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1
2pR
F
C
F
+
GBW
4pR
F
C
D
Ǹ
(1)
OPTIMIZING THE TRANSIMPEDANCE
f
*3dB
+
GBW
2pR
F
C
D
Ǹ
Hz
(2)
OPA727
V
OUT
10MW
V+
R
F
C
F
(1)
<1pF
l
NOTE:(1)C isoptionaltopreventgainpeaking.
F
Itincludesthestraycapacitanceof .
F
R
+V
Bias
OPA727 , , OPA2727
OPA4727 , OPA728
SBOS314H – SEPTEMBER 2004 – REVISED APRIL 2007
To achieve a maximally-flat, 2nd-order Butterworth For additional information, refer to Application
frequency response, the feedback pole should be set Bulletin (SBOA055 ), Compensate Transimpedance
to: Amplifiers Intuitively, available for download at
www.ti.com .
CIRCUIT
Bandwidth is calculated by:
To achieve the best performance, components
should be selected according to the following
guidelines:
1. For lowest noise, select R
F
to create the total
For even higher transimpedance bandwidth, the
required gain. Using a lower value for R
F
and
high-speed CMOS OPA380 (90MHz GBW), OPA354
adding gain after the transimpedance amplifier
(100MHz GBW), OPA300 (180MHz GBW), OPA355
generally produces poorer noise performance.
(200MHz GBW), or OPA656 , OPA657 (400MHz
The noise produced by R
F
increases with the
GBW) may be used.
square-root of R
F
, whereas the signal
For single-supply applications, the +IN input can be
increases linearly. Therefore, signal-to-noise
biased with a positive dc voltage to allow the output
ratio is improved when all the required gain is
to reach true zero when the photodiode is not
placed in the transimpedance stage.
exposed to any light, and respond without the added
2. Minimize photodiode capacitance and stray
delay that results from coming out of the negative
capacitance at the summing junction (inverting
rail; this is shown in Figure 33 . This bias voltage also
input). This capacitance causes the voltage
appears across the photodiode, providing a reverse
noise of the op amp to be amplified
bias for faster operation.
(increasing amplification at high frequency).
Using a low-noise voltage source to
reverse-bias a photodiode can significantly
reduce its capacitance. Smaller photodiodes
have lower capacitance. Use optics to
concentrate light on a small photodiode.
3. Noise increases with increased bandwidth.
Limit the circuit bandwidth to only that
required. Use a capacitor across the R
F
to
limit bandwidth, even if not required for
stability.
4. Circuit board leakage can degrade the
performance of an otherwise well-designed
amplifier. Clean the circuit board carefully. A
circuit board guard trace that encircles the
summing junction and is driven at the same
voltage can help control leakage.
For additional information, refer to the Application
Bulletins Noise Analysis of FET Transimpedance
Amplifiers (SBOA060 ), and Noise Analysis for
High-Speed Op Amps (SBOA066 ), available for
download at the TI web site.
Figure 33. Single-Supply Transimpedance
Amplifier
13
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