EMRFD Message Archive 3172

Message Date From Subject
3172 2009-06-10 10:42:26 Glen Leinweber I.F. amplifier (AGC)
Years ago, when design with discrete transistors was replacing
vacuum tubes, intermediate-frequency stages consisted of a
single transistor, with tuned input and tuned output. Automatic
gain control was accomplished by changing DC bias conditions.
Data sheets often described these transistors as desirable I.F.
amplifiers, implying that they had good AGC characteristics.
Such circuits were very common in battery-powered AM
broadcast-band "transistorized" radios.

What transistor characteristics marked it as a superior
device in this application? Perhaps current gain that varied
widely with Ic? Stable, at high & low gain?
I know there are better circuits to do I.F. AGC - this is
more of a historical curiosity.
3174 2009-06-10 11:18:55 Harold Smith Re: I.F. amplifier (AGC)
There used to be transistors whose gain dropped as collector current
increased, and these were used for AGC in TV IFs, at least. I do not recall
any use of these 'forward AGC' transistors in AM radios, though, which were
usually only about being cheap.

I believe they were discussed briefly in *Solid State Design* *for the Radio
Amateur*, although I haven't seen any mention of them in the last decade or

The only one of that class of transistors I can find still being
manufactured is the Fairchild KSC1187. I bought a handful a year or so ago,
intending to experiment with them, but never got around to it.

de KE6TI, Harold

3179 2009-06-10 22:28:25 leon Heller Re: I.F. amplifier (AGC)
----- Original Message -----