Datasheet

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
25
1
100 10k 1M
FREQUENCY (Hz)
-20
-5
10
GAIN (dB)
100k1k
10
20
15
5
0
-10
-15
C
F
= 10 pF
C
F
= 100 pF
C
F
= 1 nF
LM6211
www.ti.com SNOSAH2C –FEBRUARY 2006–REVISED MARCH 2013
Figure 46. Frequency Response of the Non-Inverting Preamplifier
TRANSIMPEDANCE AMPLIFIER
A transimpedance amplifier converts a small input current into a voltage. This current is usually generated by a
photodiode. The transimpedance gain, measured as the ratio of the output voltage to the input current, is
expected to be large and wide-band. Since the circuit deals with currents in the range of a few nA, low noise
performance is essential. The LM6211, being a CMOS input op amp, provides a wide bandwidth and low noise
performance while drawing very low input bias current, and is hence ideal for transimpedance applications.
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 47. 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 LM6211 has a large gain-bandwidth product (20 MHz), which enables high gains at wide bandwidths. A rail-
to-rail output swing at 24V 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 LM6211 to provide high
fidelity amplification for wide bandwidths. These properties make the LM6211 ideal for systems requiring wide-
band transimpedance amplification.
Figure 47. Photodiode Transimpedance Amplifier
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. The input common-mode capacitance with respect to V
CM
for the LM6211 is give in Figure 48.
Copyright © 2006–2013, Texas Instruments Incorporated Submit Documentation Feedback 19
Product Folder Links: LM6211