Datasheet

R
F
C
IN
A
0
C
F
=
2S
R
F
A
0
1 +
1 + 4S
C
CM
I
IN
R
F
V
OUT
+
-
+
-
V
B
C
F
C
D
V
OUT
I
IN
- R
F
=
C
IN
= C
D
+ C
CM
LMV791, LMV792
SNOSAG6F –SEPTEMBER 2005–REVISED MARCH 2013 www.ti.com
Usually, a transimpedance amplifier is designed on the basis of the current source driving the input. A
photodiode is a very common capacitive current source, which requires transimpedance gain for transforming its
miniscule current into easily detectable voltages. The photodiode and amplifier’s gain are selected with respect to
the speed and accuracy required of the circuit. A faster circuit would require a photodiode with lesser
capacitance and a faster amplifier. A more sensitive circuit would require a sensitive photodiode and a high gain.
A typical transimpedance amplifier is shown in Figure 57. The output voltage of the amplifier is given by the
equation V
OUT
= −I
IN
R
F
. Since the output swing of the amplifier is limited, R
F
should be selected such that all
possible values of I
IN
can be detected.
The LMV791/LMV792 have a large gain-bandwidth product (17 MHz), which enables high gains at wide
bandwidths. A rail-to-rail output swing at 5.5V supply allows detection and amplification of a wide range of input
currents. A CMOS input stage with negligible input current noise and low input voltage noise allows the
LMV791/LMV792 to provide high fidelity amplification for wide bandwidths. These properties make the
LMV791/LMV792 ideal for systems requiring wide-band transimpedance amplification.
Figure 57. Photodiode Transimpedance Amplifier
As mentioned earlier, the following parameters are used to design a transimpedance amplifier: the amplifier gain-
bandwidth product, A
0
; the amplifier input capacitance, C
CM
; the photodiode capacitance, C
D
; the
transimpedance gain required, R
F
; and the amplifier output swing. Once a feasible R
F
is selected using the
amplifier output swing, these numbers can be used to design an amplifier with the desired transimpedance gain
and a maximally flat frequency response.
An essential component for obtaining a maximally flat response is the feedback capacitor, C
F
. The capacitance
seen at the input of the amplifier, C
IN
, combined with the feedback capacitor, R
F
, generate a phase lag which
causes gain-peaking and can destabilize the circuit. C
IN
is usually just the sum of C
D
and C
CM
. The feedback
capacitor C
F
creates a pole, f
P
in the noise gain of the circuit, which neutralizes the zero in the noise gain, f
Z
,
created by the combination of R
F
and C
IN
. If properly positioned, the noise gain pole created by C
F
can ensure
that the slope of the gain remains at 20 dB/decade till the unity gain frequency of the amplifier is reached, thus
ensuring stability. As shown in Figure 58, f
P
is positioned such that it coincides with the point where the noise
gain intersects the op amp’s open loop gain. In this case, f
P
is also the overall 3 dB frequency of the
transimpedance amplifier. The value of C
F
needed to make it so is given by Equation 3. A larger value of C
F
causes excessive reduction of bandwidth, while a smaller value fails to prevent gain peaking and instability.
(3)
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