THS4601_14 Datasheet, PDF (14/32 Pages) Texas Instruments – WIDEBAND, FET-INPUT OPERATIONAL AMPLIFIER

THS4601
SLOS388B − OCTOBER 2001 − REVISED JUNE 2002
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.
Gain
AOL
−20 dB/
Decade
20 dB/Decade
Rate-of-Closure
Noise Gain
20 dB/
Decade
GBP
0
f
Zero
Pole
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
should be used as a starting point for the design, with final values for C_F optimized in the laboratory.
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