handbook of modern sensors

14.2 Photodiodes 415
(A)
(B)
(C)
Fig. 14.7. Connection of a photodiode in a photovoltaic mode to a noninverting amplifier (A);
the equivalent circuit (B); and a loading characteristic (C).
When using a photodiode in a photovoltaic mode, its large capacitance C j may
limit the speed response of the circuit. During the operation with a direct resistive
load, as in Fig. 14.7A, a photodiode exhibits a bandwidth limited mainly by its internal
capacitance C j . Figure 14.7B models such a bandwidth limit. The photodiode acts
primarily as a current source. A large resistance R and the diode capacitance shunt
the source. The capacitance ranges from 2 to 20,000 pF depending, for the most part,
on the diode area. In parallel with the shunt is the amplifier’s input capacitance (not
shown) which results in a combined input capacitance C. The diode resistance usually
can be ignored, as it is much lower than the load resistance R b . The net input network
determines the input circuit response rolloff. The resulting input circuit response has
a break frequency f 1 = 1/2πR L C, and the response is [4]
V out = −R Li p
1 + j f f 1
. (14.11)
For a single-pole response, the circuit’s 3-dB bandwidth equals the pole frequency.
The expression reflects a typical gain-versus-bandwidth compromise. Increasing R b
gives a greater gain, but reduces f 1 . From a circuit perspective, this compromise results
from impressing the signal voltage on the circuit capacitances. The signal voltage
appears across the input capacitance C = C j + C OPAM . To avoid the compromise, it