2SRFCT
f-3 dB
GBWP
=
CF =
CT
2SRF(GBWP)
NOISE GAIN (NG)
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
G
AI
N
(d
B)
0 dB
FREQUENCY
1 + sRF (CT + CF)
1 + sRFCF
1 +
CIN
CF
GBWP
fz #
1
2SRFCT
fP =
1
2SRFCF
SNOSAV7E – AUGUST 2007 – REVISED OCTOBER 2012
Figure 72. Bode Plot of Noise Gain Intersecting with Op Amp Open-Loop Gain
Figure 72 shows the bode plot of the noise gain intersecting the op amp open loop gain. With larger values of
gain, CT and RF create a zero in the transfer function. At higher frequencies the circuit can become unstable due
to excess phase shift around the loop.
A pole at fP in the noise gain function is created by placing a feedback capacitor (CF) across RF. The noise gain
slope is flattened by choosing an appropriate value of CF for optimum performance.
Theoretical expressions for calculating the optimum value of CF and the expected 3 dB bandwidth are:
(3)
(4)
3 dB bandwidth of the TIA is inversely proportional to the feedback resistor.
Therefore, if the bandwidth is important then the best approach would be to have a moderate transimpedance
gain stage followed by a broadband voltage gain stage.
Table 3 shows the measurement results of the LMH6618 with different photodiodes having various capacitances
(CPD) and a feedback resistance (RF) of 1 k.
Table 3. TIA (Figure 1) Compensation and Performance Results
CPD
CT
CF CAL
CF USED
f 3 dB CAL
f 3 dB MEAS
Peaking
(pF)
(MHz)
(dB)
22
24
7.7
5.6
23.7
20
0.9
47
49
10.9
10
16.6
15.2
0.8
100
102
15.8
15
11.5
10.8
0.9
222
224
23.4
18
7.81
8
2.9
28
Copyright 2007–2012, Texas Instruments Incorporated