[RTTY] QUESTION & ANSWER

[Bill Turner]

On Fri, 31 Dec 2004 00:25:22 +0100, Marijan Miletic, S56A wrote:

>It would be healthy to add another 1 kOhm resistor from base to ground in
>order to conform with -5 V max inverse voltage from PC COM port.
>Shunt capacitor of 10 nF is also highly recommended at collector output in
>order to prevent RF feedback to base and subsequent blocking.
>This solves 99% of hamradio problems but it is missing in 99% of schematics!
___________________________________________________________

I agree about the capacitor from collector to ground, but the resistor
from base to ground isn't really necessary.  Many hams don't know it,
but the base-emitter junction of a typical small transistor acts like
a five to seven volt zener diode when reverse biased, and it will
simply conduct a tiny amount of reverse current, limited by the 1k
resistor.  Completely harmless.

I have used the identical circuit built into a DB-9 connector for over
ten years and it's still going strong, 45,000 QSOs later.  :-) 

-- 
Bill, W6WRT



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