Transimpedance Amplifier
3-6
Z5
+
1
p
(R4)GBP
)
ǒ
1
p
(R4)GBP
Ǔ
2
)
4C
S
p
(R4)GBP
Ǹ
2
Once the optimal feedback capacitor has been selected, the transimpedance
bandwidth can be calculated:
F
–3dB
+
GBP
2
p
(R4)
ǒ
C
S
)
Z5
Ǔ
Ǹ
The total source capacitance C
S
is the sum of several distinct capacitances
as shown in Figure 3-6.
Figure 3-6. Parasitic Capacitances in Photodiode Applications
_
+
C
IDIFF
C
ICM
C
P
R
F
C
F
C
D
I
DIODE
C
S
= C
ICM
+ C
IDIFF
+ C
P
+ C
D
where
C
ICM
is the common-mode input capacitance.
C
IDIFF
is the differential input capacitance.
C
D
is the diode capacitance.
C
P
is parasitic capacitance at the inverting node.
As shown in Figure 3-7, 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.