"TRABEM" bravely wrote to "All" (03 Oct 05 00:13:54)
--- on the heady topic of " Preamp"

TR From: TRABEM
TR Xref: core-easynews rec.radio.amateur.homebrew:88137

TR On Mon, 03 Oct 2005 01:09:33 GMT, wrote:

I put your tirade on the end of this as it's not relevant to the
orininal question.


TR The original post (by A*s*i*m*o*v) mentioned back to back diodes as
TR protection, so my reply was quite relevant.
[,,,]
TR Actually, I hope the original poster (A*s*i*m*o*v) got the message
TR about introducing gross non linear back to back diodes across the
TR antenna terminals.

TR Have fun.

I have followed both your points with great interest. My guess is that
the diodes are benign in a weak signal area. I expect the original
design used them more for static rather than for overload protection
because the input was intended to expect microvolt levels.

My choice would be not to use anti-parallel diodes in a design but this
is what I found in the circuit so some engineer thought about it enough
to use them. I would have preferentially used a reverse biased diode
across the active device's junction because it is usually the reverse
polarity in a pulse that damages a junction.

In this question I was more interested on the effect of the protection
diodes' capacitance because as you know it is greatest at zero bias.
This wideband pre-amp's input stage uses feedback to define the input
impedance and the diodes' capacitance is at the node point, so I
wondered if there was little effect. As you know, in an inverter the
node point is ideally a voltage null.

Thank you both for the great debate!

A*s*i*m*o*v

.... As I suspected, you're a rank sentimentalist! --Cpt. Louis Renault