International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
Precision AC/DC Converter using OP-Amp Saturation
Robin Dixit1, C. K. Dwivedi 2
1,2
Department of Electronics and Communication, University of Allahabad
The circuit does not use any diode for rectification but
the zero voltage output saturation of single supply
operational amplifier is used for rectification. This gives
considerable improvement as there is no change in the
output due to temperature dependent characteristics of
diode. Transients due to switching noise of the diode are
also eliminated.
Output of a single supply operational amplifier can
swing to –ve supply (ground terminal) within few
millivolts and common mode input range also includes –ve
supply voltage, thus such operational amplifier can be used
as saturating inverter amplifier to generate nonlinear
transfer function similar to Ideal diode.
Abstract— A simple full wave precision rectifier using a
single supply operational amplifier in saturation mode is
described. The rectification is achieved by non-linearity
arising due to output saturation at ground voltage. This
circuit does not require the use of any diode for rectification.
One such circuit implemented using LM358 Operational
Amplifier has been tested
Keywords— LM358 Operational Amplifier, Precision
rectifier, non-linearity, Rectification.
I. INTRODUCTION
It is often necessary to precisely rectify available AC
signal to obtain proportionate DC voltage. Conventional
rectifiers used for instrumentation use diodes. Which
suffers the initial zero offset corresponding to diode cut-in
voltages that can not be easily offset because of their
temperature dependence? The improvement over such
rectifiers was proposed by various manufactures of the
operational amplifier in their application notes. In such
circuits diodes are used in the feedback loop of operational
amplifier as shown in Figure 1. And two separate circuits
are used for each half of full AC cycle and combined signal
is used to get the full wave rectified output.
Figure 2. Block Diagram of proposed AC/DC converter
II. PRINCIPLE
The principle of the proposed precision AC/DC
converters can be explained with reference to the block
diagram shown in Figure 2. The converter consists of a
saturating inverter amplifier having transfer function as
shown in diagram having VOUT = K . [|VIN| – VIN], a
summing circuit and a low pass filter with buffer amplifier.
Figure I. Conventional Precision Full Wave Rectifier
In the present paper a novel precision rectifier using
single supply operational amplifier in saturated mode is
described.
388
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
Timing diagram shows the signal at various stages for
sinusoidal input signal, figure 3(a). Output from saturating
inverter amplifier is zero for all positive values of input and
is proportional to absolute value of input for negative
values of input signal as shown in figure 3(b). Output
signal, figure 3(c), is obtained by superposition of 3(a) and
3(b). For output to have equal values for +ve and –ve
values of input signal it is required that gain of absolute
value of signal at output of saturating inverting amplifier
must be twice the direct signal from the input.
III. THEORY
Figure 4 Proposed circuit
The circuit diagram for proposed AC/DC converter is
shown in figure 4. The output from proposed circuit can be
calculated for positive values of VINPUT & for negative
values of VINPUT Separately by considering the equivalent
circuit for both cases as shown in figure 5(a) and 5(b).
Figure 3 Timing Diagram
389
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
V3+ =VIN . R2/(R1+R2) …………………………………. (1)
And for negative values of V INPUT the output voltage V3
is given by the expression
V3– = VIN . R2/(R1+R2) + V2. R1/(R1+R2) ………………….. (2)
Where V2 is output of saturating inverter amplifier that
equals – VIN, (for – ve values of VIN). Thus
V3– = VIN . R2/(R1+R2) – VIN. R1/(R1+R2) =VIN. (R2–R1) /
(R1+R2) ………………….. (3)
Equating the value of V3+ and V3– so that rectified output
is same for positive and negative half cycles of AC
sinusoidal signal. We getVIN.R2 / (R1+R2) = VIN. (R2–R1) /
(R1+R2) R1 = 2. R2
Figure 5. Equivalent Circuit
For positive values of VINPUT the output voltage V3 is
given by the expression
R1 440K
IC2
R3 220K
VIN
R4
220K
R2
220K
C
VOUT
10MFD
IC1
R5 110K
R6 220K
R7 440K
Figure 6 : Proposed Precision AC/DC Converter
IV. CIRCUIT DESCRIPTION
V. CONCLUSION
The Circuit shown in Figure 6 has been tested using
single supply dual operations amplifier LM 358.
Resistances in the circuit are so chosen that R1=2. R2 and
R4=R3. Value of R5 is chosen equal to R3 II R4 to have
minimum error due to bias current of ICI. R 7 is chosen to
have R7=R6 so that buffer gain is equal to attenuation in
converter circuit which in 3 as calculated above. R6 II R7
should be equal to R1 II R2 to minimize bias current error
of IC2. Value of C was selected to obtain time constant of
filter circuit to be the order of a second or so.
The circuit is tested to work satisfactorily over a wide
supply range (3 to 15 volts) and for AC signal of few
millivolts to few volts the output is equal to absolute
average value of input signal. For 3 volts supply voltage the
circuit can accommodate input sinusoidal signal little more
than 1 volt. The performance of the circuit has been
checked for frequencies from 10Hz up to 5khz, the error is
with in 2 to 3% the performance is having very less effect
of temperature variations and is very stable.
390
International Journal of Emerging Technology and Advanced Engineering
Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 4, Issue 3, March 2014)
[3]
The scheme may find useful application in electronic
measurements, development of simple wide voltage range
electronic multimeters and audio detector circuits.
Frequency range of opration can be improved by using C-7
type capacitor in place of C in above circuit
[4]
[5]
REFERENCES
[1]
[2]
[6]
JIM SHERWIN, 1986 Use of Single supply operational amplifier
LM358, National Semiconductor Linear Application data book, AN116.
National Semiconductor Corporation, 1986, The full wave rectifier
and averaging filter. Linear Applications Data book, AN-20.
[7]
391
Signetics, 1977, Precision full wave rectifier, Analog data manual,
Sec 20.
WILSON B., MANNAMA V.:Current-mode rectifier with improved
precision. IEE 1995, Electronics Letters Online No: I9950185.
KOTON J., HERENCSAR N., VRBA K.: Current and Voltage
Conveyors in Current and Voltage-Mode Precision Full-Wave
Rectifiers, RADIOENGINEERING, VOL. 20, NO. 1, APRIL 2011.
TIETZE.U, SCHENK,CH. GRAMM.E. Electronics Circuit
Handbook for design and application. Berlin (Germany)Springer,
2008
MINHAJ, N. OTA-based non inverting and inverting precision full
wave rectifier circuit without diodes. Int.J. Recent Trends in
engineering 2009, Vol 1, no 3 P. 72-75