NOISE GAIN (NG)
OP AMP OPEN
LOOP GAIN
I-V GAIN (:)
GAIN (dB)
0 dB
FREQUENCY
1 + sR
F
(C
T
+ C
F
)
1 + sR
F
C
F
1 +
C
IN
C
F
GBWP
f
z
#
1
2SR
F
C
T
f
P
=
1
2SR
F
C
F
2SR
F
C
T
Where, f
Z
1
#
and f
P
=
2SR
F
C
F
1
NG =
1 + sR
F
(C
T
+ C
F
)
1 + sC
F
R
F
LMH6619Q
SNOSC78A –JUNE 2012–REVISED NOVEMBER 2012
www.ti.com
Figure 9. Photodiode Modeled with Capacitance Elements
Figure 9 shows the LMH6619Q modeled with photodiode and the internal op amp capacitances. The LMH6619Q
allows circuit operation of a low intensity light due to its low input bias current by using larger values of gain (R
F
).
The total capacitance (C
T
) on the inverting terminal of the op amp includes the photodiode capacitance (C
PD
) and
the input capacitance of the op amp (C
IN
). This total capacitance (C
T
) plays an important role in the stability of
the circuit. The noise gain of this circuit determines the stability and is defined by:
(1)
(2)
Figure 10. Bode Plot of Noise Gain Intersecting with Op Amp Open-Loop Gain
Figure 10 shows the bode plot of the noise gain intersecting the op amp open loop gain. With larger values of
gain, C
T
and R
F
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 f
P
in the noise gain function is created by placing a feedback capacitor (C
F
) across R
F
. The noise gain
slope is flattened by choosing an appropriate value of C
F
for optimum performance.
Theoretical expressions for calculating the optimum value of C
F
and the expected −3 dB bandwidth are:
(3)
(4)
Equation 4 indicates that the −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.
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