APD Bias Circuit Has Adjustable Output

Abstract

This design note discusses the role of an avalanche photodiode detector (APD) and the importance of APD bias generation. It shows an APD bias generator that produces a regulated, computer-adjusted output in the 30V to 70V range. The MAX1771 step-up DC-DC controller is featured.

A similar idea appeared in the May 22, 1997 issue of EDN.

The avalanche photodiode detector (APD) is used as a receiver in optical communications, as is the pin diode. The APD is more sensitive, but it must be biased properly to produce the appropriate electron flow for a given flux of photons. In Figure 1, an external digital-to-analog converter enables dynamic, computer-controlled adjustment of an APD bias generator.

Figure 1. This APD-bias generator produces a regulated, computer-adjusted output in the 30V to 70V range.

To generate the bias voltage V_OUT, a boost converter (IC1, L1, and Q1) drives a diode-capacitor charge pump (D3/C4, D2/C3, and D1/C5). IC1 regulates V_OUT with respect to a set point established by the external control voltage V_DAC: As V_DAC ranges from 2V to zero, V_OUT varies from 28V to about 71V (Figure 2).

At 70V with a 0.5mA output current, the limitation on ripple is typically 0.5V (max) (0.7%). The circuit shown produces less than 0.3% ripple at 1mA, and its maximum output current is about 3mA. The output capacitor (C5) should be a low-ESR type.

Figure 2. The DC-output level in Figure 1 is a linear function of the control-voltage V<sub>DAC</sub>

Figure 2. The DC-output level in Figure 1 is a linear function of the control-voltage V_DAC.