Simplified circuit uses one operational amplifier and two precision resistors. The amplifier is operated open loop for switching and closed loop for linear gain, both simultaneously

Deboo, G. J.

Hedlund, R. C.

English

NASA Technical Reports Server

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In e Out 
April 1970	 Brief 70-10161 
NASA TECH BRIEF Ex 
NASA Tech Briefs announce new technology derived from the U.S. space program. They are issued to encourage commercial 
application. Tech Briefs are available on a subscription basis from the Clearinghouse for Federal Scientific and Technical 
Information, Springfield, Virginia 22151. Requests for individual copies or questions relating to the Tech Brief program may 
be directed to the Technology Utilization Division, NASA, Code UT, Washington, D.C. 20546. 
Precision Full-Wave Rectifier 
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Qi UC451 
Full-Wave Rectifiei Circuit 
The problem: 
To simplify the circuits of precision full-wave 
rectifiers which use operational amplifiers. Conven-
tional circuits require at least two operational ampli-
fiers and six precision resistors. 
The solution: 
A circuit using only one operational amplifier 
(acting as a linear amplifier and a switch driver) and 
two precision resistors. The operational amplifier is 
operated open loop for switching and closed loop for 
linear gain, both simultaneously. 
How it's done: 
In the figure, precision resistors RI and R2 are of 
equal value, and function as the conventional input-
feedback loop for the operational amplifier; the other 
resistors can be of the ordinary ± 20% carbon types. 
When the input voltage is positive, the voltage at 
point B will be equal in magnitude but 180° out of 
phase with the input. This requires the output of the 
operational amplifier at point A to be more negative 
by an amount equal to the sum of the forward and 
reverse potentials of the zener diodes CR3 and CR4; 
this value will be of the order of 6 volts for diodes 
which have zener breakdown potentials of about 5.5 
volts. Thus, even though the input voltage may be a 
very small positive value, the amplifier generates at 
least —5.5 volts at point A, and this potential is more 
than sufficient to turn off the field effect transistor 
(FET) switch Q2 via a forward biased diode CR2. 
(continued overleaf) 
This document was prepared under the sponsorship of the National
	 Government assumes any liability resulting from the use of the 
Aeronautics and Space Administration. Neither the United States
	 information contained in this document, or warrants that such use 
Government nor any person acting on behalf of the United States
	 will be free from privately owned.rights.
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Diode CR! prevents transfer of the voltage at point 
A to the gate of the FET switch Ql, and since the gate 
and source of Q! are at the same potential (the condi-
tion for minimum source-to-drain channel resistance), 
QI is on and the input is coupled directly to the out-
put. 
When the input voltage is negative, it is evident that 
the FET switch Q2 is turned on and Qi is turned off. 
Hence, full-wave rectification is effected (positive 
output voltage) by coupling the input to the output 
terminal in one mode of operation (via Qi), and the 
output of the amplifier appearing at point B through 
Q2 in the other mode. Since the FET switches in the 
signal paths have no offsets and very low values of 
resistance, the circuit provides linear full-wave 
rectification. Frequencies up to a few kHz can be 
handled by the components; higher frequency opera-
tion is possible by replacing the FET switches with 
MOSFET devices of superior high-speed switching 
characteristics and using high-bandwidth operational 
amplifiers. 
If Q1 and Q2 are interchanged, and CR! and CR2 
are each reversed, the polarity of the output terminal 
will be negative. FET switches minimize temperature-
sensitive offsets.
Notes: 
1. Additional information is contained in the follow-
ing paper: Deboo, G. J. and Hedlund, R. C.: Pre-
cision Full-Wave Rectifier Uses Only One Opera-
tional Amplifier, EEE Magazine, June 1968, p. 120. 
2. Requests for further documentation may be di-
rected to:
Technology Utilization Officer 
Ames Research Center 
Moffett Field, Calif. 94035 
Reference: B70-10161 
Patent status: 
Inquiries about obtaining rights for the commercial 
use of this invention may be made to National Aero-
nautics and Space Administration, Code GP, Wash-
ington, D.C. 20546.
Source: Gordon J. Deboo
and Roger C. Hedlund 
Ames Research Center
(ARC-10101) 
Brief 70-10161	 Category 02


Precision full-wave rectifier

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