1 Photovoltaic mode

2 A simple I to U converter Basic building block: I to U converter U = R * I

3 PD bias increases linear range

4 + voltage output buffer... Simple standard circuit. Can be very sensitive, but has generally limited dynamic range.

5 Transimpedance configuration Advantage: larger dynamic range Disadvantage: OpAmp has to take the PD current

6 Transimpedance configuration Noises: R Thermal noise: ~sqrt(R)‏ OP voltage noise: fixed by OP type OP current noise: fixed by OP type, but scales with R Photocurrent shot noise: ~sqrt(I)‏

7 Noise sources as function of R

8 Resonant PD A resonant circuit around the PD (here: C_pd and L1)‏ can increase the dynamic range, depending on the signal content. It can have other advantages, as avoiding non-linearities Z = L1 / (R_pd * C_pd)‏ on resonance Thermal noise according to Z

9 GEO PD circuit

10 New style GEO PD circuit Enhanced dynamic range, multiple resonant outputs

11 LIGO PD circuit variable bias

12 Virgo PD circuit DC current combined to output