[Ronja] worst-case with Q103 in the receiver?

[Thomas Egenhofer]

hi again,
i'm back with another few and annoying question but please bear with it.
(i'm just asking because i had the very same problem with my own
receiver circuit of a slower, low-cost fso)
it's about Q103 in the receiver.
from what i figured out it's why you have to limit the receiver for
short-distance links.
well isn't it possible that the BE-voltage of Q103 can swing into a
negative value ( if the signal is strong enough.. means about
2xVbe-pp(>~1,4V)and more).
in my case this caused the current to flow through the BE-diode part of
the transistor right onto the base-capacitor and "flooding" it.
than the be differential voltage on the base of Q102 and Q103 would be
unfortunate and causing the circuit to not work like it's supposed to
do.

well in case this IS the case and you want to correct it (thus making
low-distance tuning not neccessary anymore... you could either use the 2
transistors in a cascode wiring (little bit bad to tune the needed
voltages- i did this in my circuit(remember mine is not the ronja rec.)
or add 2 diodes. one in front of each transistor base. (the one of Q102
might be better build between the 2 6.8k and the 1k resistor.

jep this is pure theory and i won't stop you from blaming me for.
but can anyone confirm this behavior of the circuit?

oh and another small addition. from my experiments it's very good to
highpass the low-frequency's out of the signal after every (reasonable
high) amplification. so adding a second capacitor after C167 and ground
the wire between them over an ~500 Ohm resistor might improve signal
quality,too.

greetings
thomas e

ps:this is the only part of the receiver which worries me. the rest is a
prefect design as far as i can tell.
ps:ps:if you use 2 diodes for reverse-voltage protection you can avoid
to short-circuit the power-supply :)