Hi All,

Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.

C.W.

On 17 Mar 2005 22:45:11 -0800, wrote:

Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.

You have to look at the fT parameter (which specifies the frequency at
which _current_ gain drops to unity), since a typical power darlington
is a very slow device.

Paul OH3LWR

A major problem with the speed is that a common three-terminal
Darlington doesn't have any pulldown resistor or current sink to the
Q1e-Q2b connection. That leaves no good way to remove charge from Q2b
quickly when you want Q2 to turn off. If you can find one with an
internal Q2b-Q2e resistor of pretty low value, or better yet have access
to that junction so you can long-tail it down to a negative supply or
even to ground, you'll have a much faster switching device. I still
suspect you'll have trouble getting one to go fast enough for efficient
class C operation at VHF.

Roy Lewallen, W7EL

Paul Keinanen wrote:
On 17 Mar 2005 22:45:11 -0800, wrote:


Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.


You have to look at the fT parameter (which specifies the frequency at
which _current_ gain drops to unity), since a typical power darlington
is a very slow device.

Paul OH3LWR

On Fri, 18 Mar 2005 09:45:02 -0800, Roy Lewallen
wrote:

A major problem with the speed is that a common three-terminal
Darlington doesn't have any pulldown resistor or current sink to the
Q1e-Q2b connection. That leaves no good way to remove charge from Q2b
quickly when you want Q2 to turn off. If you can find one with an
internal Q2b-Q2e resistor of pretty low value, or better yet have access
to that junction so you can long-tail it down to a negative supply or
even to ground, you'll have a much faster switching device. I still
suspect you'll have trouble getting one to go fast enough for efficient
class C operation at VHF.

Roy Lewallen, W7EL


perhaps not so much for RF, but I wonder why you don't see NPN/PNP
combination for darlington transistors - with Vbe = 0.55-0.6V

Jan-Martin, LA8AK
---
J. M. Noeding, LA8AK, N-4623 Kristiansand
http://home.online.no/~la8ak/c.htm

Roy Lewallen ) writes:
A major problem with the speed is that a common three-terminal
Darlington doesn't have any pulldown resistor or current sink to the
Q1e-Q2b connection. That leaves no good way to remove charge from Q2b
quickly when you want Q2 to turn off. If you can find one with an
internal Q2b-Q2e resistor of pretty low value, or better yet have access
to that junction so you can long-tail it down to a negative supply or
even to ground, you'll have a much faster switching device. I still
suspect you'll have trouble getting one to go fast enough for efficient
class C operation at VHF.

Roy Lewallen, W7EL

I thought darlington transistors were just two transistors connected
that way inside the same case, ie not particularly different from
connecting two discrete transistors in the same configuration.

Hence, one could play with darlington transistors, but just
use two distinct transistors, so you have that junction.

Of course, there are other reaosn for having an amplifier chain, besides
overall gain. Those intermediate stages often help to filter the signal.

Michael VE2BVW

Paul Keinanen wrote:
On 17 Mar 2005 22:45:11 -0800, wrote:


Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.


You have to look at the fT parameter (which specifies the frequency at
which _current_ gain drops to unity), since a typical power darlington
is a very slow device.

Paul OH3LWR

"Roy Lewallen" bravely wrote to "All" (18 Mar 05 09:45:02)
--- on the heady topic of " Darlington Transistors At RF?"

Hmmm, what if the darlington is used in the bottom end of a cascode?
Use as the top end a high ft but low gain transistor. This way the
darlington collector voltage is stable and it relies more on the
common base fae for the gain limit, somewhat higher than ft, I think.
Darlingtons are only mostly good as emitter followers and DC switching
drivers anyways.

A*s*i*m*o*v


RL From: Roy Lewallen
RL Xref: aeinews rec.radio.amateur.homebrew:8924

RL A major problem with the speed is that a common three-terminal
RL Darlington doesn't have any pulldown resistor or current sink to the
RL Q1e-Q2b connection. That leaves no good way to remove charge from Q2b
RL quickly when you want Q2 to turn off. If you can find one with an
RL internal Q2b-Q2e resistor of pretty low value, or better yet have
RL access to that junction so you can long-tail it down to a negative
RL supply or even to ground, you'll have a much faster switching device.
RL I still suspect you'll have trouble getting one to go fast enough for
RL efficient class C operation at VHF.

RL Roy Lewallen, W7EL

RL Paul Keinanen wrote:
On 17 Mar 2005 22:45:11 -0800, wrote:


Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.


You have to look at the fT parameter (which specifies the frequency at
which _current_ gain drops to unity), since a typical power darlington
is a very slow device.

Paul OH3LWR

.... - Grains Of Salt. Take As Needed With Above Message.

The most significant problem with the standard Darlington connection is
the amplified Miller capacitance of the base-collector junctions. The
high gain of the darlington pair effectively multiplies this capacitance a
lot more resulting in poor frequency response (it won't go to VHF).
One approach is to tie the collector of the first transistor to an
AC-grounded
DC bias source, reducing this effect a lot.

This approach is described by Alan Grebene (founder of Exar) in his very
readable book "Bipolar and MOS Analog Integrated Circuit Design".

-- Tom




wrote in message
oups.com...
Hi All,

Does anyone know if a Darlington transistor (IC of two transistors in
Darlington congifuration) can be biased at class C for use in an output
stage of a VHF transmitter? Darlingtons seem to have pretty high gain,
so I supposed they could reduce the number of stages required to
amplify an RF signal? Thanks in advance.

C.W.

Jan-Martin wrote, "perhaps not so much for RF, but I wonder why you
don't see NPN/PNP combination for darlington transistors - with Vbe =
0.55-0.6V"

Actually, I think if you look at schematics of transistorized audio
amplifiers, you'll find examples of about every practical combination
of two and three transistors, used to build complimentary output
stages. If I understand correctly what you are suggesting, I'm quite
sure I've seen that configuration. In the 1960's and into the '70's,
there were a lot of games played to try to pair up NPN power
transistors with PNP drivers to get PNP-equivalents and NPN-equivalents
that were as nearly symmetrical as possible while giving high current
gain, since PNP power transistors weren't up to performing as well as
the NPNs in that era.

Cheers,
Tom

There are a few darlington transistors with an fT ratings of 20-200
MHz. IIRC, they are 2N6576, 2N6577 and 2N6578, all NPN devices with
Pd's of 120 watts. I'm not sure if 20-200 is a misprint. Looks like
one could get a lot of output with very little drive! Is anyone
familiar with the 2N657x series? Are they suitable for VHF?

K7ITM wrote:
Jan-Martin wrote, "perhaps not so much for RF, but I wonder why you
don't see NPN/PNP combination for darlington transistors - with Vbe =
0.55-0.6V"

Actually, I think if you look at schematics of transistorized audio
amplifiers, you'll find examples of about every practical combination
of two and three transistors, used to build complimentary output
stages. If I understand correctly what you are suggesting, I'm quite
sure I've seen that configuration. In the 1960's and into the '70's,
there were a lot of games played to try to pair up NPN power
transistors with PNP drivers to get PNP-equivalents and NPN-equivalents
that were as nearly symmetrical as possible while giving high current
gain, since PNP power transistors weren't up to performing as well as
the NPNs in that era.

Cheers,
Tom

You can cascade NPN-PNP transistors by connecting the collector of
on transistor to the base of another (of different polarity). I recall
a circuit that used two pnp and one npn transistors in a three way
cascade, the final transistor was a PNP power type (all Ge).
It used 2n107, 2n170, and 2n301 transistors as a simple audio
amplifier for a phonograph.