9.2.1.2 Single-Supply, Low-Power Photodiode Amplifier

The circuit shown in Figure 55 is used to amplify the current from a photodiode into a voltage output. In this circuit, the emphasis is on achieving high bandwidth and the transimpedance gain setting is kept relatively low. Because of its high slew rate limit and high speed, the LMH6644-MIL lends itself well to such an application.

This circuit achieves approximately 1V/mA of transimpedance gain and capable of handling up to 1mA_pp from the photodiode. Q1, in a common base configuration, isolates the high capacitance of the photodiode (C_d) from the Op Amp input in order to maximize speed. Input is AC coupled through C1 to ease biasing and allow single-supply operation. With 5-V single supply, the device input/output is shifted to near half supply using a voltage divider from V_CC. Note that Q1 collector does not have any voltage swing and the Miller effect is minimized. D1, tied to Q1 base, is for temperature compensation of Q1’s bias point. Q1 collector current was set to be large enough to handle the peak-to-peak photodiode excitation and not too large to shift the U1 output too far from mid-supply.

No matter how low an R_f is selected, there is a need for C_f in order to stabilize the circuit. The reason for this is that the Op Amp input capacitance and Q1 equivalent collector capacitance together (C_IN) will cause additional phase shift to the signal fed back to the inverting node. C_f will function as a zero in the feedback path counter-acting the effect of the C_IN and acting to stabilized the circuit. By proper selection of C_f such that the Op Amp open loop gain is equal to the inverse of the feedback factor at that frequency, the response is optimized with a theoretical 45° phase margin.

Equation 1.

where

• GBWP is the Gain Bandwidth Product of the Op Amp

Optimized as such, the I-V converter will have a theoretical pole, f_p, at:

Equation 2.

With op amp input capacitance of 3 pF and an estimate for Q1 output capacitance of about 3 pF as well, C_IN = 6 pF. From the typical performance plots, GBWP is approximately 57 MHz. Therefore, with R_f = 1k, from Equation 1 and Equation 2:

For this example, optimum C_f was empirically determined to be around 5 pF. This time domain response is shown in Figure 58 below showing about 9-ns rise/fall times, corresponding to about 39 MHz for f_p. The overall supply current from the +5 V supply is around 5 mA with no load.