A photodiode generates current proportional to incident light, but how you bias it dramatically changes its behavior. The same physical diode can be a quiet, slow precision detector or a fast, noisy pulse catcher — your choice.

Photovoltaic mode (zero bias): The photodiode sits across the inverting input and output of a transimpedance amplifier with no reverse bias. Both terminals are at virtual ground, so the diode sees 0V across it. This eliminates dark current almost entirely (typically picoamps), making it ideal for low-light measurements like fluorescence detection, pulse oximetry, or precision colorimetry. The penalty: the diode's junction capacitance is at its maximum (often 50-500 pF for a large-area device), which limits bandwidth and forces you to use modest feedback resistors or add compensation.

Photoconductive mode (reverse biased): Apply 5-15V of reverse bias to the cathode while the anode connects to the TIA summing junction. The depletion region widens, slashing junction capacitance by 3-10x. Bandwidth jumps accordingly — you can now detect nanosecond optical pulses for LIDAR, optical communications, or laser rangefinding. The cost: dark current rises by orders of magnitude (nanoamps instead of picoamps), shot noise increases, and you've added a voltage source that must be clean.

Rule of thumb for TIA bandwidth: The noise gain peaks at f_p = 1/(2π·R_f·C_in), where C_in is diode capacitance plus op-amp input capacitance. To stabilize, add a feedback capacitor C_f where C_f = √(C_in / (2π·R_f·GBW)). This sets your -3dB bandwidth at roughly 1/(2π·R_f·C_f).

Concrete example: A Hamamatsu S1223 (large area silicon PIN) has C_j = 40 pF at 0V, dropping to 7 pF at 10V reverse bias. With R_f = 1 MΩ and a 10 MHz GBW op-amp like the OPA656:

• Photovoltaic: C_f ≈ √(40pF / (2π·1MΩ·10MHz)) ≈ 0.8 pF, bandwidth ≈ 200 kHz
• Photoconductive: C_f ≈ 0.34 pF, bandwidth ≈ 470 kHz — more than 2x faster

Practical tips: Always shield the photodiode and TIA input — they're high-impedance and pick up everything. Use Teflon standoffs for sub-picoamp work. For photoconductive mode, decouple the bias voltage with a series resistor and large electrolytic plus ceramic to ground; bias noise becomes signal noise directly through the diode capacitance. And remember: bigger photodiodes catch more light but have more capacitance — there's no free lunch.