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Precision Rectifier Circuits
Rectifier circuits are used in the design of power supplycircuits. In such applications, the voltage being rectifiedare usually much greater than the diode voltage drop,
rendering the exact value of the diode drop unimportant to the proper operation of the rectifier.Other applications exists, however, where this is not the case. For example, in instrumentation applications,
the signal to be rectified can be of very small amplitude, say 0.1 V, making it impossible to employthe conventional rectifier circuits. Also the need arises
for very precise transfer characteristics.
Precision Half-Wave Rectifier- The Superdiode There are many applications for precision rectifiers, and most aresuitable for use in audio circuits. A half wave precision rectifier is implemented using an op amp, and includes the diode in the feedbackloop. This effectively cancels the forward voltage drop of the diode, sovery low level signals (well below the diode's forward voltage) can still
be rectified with minimal error.
Exercise 13.24:
Consider the operational amplifier in Figure 13.33(a), with
R
=
1 k
Ω
. For vI = 10 mV, 1 V, and -1 V, what are the voltages that result at the
rectifier output and at the output of the op am? Assume that the op amp is idealand its output saturates at
± 12 V. The diode has a 0.7-V drop at 1-mA current,
and the voltage drop changes by 0.1 V per decade of current change.
V
V
operative not is
loop
feedback
negative The
-1V
V
V
mA, 1
V
V 1
V
V
V
o
A
mA
mA 1
Given
A
mV 10
mV 10
mV 10
v^ A
vI vo
vA
vD
iD
vo
vI
R iD v
iD
vo
vI
Another Circuit
The circuit below accepts an incomimng waveform and as usual with op amps,inverts it. However, only the positive-going portions of the output waveform,which correspond to the negative-going portions of the input signal, actuallyreach the output. The direct feedback diode shunts any negative-going outputback to the "-" input directly, preventing it from being reproduced. The slightvoltage drop across the diode itself is blocked from the output by the second
diode. D
allows positive-going output voltage to reach the output. 1
Design Task:
Based on the circuit of the AC voltmeter and for a
sinusoidal input signal with an amplitude V
at a frequencyp^
ω
, find theo
output voltage
v
described by its Fourier series (average value plus the I^
harmonics of the frequency
ω
.o
State when the output signal can be described by the DC voltage components (this means reducing the amplitudes of these harmonics to
very small level).
A Basic Circuit for Precision Full-Wave Rectifier Replace
D
A^ with a superdiode and the diode
D
B^ and the inverting
amplifier with the inverting precision half-wave rectifier to get the
precision full wave rectifier in the following page.
Precision Bridge Rectifier for Instrumentation Applications
AC Voltmeter
A Precision Peak Rectifier
The capacitor retains a voltage equal to the positive peak of
the input
A Precision Clamping Circuit
By replacing the diode in the usual clamping circuit with a
superdiode the precision clamp is obtained.
