9.2.1 Design Requirements

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 1 V/mA of transimpedance gain and capable of handling up to 1 mApp from the photodiode. Q1, in a common base configuration, isolates the high capacitance of the photodiode (Cd) 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 VCC. 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.