CIRCUIT REDRESSEUR DE PRECISION

PRECISION RECTIFIER CIRCUIT

Abrégé anglais

ABSTRACT OF THE DISCLOSURE
A precision rectifier circuit using an operational
amplifier connected in a differential amplifier mode. At
least one of the rectifying feedback connections is provided
by the emitter-base path of a transistor and a half-wave
rectified version of the input signal is obtained from the
collector of the transistor. A biassing circuit is disclosed
for use when both rectifying feedback connections are
transistors to reduce the output voltage swing between one
transistor turning off and the other turning on.

Revendications

THE EMBODIMENTS OF THE INVENTION IN WHICH AN EXCLUSIVE
PROPERTY OR PRIVILEGE IS CLAIMED ARE DEFINED AS FOLLOWS:
1. A precision rectifier comprising:
an amplifier having an input and an output,
a signal input terminal connected to the amplifier
input,
a diode connected between the amplifier output and
input, and
a transistor with its base-emitter path connected between
the amplifier output and input providing a feedback path
poled in the opposite sense to that through the diode.
2. A precision rectifier as set out in claim 1 wherein
the output signal is the collector current of the transistor
and is a half-wave rectified version of the signal applied
to the signal input terminal.
3. A precision rectifier as set out in claim 1 wherein
said diode is constituted by the base-emitter path of a
second transistor.
4. A precision rectifier as set out in claim 3 further
including a biassing network connected between the amplifier
output and the base of said second transistor to promote the
turn on of said transistor when the output voltage passes
through zero.

5. A precision rectifier as set out in claim 1 wherein
said amplifier is a differential amplifier, the signal input
terminal is connected to the non-inverting input and the
signal fed back from the output is connected to the inverting
input.
6. A precision rectifier comprising a differential
amplifier having an inverting input, a non-inverting input
and an output,
a signal input terminal connected to the non-inverting
input,
first and second transistors of opposite polarity
having their base-emitter paths providing a feedback connection
between the amplifier output and the inverting input,
a biassing network connected between the amplifier
output and the base of the second transistor to promote the
turn on of the second transistor when the voltage at the
amplifier output passes through zero,
said biassing network comprising a pair of diodes
arranged in series with the base of said second transistor
and oppositely poled thereto and a resistive voltage divider
applying biassing voltages to the junction of the diodes and
to the base of the second transistor.

Description

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Background of the Invention
. .
This invention relates to a precision rectifier
circuit. Such clrcuits are used when signal waveforms are
to be measured with precision and distortion of the ~aveform
must be avoided.
It is known that when diodes are used for rectifi-
cation a significant signal loss occurs due to the forward
bias necessary before the diode conducts. Circuits using
the rectifying diodes in the feedback path of an operational
amplifier reduce this loss by a factor approximately equal
to the gain of the amplifier. Such circuits are shown in
Canadian patent No. 954,938, issued September 17, 1974 in
the names of Dammann et al. and U.S. patent No. 3,553,566,
issued January 5, 1971 in the name of Nagy.
The present invention replaces at least one of the
rectifying diodes by the emitter-base path of a transistor.
By taking the output signal from the collector of the transistor
a high output impedance source is provided suitable Eor
feeding a grounded current meter or a recorder. The invention
also includes a biassing circuit for use when both rectifying
diodes are replaced by transistors to reduce the output
voltage swing between one transistor turning off and the
other turning on.
Specifically, the invention relates to a ?recision recti-
fier comprising an amplifier having an input and an output, a
signal input terminal connected to the amplifier input,
a diode connected between the ampli~ier output and
lnput, and a transistor with its base~emitter path connected between
29 to the amplifier output and input providing a feedback path
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poled in the opposite sense to that through the diode.
In its restricted aspect, the invention relates to a
precision rectifier comprising a differential amplifier having
an inverting input, a non-inverting input and an output. A
signal input terminal is connected to the non-inverting input.
First and second transistors of opposite polarity have their
base-emitter paths provide a feedback connection between the
amplifier output and the inverting input. A biassing network
is connected between the amplifier output and the base of the
i 10 second transistor to promote the turn on of the second transistor
when the voltage at the amplifier output passes through zero.
The biassing network comprises a pair of diodes arranged in
series with the base of the second transistor and is oppositely
poled thereto. A resistive voltage divider applies biassing
voltages to the junction of the diodes and to the base of
the second transistor.
Brief Description of the Drawings
__
Figure 1 is a schematic diagram of a precision rectifier
circu:it in accordance with the present invention; and
Pigure 2 is a schematic diagram of a modification of
the circuit of Figure l.
Description of the Preferred Embodiments
... . ~
Referring now to Figure 1, a differential amplifier
10 has its non-inverting input connected to a suitable source
of biassing potential at terminal 19. Terminals ll and 12
are provided to receive tlle signal input with terminal 12 being
27 grounded and terminal 11 connected to the inverting input of
- 2 -
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differential amplifier 10 vla a resistor 13. Appropriate
voltage bias is supplied to the ampli-Eier via terminal 14.
The amplifier output is fed back to the input side of
resistor 13 via a diode 15. Another feedback connection oE
opposite polarity is provided by the base-emitter path of a
transistor 16. The precision rectifier output is obtained
from the collector of transistor 16 via a terminal 17. IE a
high impedance, current output circuit is required resistor 18
can be omitted. Conversely, if a voltage output is required
the collector current from transistor 16 is passed to ground
through resistor 18.
In operation, terminal 19 is suppl:ied with a positîve
voltage, typically 0.7 times the voltage applied to terminal
l~i. Normal operational amplifier feedback causes the current
in resistor 13 to equal the current flowing in either d:iode 15
or the emitter of transistor ]6. On positive going input
signals the amplifier output wi]l swing negatively causing
transistor 16 to conduct. Since transistor 16 is selected to
have a high current gain almost all the emitter current also
flows in the collector circuit producing a half-wave rectified
version of the input signal. Transistor 16 may be a Darlington
pair which, as is known, is the equivalent of a single transistor
of high current gain.
Figure 2 illustrates modifications to the circuit o-E
Figure 1. The same reference numerals are used for elements
already described in connection with Figure 1. As before,
transistor 16 provides a feedback path from the output of
28 amplifier 10 to its inverting input. In distinction to the
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c:ircuit of Figure 1, however, the signal input from terminal
11 is applied to the non-inverting input via a biassing
network formed by capacitor 21 and resistors 22 and 23.
This provides a high impedance input.
The feedback connection provided by diode 15 in
Figure 1 is replaced by the base-emitter path of a further
transistor 25 in Figure 2. If required, a further output of
the precision rectifier is available from the collector of
transistor 25 at terminal 27. Resistor 28 can be provided
if a voltage output is required. The performance of the
rectifier circuit is improved by including a biassing
network consisting of diodes 31, 32 and resistors 33, 3~l.
When a source of positive bias is applied to terminal 35 the
network functions to maintain the base of transistor 25
positive with respect to the output of amplifier 10. Thus,
as the output signal from amplifier 10 changes from negative
to positive the amplitude of the voltage swing between
transistor 16 turning off and transistor 25 turning on is
greatly reduced. The time required between turn off and
turn on is correspondingly reduced enabling the circuit to
operate at higher frequencies for a given precision.
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