LMH6642 Datasheet, PDF (19/33 Pages) National Semiconductor (TI) – 3V, Low Power, 130MHz, 75mA Rail-to-Rail Output Amplifiers

LMH6642, LMH6643, LMH6644
www.ti.com
SNOS966P – MAY 2001 – REVISED MARCH 2013
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)/(2S ˜ GBWP ˜ RF)}
(1)
where GBWP is the Gain Bandwidth Product of the Op Amp
Optimized as such, the I-V converter will have a theoretical pole, f_p, at:
f_P = SQRT _{GBWP/(2SRF ˜ CIN)}
(2)
With Op Amp input capacitance of 3pF and an estimate for Q1 output capacitance of about 3pF as well, C_IN =
6pF. From the typical performance plots, LMH6642/6643 family GBWP is approximately 57MHz. Therefore, with
R_f = 1k, from Equation 1 and Equation 2 above.
C_f = ∼4.1pF and f_p = 39MHz
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 63. Single Supply Photodiode I-V Converter
For this example, optimum C_f was empirically determined to be around 5pF. This time domain response is shown
in Figure 64 below showing about 9ns rise/fall times, corresponding to about 39MHz for f_p. The overall supply
current from the +5V supply is around 5mA with no load.
200 mV/DIV
20 ns/DIV
Figure 64. Converter Step Response (1V_PP, 20 ns/DIV)
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