LMH6642Q-Q1_15 Datasheet, PDF (23/34 Pages) Texas Instruments – Low Power, 130 MHz, 75 mA Rail-to-Rail Output Amplifiers

www.ti.com
LMH6642Q-Q1, LMH6643Q-Q1
SNOSC61C – JANUARY 2012 – REVISED SEPTEMBER 2014
8.2 Single Supply, Low Power Photodiode Amplifier
The circuit shown in Figure 57 is used to amplify the current from a photo-diode into a voltage output. In this
circuit, the emphasis is on achieving high bandwidth and the transimpedance gain setting is kept relatively low.
Because of its high slew rate limit and high speed, the LMH664X family lends itself well to such an application.
This circuit achieves approximately 1V/mA of transimpedance gain and capable of handling up to 1mA_pp from the
photodiode. Q1, in a common base configuration, isolates the high capacitance of the photodiode (C_d) from the
Op Amp input in order to maximize speed. Input is AC coupled through C1 to ease biasing and allow single
supply operation. With 5V single supply, the device input/output is shifted to near half supply using a voltage
divider from V_CC. Note that Q1 collector does not have any voltage swing and the Miller effect is minimized. D1,
tied to Q1 base, is for temperature compensation of Q1’s bias point. Q1 collector current was set to be large
enough to handle the peak-to-peak photodiode excitation and not too large to shift the U1 output too far from
mid-supply.
No matter how low an R_f is selected, there is a need for C_f in order to stabilize the circuit. The reason for this is
that the Op Amp input capacitance and Q1 equivalent collector capacitance together (C_IN) will cause additional
phase shift to the signal fed back to the inverting node. C_f will function as a zero in the feedback path counter-
acting the effect of the C_IN and acting to stabilized the circuit. By proper selection of C_f such that the Op Amp
open loop gain is equal to the inverse of the feedback factor at that frequency, the response is optimized with a
theoretical 45° phase margin.
C_F = _{ SQRT (CIN)/(25bGBWP bRF)}
where GBWP is the Gain Bandwidth Product of the Op Amp
(1)
Optimized as such, the I-V converter will have a theoretical pole, f_p, at:
f_P = SQRT _{GBWP/(25RF bCIN)}
(2)
With Op Amp input capacitance of 3pF and an estimate for Q1 output capacitance of about 3pF as well, C_IN =
6pF. From Typical Performance Characteristics, LMH6642/6643 family GBWP is approximately 57 MHz.
Therefore, with R_f = 1k, from Equation 1 and Equation 2 above.
C_f = ∼4.1 pF, and f_p = 39 MHz
C_f
5pF
V_bias
Photodiode
Equivalent
R_f
Circuit
1k:
R_bias
C1
100nF
Q1
2N3904
-1mA_PP
V_CC =
+5V
-
Photodiode
I_d
C_d
10
-
200pF
R_d
×^100k:
R5
x
510:
R2
1.8k:
+
V_out
R11
D1 910
R10
1N4148 :
1k:
R3
1k:
+5V
Figure 57. Single Supply Photodiode I-V Converter
Copyright © 2012–2014, Texas Instruments Incorporated
Submit Documentation Feedback
23
Product Folder Links: LMH6642Q-Q1 LMH6643Q-Q1