Download operational amplifier design with rail to rail supply voltage output

IJVD: 3(1), 2012, pp. 11-14
OPERATIONAL AMPLIFIER DESIGN WITH RAIL TO RAIL SUPPLY
VOLTAGE OUTPUT HAVING REDUCED POWER DISSIPATION
Raj Kumar Tiwari1 and Anil Kumar Shukla2
1,2
Circuit Design and Simulation Lab, Department of Physics and Electronics, Dr.R.M.L. Avadh University,
Faizabad (U.P), India, 1E-mail: [email protected] 2E-mail: [email protected],
Abstract: In this paper design for 1 volt rail to rail supply voltage output of operational amplifier with
common mode feedback amplifier has been presented. The circuit has been simulated using BSIM4 model
with 50nm CMOS process. The simulation results show that output voltages for the modified operational
amplifier are swinging from 10mV to 990mV i.e. between the rail to rail supply voltages (0 to 1V). Results
also show that current drawn by the modified operational amplifier is 310µA in comparison to the circuit
for which it is 470 µA and thus shows the reduced power dissipation for the operational amplifier.
Keywords: Operational amplifier, CMFB, power dissipation, differential amplifier.
1. INTRODUCTION
In the recent years developments in the field of
wireless communication and biomedical signal
processing demand of analog circuits working on
low supply voltages and reduced power dissipation
has been increased.[1-2]. Operational amplifiers are
the main analog building block required for these
applications. Number of rail to rail operational
amplifier design based on the complementary
PMOS and NMOS transistors and constant-gm has
been reported [3-5]. Several approaches have been
reported for operational amplifier design by using
both fully differential and pseudo-differential
configurations respectively. The pseudo-differential
structure requires the common mode feedback
circuit (CMFB) for designing the operational
amplifier [6-8].
In this paper design of a rail to rail operational
amplifier has been presented. The paper is
organized in the following way. Section II contains
the circuit of two stage operational amplifier with
common mode feed back amplifier. Section III
contains the proposed circuit structure for rail to rail
voltage operational amplifier. Finally section IV
shows the simulation results for the operational
amplifier circuits.
2. TWO-STAGE OPERATIONAL AMPLIFIER
WITH CMFB AMPLIFIER
Figure 1 shows the schematic of a two stage
operational amplifier design with common mode
feedback amplifier. In this circuit the gate-drain
connected PMOS transistors behave like MOSFETs
with twice the width of other PMOS transistors. The
NMOS transistors which are biased from the voltage
Vbias behave like MOSFETs with four times the
width of the other NMOS transistors. The current
flowing into all the MOSFET will be same for the
equal differential amplifier inputs. When the input
voltage at the positive terminal of differential
amplifier is raised above the voltage at the negative
terminal, all of the bias current flows into the left
NMOS transistors of the differential amplifier. In
this circuit the second stage of the operational
amplifier operates as class AB. Capacitors of the
value 50fF are used for the compensation. The
common mode feedback amplifier (CMFB) used in
the circuit balances the output over the entire range
of the differential amplifier output voltages.
3.
CIRCUIT STRUCTURE FOR RAIL TO
RAIL SUPPLY VOLTAGE OUTPUT IN
TWO STAGE OP-AMP WITH CMFB
AMPLIFIER
Figure 2 Shows the modified circuit structure for
rail to rail supply voltage output. In this circuit the
NMOS transistors in the output buffer of the
operational amplifier are cascaded by the
combination of NMOS and PMOS transistors
respectively. The added transistors will operate near
or in the triode region and allow the operational
amplifier’s output to swing more freely.
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Raj Kumar Tiwari and Anil Kumar Shukla
Figure 1: Two -Stage Op-Amp with Common Mode Feedback Amplifier
Figure 2: Circuit Structure for Rail to Rail Supply Voltage Operational Amplifier Design
4. RESULTS AND DISCUSSIONS
Figure 3 shows the variation in differential output
voltage of the two stage operational amplifier given
by figure 1 with respect to the input voltage sweep.
