Data Sheet

ADA4620-1, ADA4620-2

analog.com

Rev. 0

36 of 49

Figure 116. Gain and Phase Plot of the Transimpedance Amplifier Design, Without Compensation

Adding CF creates a zero in the loop transmission that compensates for the effect of the input pole, which stabilizes

the photodiode preamplifier design because of the increased phase margin. Adding CF also sets the signal

bandwidth (see Figure 117). The signal bandwidth and the zero frequency are determined by:

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1

2������������������������������

(4)

where, fZ is the zero frequency. Setting the zero at the fX frequency maximizes the signal bandwidth with a 45°

phase margin. Because fX is the geometric mean of fP and fGBP, it can be calculated by:

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(5)

Combining these equations, the CF value that produces fX is:

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������������

2������ × ������������ × ������

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(6)

The frequency response in this case shows approximately 2 dB of peaking and 15% overshoot. Doubling CF and

halving the bandwidth results in a flat frequency response with approximately 5% transient overshoot. The

dominant sources of output noise in the wideband photodiode preamp design are the input voltage noise of the

amplifier, VNOISE, and the resistor noise due to RF.

OPEN-LOOP GAIN

G = 1

G = RFCSs

fX

fGBP

fP

log f

log f

0

–45

–90

–135

–180