[sdiy] Summing resistors in transistor circuit

David Ingebretsen dingebre at 3dphysics.net
Mon Dec 2 05:58:32 CET 2013

Is there a rule of thumb for picking a summing resistor for mixing inputs in
a transistor based amplifier? I've been experimenting with different values
and am trying to understand the advantages/disadvantages of picking higher
values (47K) as opposed to lower (4.7K) or whatever values make sense.

The circuit I'm looking at is a "vintage" design with an emitter follower
for input conditioning. There is a 2.5uF cap in series with the input to the
base, a 100k resistor on the "input" side of the cap to ground, and then two
100k biasing resistors on the "output" side of the cap connected to the
base, one goes to +12 and one to ground. The emitter basically connects to
ground through a 2.2k resistor. There is a 220 ohm resistor in series with
the collector to +12 volts.

I'd like to be able to sum two or three inputs to this and so would
appreciate any thoughts on how to pick the summing resistor values.


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