THS4601

SLOS388B – OCTOBER 2001 – REVISED JUNE 2002

14

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APPLICATION INFORMATION

designing the transimpedance circuit (continued)

The feedback capacitor provides a pole in the noise gain of the circuit, counteracting the zero in the noise gain

caused by the source capacitance. The pole is set such that the noise gain achieves a 20 dB per decade

rate-of-closure with the open-loop gain response of the amplifier, resulting in a stable circuit. As indicated, the

formula given provides the feedback capacitance for maximally flat bandwidth. Reduction in the value of the

feedback capacitor can increase the signal bandwidth, but this occurs at the expense of peaking in the AC

response.

20 dB/Decade

Rate-of-Closure

GBP

20 dB/

Decade

–20 dB/

Decade

AOL

Noise Gain

0

Gain

Zero

Pole

f

Figure 28. Transimpedance Circuit Bode Plot

The performance of the THS4601 has been measured for a variety of transimpedance gains with a variety of

source capacitances. The achievable bandwidths of the various circuit configurations are summarized

numerically in the table. The frequency responses are presented in the Figures 27, 28, and 29.

Note that the feedback capacitances do not correspond exactly with the values predicted by the equation. They

have been tuned to account for the parasitic capacitance of the feedback resistor (typically 0.2 pF for 0805

surface mount devices) as well as the additional capacitance associated with the PC board. The equation