WIDER_BANDWIDTH_PHOTODIODE_AMPLIFIER

Published:2009/7/13 2:03:00 Author:May | From:SeekIC

Many photodiode applications require high gain and wide bandwidth. A dc-restored photodiode amplifier (Fig. 1) is useful in situations where a time-varying light signal of interest competes with unwanted ambient background light. To extend the amplifier's bandwidth, the design can be modified to provide current-to-voltage conversion with subsequent voltage gain (Fig. 2). Current from the low-capacitance photodiode flows through Rg and establishes a voltage at the noninverting terminal. This voltage then receives a gain equal to 11Rf/R1. With the values shown in Fig. 2, the equivalent transimpedance Rg(11Rf/R1) is equal to 10Ω. The design uses an invertingintegrator to drive a restoration current through Rg This low-frequency current cancels current from the photodiode at frequencies below the low-frequency cutoff pole. The low-frequency pole in Fig. 2 is given by f-3dB5 (1/2πRC) (11Rf/R1). For the values shown, this is equal to 159 Hz. Rg helps determine the equivalent transimpedance and provides the path for the low-frequency restoration current. The measured high-frequency cutoff for the circuit in Fig. 1 is 4.9 kHz, with a total in-bartd noise of 370 μV rms. In Fig. 2, the high-frequency cutoff is extended to 11.7 kHz, with a total in-band noise of 589 μV rms.



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