Post by Karel Kulhavy
Is it possible to make a passive network from coils and capacitors that
would approximate negated reactance of 6pF capacitor in the frequency
range from 1MHz to 10MHz?
It has to be passive device because I want to use it to compensate out
capacitance of photodiode in low-noise broadband preamplifier.
Short answer is probably not. Depends on the rest of the circuit. If
you have a low resistance driving the 6pF so it's only a minor pain,
then yes. If the 6pF is driven by a high resistance, then no.
Longer answer is Bode showed how to compute such things in his book
"Network Analysis and Feedback Amplifier Design". I doubt the problem
he solved is 100% identical to yours, but it is similar. Basically what
he proved is that for a circuit with a resistive source driving a
capacitive load, no matter what LC network you stick in between, you
can't exceed a certain value for the integral of gain over frequency.
If you like math and circuit theory, Bode's original proof of this is a
pretty straightforward read. Fano then expanded and generalized Bode's
result, but Fano's paper is a harder read.
Bottom line, you can always do such things (with ideal components)
perfectly at one frequency and the wider the frequency range, the worse
you will do no matter how many elements you throw at the problem.