-=> Mike Ross said to Jasen Betts
 -=> about "I/F" on 01-07-03  10:16.....

 MR> That's true but, as you mention, the gain is often about 100 times
 MR> less. Speaking of gain, I once measured an oddball switching transistor
 MR> type in which the gain was the same on either terminal. Worse still the

There were several types that were intentionally made symmetrical
although I'm not sure why. But one application was for switching
of audio signals.

At the Australian Broadcasting Corp where I worked for many
years, we had some program switches that used them. From memory,
the circuit was roughly as follows...

.  ------------- c b e -------------
.  balanced        |                ) bridging
.   signal         R                ) amplifier
.   input          |- bias      gnd-|  input
.   600 ohm        R                ) transformer
.   source         |                ) (20k-ohms)
. -------------- c b e -------------

Of course the collector and emitter were interchangeable, but the
above is how we wired them IIRC.

The output of the switch itself was paralleled with the output of
about 60 other switches feeding the amplifier input. From memory
we had about 20 of these selectors, with two in each of the
five transmission studios.

These were PNP transistors. They were switched off by applying a
positive voltage to the "bias" point above, and turned on with a
negative voltage. These voltages had to be greater than the peak
signal voltages and the base current had to be greater than the
peak signal current into the load divided by the gain, hence the
need for a high impedance load.

The distortion of the switch was quite low, and the isolation
ratio was excellent, provided that at least one switch was turned
on to give a low impedance across the input of the amplifier.

This was a system that we used for quite a few years. I can't
remember the type number of the original transistors, I think
they were from Siemens. When the supply dried up we used Philips
type OC80, which IIRC were not specifically made as symmestrical
transistors, but they had a high enough reverse gain to do the
job.

From Greg Mayman, in beautiful Adelaide, South Australia

... Go drink some chamomile tea if someone gets on your nerves.
___ Blue Wave/386 v2.30

--- FLAME v2.0/b