors have an extemely
low inductance. They are NOT wirewound. They are an integral part
of
the transistor construction and material. The manufacturer may not
mention their existence in his catalogue or data book.

RF power transistors are usually multi-emitter devices - all in
parallel. Each emitter has its own individual series resistor.

Such low-value external resistors are not available as discrete
wire-ended circuit components. Inductance would be too high at HF
and
an invitation to self oscillation. Don't attempt to use normal
circuit
components in emitter leads unless a transistor manufacturer
specifically recommends them.

HF and VHF power transistors are very fast fuses. They are worked
very
close to their maximum voltage and current ratings.
----
Reg, G4FGQ

yup!