Applying a basic op amp current amplifier to photodiodes
presents three severe problems: high nonlinearity, oscilla-
tions, and a latch condition. All three result from the pres-
ence of load-signal voltage feedback to the photodiode. A
simple bootstrapping arrangement can remove them all.
In the basic circuit, Figure 1, two resistors, R
1
and R
2
,
control the positive and negative feedback, respectively.
Consequently, they also control the current amplifier鈥檚 gain.
All the signal current, i
P
, from the photodiode flows through
R
1
(negligible current flows into the op amp鈥檚 input), thereby
defining the input-to-output voltage drop of the op amp.
Because of the op amp鈥檚 very high open-loop gain and the
feedback arrangement, the circuit replicates that voltage
across R
2
to keep the differential voltage between the op
amp inputs close to zero. As a result, feedback current i
O
flows into the load Z
L
through R
2
. Thus, the current gain
i
O
/i
P
, or A
i
, equals R
1
/R
2
.
Because the photodiode鈥檚 responsivity changes as its volt-
age changes with light input, voltage variation across Z
L
,
which is also across the photodiode, causes nonlinearity.
Even worse, the photodiode鈥檚 capacitance, C
D
, rolls off the
negative feedback from R
1
at high frequencies. Conse-
quently, the positive feedback from R
2
can dominate, and
oscillations, can result. In fact, C
D
inadvertently converts the
circuit to a conventional op amp square-wave generator. If
large enough to stop oscillations, a dominant roll-off bypass
capacitor C
B
added across the load would devastate the
circuit鈥檚 bandwidth.
Moreover, under the condition of input overloads, which can
occur during turn-on, a high impedance load could create a
latch state in conjunction with the diode. If the load imped-
I
P
C
D
44BH05M
270pF
R
1
100M鈩?/div>
OPA128
44BH05M
C
D
OPA128
270pF
C
1
0.068碌F
R
2
I
O
100k鈩?/div>
I
O
= 1 + (2 +
E
L
Z
L
C
B
for f >
1
2R
3
C
1
R
1
+ R
2
R
3
)
R
1
R
2
I
P
ance supports a great enough voltage, positive feedback
takes continuous control at the amplifier鈥檚 noninverting
input. At the inverting input, the photodiode clamps the
voltage and prevents negative feedback recovery.
Bootstrapping, though, removes each of the problems caused
by load voltage on the photodiode (see Figure 2). In the new
circuit, the load voltage drives the end of the photodiode
that鈥檚 grounded in the basic circuit. Also, a feedback-tee
circuit option becomes possible. With only the very small op
amp differential input error signal across the photodiode, its
response is essentially linear. Moreover, the canceled-out
positive feedback signal on C
D
avoids the square-wave
generator action.
Through its effect on feedback, bootstrapping preserves
bandwidth in two ways. The negative-feedback network
riding on top of the positive-feedback signal always ensures
a net negative feedback. The circuit requires little, if any,
load bypassing. As a result, this arrangement reduces the
bandwidth-limiting bypassing effect of the load and its
capacitance comparable to that of traditional current-to-
voltage conversion circuits. Also, because positive feedback
can no longer dominate, the circuit eliminates input clamp-
ing by the photodiode and the latch state.
The bootstrapping circuit also benefits from the use of a
feedback-tee network. In the bootstrapping circuit, the tee,
like the photodiode, also rides atop the load to similarly
avoid the positive-feedback effects. Tee networks offer a
degree of frequency-response control. In the tee, capacitor
C
1
blocks the low-frequency shunting effects of R
3
to pro-
duce a high-pass response without an amplified offset volt-
age. (Request PDS-653.)
I
P
R
1
1M鈩?/div>
R
1
1M鈩?/div>
R
3
20.5鈩?/div>
R
2
I
O
100k鈩?/div>
I
O
=
E
L
Z
L
C
B
C
B
>
R
1
R
2
R
1
R
2
I
P
C
D
FIGURE 1. Basic Photodiode Circuit.
FIGURE 2. Bootstrapped Photodiode Circuit.
The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN assumes
no responsibility for the use of this information, and all use of such information shall be entirely at the user鈥檚 own risk. Prices and specifications are subject to change
without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not authorize or warrant
any BURR-BROWN product for use in life support devices and/or systems.
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