LMH6642MF National Semiconductor, LMH6642MF Datasheet - Page 17

• Manufacturer Part Number: LMH6642MF
• Description: OPERATIONAL AMPLIFIER OP AMP IC
• Manufacturer: National Semiconductor

Specifications of LMH6642MF

• No. Of Amplifiers: 1
• Bandwidth: 130MHz
• Slew Rate: 135V/µs
• No. Of Pins: 5
• Peak Reflow Compatible (260 C): No
• Leaded Process Compatible: No
• Mounting Type: Surface Mount
• Package / Case: 5-SOT-23
• Lead Free Status / RoHS Status: Contains lead / RoHS non-compliant

Application Notes
SINGLE SUPPLY, LOW POWER PHOTODIODE
AMPLIFIER
The circuit shown in Figure 3 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 transim-
pedance 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 transimped-
ance gain and capable of handling up to 1mA
photodiode. Q1, in a common base configuration, isolates
the high capacitance of the photodiode (C
Amp input in order to maximize speed. Input is AC coupled
through C1 to ease biasing and allow single supply opera-
tion. With 5V single supply, the device input/output is shifted
to near half supply using a voltage divider from V
that Q1 collector does not have any voltage swing and the
Miller effect is minimized. D1, tied to Q1 base, is for tem-
perature compensation of Q1’s bias point. Q1 collector cur-
rent 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
order to stabilize the circuit. The reason for this is that the Op
f
is selected, there is a need for C
(Continued)
FIGURE 3. Single Supply Photodiode I-V Converter
d
) from the Op
pp
CC
from the
. Note
f
in
17
Amp input capacitance and Q1 equivalent collector capaci-
tance together (C
signal fed back to the inverting node. C
zero in the feedback path counter-acting the effect of the C
and acting to stabilized the circuit. By proper selection of C
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.
where GBWP is the Gain Bandwidth Product of the Op Amp
Optimized as such, the I-V converter will have a theoretical
pole, f
With Op Amp input capacitance of 3pF and an estimate for
Q1 output capacitance of about 3pF as well, C
the typical performance plots, LMH6642/6643 family GBWP
is approximately 57MHz. Therefore, with R
tion 1 and 2 above.
C
f
= ∼4.1pF, and f
p
, at:
IN
p
) will cause additional phase shift to the
= 39MHz
f
f
will function as a
= 1k, from Equa-
IN
20018564
= 6pF. From
www.national.com
(1)
(2)
IN
f