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single-supply precision rectifier
035
An Analog Devices application
d.c. to 2 kHz. The single-supply operation at very low quiescent
current drain makes the circuit particularly useful for battery-pow-
ered equipment.
When the input voltage, V
IN
is positive, A
1
drives T
1
and D
2
to
make output voltage V
O
equal to the input voltage. The output
swing at V
O
is about three diode drops below the supply voltage,
so that the peak output voltage is around +3 V. The output of
The precision full-wave rectifier circuit shown in the diagram
accepts a.c. inputs of up to ±3 V, yet operates from a single +5 V
supply voltage. The quiescent current is only 320 µA. Rectifier gain
is unity, with the gain accuracy almost entirely dependent on the
match between the two resistors 2R
1
. The frequency range is about
62
Elektor Electronics
7-8/99
amplifier A
2
goes to negative saturation, which is about +0.8 V;
T
2
is then reverse-biased and off.
When the input voltage is negative, the output of A
1
goes into
negative saturation so that T
1
is switched off. Amplifier A
2
then
serves as a unity-gain inverter. Since V
O
is equal to V
IN
in magnitude
but opposite in polarity, V
O
will be equal to the absolute value of
V
IN
.
The quiescent current is determined by the the set current,
I
SET
.
With a 5 V supply, the set current is 3.7/
R
SET
. Slew rate and band-
width vary directly with the set current.
Amplifier A
1
essentially operates with unity-gain feedback ,
while A
2
operates with a feedback gain of 0.5. The closed-loop
gain-bandwidth is therefore made equal, and the frequency
response symmetrical, by making the set current of A
2
twice that of
A
1
. Amplifier A
2
has a set current of 3.7/2×10
5
, that is, 18.5 µA,
and amplifier A
1
has a set current of 3.7/39
5V
5V
R1
100k
2
IC1
7
T1
T2
6
OP22
R1
100k
3
2x
4
2N2222
8
R
D1
SET
1N4148
D2
U
IN
t
U
O
t
2R1
200k
2R1
200k
1N4148
1%
5V
1%
10
4
, which is 9.5 µA.
These set currents result in quiescent currents of 100 µA for ampli-
fier A
1
and 220 µA for amplifier A
2
.
The input stage of the amplifiers is a p-n-p Darlington, so that
a negative input voltage can forward-bias the collector-base junc-
tion of the input transistor. This potential problem is prevented by
adding resistor R
1
and diode D
1
at the A
1
input to limit the nega-
tive input voltage.
IC2
7
2
OP22
6
3
R
L
4
R1
8
R
SET
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