[Ronja] Transreactance amplifier

[Karel Kulhavy]

Hello

I have invented (not sure if I am the first) a photodiode preamplifier that has theoretically better noise than the usual transimpedance amplifier found in commercial FSO.

The transimpedance amplifier is an opamp which has a
resistor between the output and input - like 100kOhm - the transimpedance. The noise of
a transimpedance amplifier is dominated by this resistor. The resistor cannot be increased because the amplifier output would saturate on a sunny day.

I split the amplifier into two stages. The first one uses a capacitor for transimpedance, the other inductor. Neither of them produce any noise. Capacitor reactance is 1/wC, inductor wL. When multiplied, you end up with L/C - a frequency independent response.

I have quickly tested a transreactance frontend and it seems to work fine. It produces a nice signal without distortion.

I have C=5.86pF, L=2.63uH and intermediate gain 25mS. That gives transimpedance of
12.3 kOhm.

Karel