Simulation result shows that both the differential
outputs cross at the ideal common mode voltage of
500mV. In this case outputs are swinging from
200mV to 800mV and not between rail to rail supply
voltages. Figure 4 shows the variation in the
difference between the differential output voltages
with respect to the input voltage. Figure 5 shows
variation of the derivative of the difference between
the differential output voltages with respect to the
input voltage and thus the gain of the operational
amplifier. Figure 6 shows the current drawn by the
operational amplifier and at the common mode
voltage of 500mV the current drawn by the
operational amplifier is 470µA.
Operational Amplifier Design with Rail to Rail Supply Voltage Output Having Reduced Power Dissipation
13
Figure 3: Variation in the Differential Output Voltages with
Respect to Input Voltage for the Operational
Amplifier Given by Figure (1)
Figure 4: Variation in the Difference between the Differential
Output Voltages with Respect to Input Voltage for
the Operational Amplifier Given by Figure (1).
Figure 5: Variation in the Gain with Respect to Input Voltage
for the Operational Amplifier Given by Figure (1).
Figure 6: Variation in the Current Drawn with Respect to
Input Voltage for the Operational Amplifier Given
by Figure (1)
Figure 7 shows the variation of differential
output voltage of the modified circuit given in the
figure 2. The simulation results show that the output
voltages are now swinging from 10mV to 990mV
i.e. between the rail to rail supply voltages (0 to 1V).
Figure 8 Shows the variation in the difference
between the differential output voltages with
respect to the input voltage. Figure 9 Shows the
variation in the gain of the modified circuit with
respect to the input voltage. Simulation result shows
that the modified circuit posses the higher gain.
Figure 10 shows the current drawn by the
operational amplifier and at the common mode
voltage of 500mV the current drawn by the modified
circuit of operational amplifier is approximately
310µA. It shows that the power dissipation in the
modified circuit has been reduced.
Figure 7: Variation in the Differential Output Voltages with
Respect to Input Voltage for the Operational
Amplifier Given by Figure 2.
Figure 8: Variation in the Difference between the Differential
Output Voltages with Respect to Input Voltage for
the Operational Amplifier Given by Figure 2.
14
Raj Kumar Tiwari and Anil Kumar Shukla
Figure 9: Variation in the Gain with Respect to Input Voltage
for the Operational Amplifier Given by Figure (2).
Figure 10: Variation in the Current with Respect to Input
Voltage for the Operational Amplifier Given by
Figure (2).
5. CONCLUSION
[5]
C.J.B. Fayomi, M. Sawan, and G.W. Roberts, “A
Design Strategy for a 1-V Rail-to-Rail Input/Output
CMOS Opamp”, in Proc. of the IEEE International
Symposium on Circuits and Systems Conference, 639642, (2001).
[6]
Y. Lu, and R.H. Yao, “Low-voltage Constant-gm
Rail-to-rail CMOS Operational Amplifier Input
Stage”, Solid State Electronics, 52, 957-961, (2008).
[7]
M. Waltari, K. Halonen, “A Switched-opamp with
Fast Common Mode Feedback”, ICECS ‘99, 3, 5-8
Sept. 1999, pp. 1523-1525.
[8]
M. Banu, J.M. Khoury, Y. Tsividis, “Fully
Differential Operational Amplifiers with Accurate
Output Balancing”, IEEE J.Solid-State Circuits, 23,
Dec. 1988, pp. 1410-1414.
[9]
A.H. Maarefi, H. Parsa, Hatamkhani and D. Shiri,
“A Wide Swing 1.5V Fully Differential OP-AMP
Using A Rail-to-Rail Analog CMFB Circuit”, IEEE
Trans. Instrum. Meas, 40, Aug. 1991, pp. 699-702.
In this paper design of an operational amplifier
with rail-to rail supply voltage output having
reduced power dissipation has been presented.
Simulation results show that for the designed
operational amplifier output voltages are swinging
from 10mV to 990mV which is very close to the
supply voltages (0 to 1V). Results also show that
current drawn by the designed operational amplifier
is approximately 310µA and thus indicating the
reduced power dissipation in the circuit.
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