LMH6643MA datasheet(17/22 Pages) NSC

17 / 22 page

Application Notes (Continued)

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

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 opera-

tion. 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 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

f is selected, there is a need for Cf in

order to stabilize the circuit. The reason for this is that the Op

Amp input capacitance and Q1 equivalent collector capaci-

tance 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

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.

(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:

(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 Equa-

tion 1 and 2 above.

f =

∼4.1pF, and f

p = 39MHz

20018564

FIGURE 3. Single Supply Photodiode I-V Converter

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