Electronic Design

Improved Photodiode Pre-Amp Uses Current-Feedback Amplifier

A significant problem with using photodiodes in fast applications like barcode scanners, CD-ROMs, and DVDs is that the diode has a high output capacitance. Combined with the input impedance of the pre-amp stage, this capacitance generally limits the speed at which the photodiodes can be used. One technique to minimize this effect exploits the low input impedance of current-feedback amplifiers. Unfortunately, typical current-feedback amplifiers have high input-bias current, offset-voltage drift, and input-current noise. Such qualities render them useful for only the crudest applications.

Featuring dramatically enhanced input characteristics, the AD8014 also boasts high speed and low quiescent supply current. As a result, the device is suitable for the most demanding photodiode-amplifying applications. The improved input capabilities are due to the low bias currents of the input stage. These are made possible by the low parasitic capacitance of the proprietary eXtra fast complementary bipolar (XFCB) process, and a patented circuit architecture.

The benefits of current-feedback amplifiers can be shown by comparing these devices with voltage-feedback amplifiers. Figure 1 shows a typical pre-amp configuration for a photodiode with the amplifier configured as a current-to-voltage converter. If, for example, the photodiode maximum current is 1 mA, a feedback resistor of 1k is needed to generate a 1-V output signal. Photodiodes have a high output capacitance, on the order of 8 to 10 pF. The voltage-feedback amplifier maintains a very high input impedance (RI). Because of this, the equivalent impedance at the summing node is dominated by RF and CD and restricts the bandwidth to about 16 to 20 MHz. Excessive overshoot and ringing stem from the high input capacitance. To compensate for this, a feedback capacitor (CF) of approximately the same value as CD is needed.

If a current-feedback amplifier is used in place of the voltage-feedback amplifier, RI is sufficiently lower. (For the AD8014, the input impedance at high frequency is on the order of 100 Ohms.) In addition, the impedance at the summing node is now dominated by RI and CD. This results in a greater bandwidth of 160 MHz at that node. A much smaller feedback capacitance, 1 to 2 pF, is required to compensate the input zero created by the photodiode capacitance.

One common misconception about current-feedback amplifiers is that the circuit will oscillate if feedback capacitors are used. The impedance of the feedback loop determines the bandwidth of current-feedback amplifiers. Therefore, caution must be used to ensure that the impedance at the crossover frequency is high enough to ensure stability. For the AD8014, 1 to 2 pF will not cause any problems.

Figure 2 demonstrates a 2-Vpp square-wave output with rise and fall times of less than 5 ns. Here, the AD8014 is used as a current-to-voltage converter with a 10-pF source capacitance. The AD8014 has a slew rate of up to 4000 V/µs, making it ideal for high-bit-rate applications. Individual results depend on the type of photodiode used.

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