This document proposes a novel structure to improve the common mode rejection ratio (CMRR) of circuits like current buffers and folded cascode amplifiers. The proposed structure uses only four transistors and a current source to deviate common mode signals without affecting differential mode signals. This improves the CMRR by at least 12dB while preserving the CMRR bandwidth, which is a novel technique. The structure was applied to both a current buffer and folded cascode amplifier based on simulation results, demonstrating its effectiveness in improving CMRR.

1. A Novel Simple And High Performance Structure For
Improving CMRR: Application to Current Buffers
and Folded Cascode Ampilifier
Amir Hossein Miremadi Hassan Faraji Baghtash
Islamic Azad University, West Tehran branch Iran University of science and technology (IUST) electrical
Tehran, Iran and electronic engineering faculty/ Electronics research
amirh_miremadi@yahoo.com center
Tehran, Iran
hfarajii@gmail.com
Abstract—A novel and simple structure for improving CMRR is process signals in differential form rather than ground-
introduced. This structure can be added to the circuits like folded reference form. Another advantage of differential operation
cascode amplifier, telescopic amplifier, current buffers, .etc to over the single-ended case is that the amplitude of the signal
improve the CMRR of these circuits. This simple and effective increases by the factor of 2[2]. An important parameter of
circuit uses common mode deviating technique to improve differential active structures is the CMRR. Differential signals
CMRR at least 12dB while preserves CMRR bandwidth which is have the advantage of canceling common-mode interference
a novel technique in order to improve CMRR. Application of this from unwanted signals and/ or noise. So CMRR is one of the
structure in both current buffer and folded cascode structures most significant parameters in many of the circuits which are
are shown. Simulation results in TSMC 0.18µm CMOS
processing differential signals; such as Op-amps, OTAs,
technology with HSPICE are presented to demonstrate the
validity of the proposed circuit. In addition Monte Carlo analysis
current buffers, etc; hence its improving is a critical issue in
is performed to simulate the fabrication condition. these circuits. Any variation of the input stage tail current
source in these circuits can significantly harm both
Keywords- High cmrr; current buffer; analog circuits; mixed deterministic and random components of the CMRR [3], [4].
mode; low voltag;, low power There have been numerous attempts to improve CMRR [5]-[7].
Most of these references used schemes to increase tail output
impedance such as using cascode current mirror. However
I. INTRODUCTION
most of these structures cause to increase minimum voltage
Today's Digital Signal Processing (DSP) is extremely on requirement. Some other structures that used techniques which
demand because of its natural benefits like reduced sensitivity didn’t need high output impedance tail need complicated
to analog noise, enhanced functionality and flexibility, implementation. On the other hand these structures need more
automated design and test, shorter design cycle, direct benefit power consumption and cause reduction in CMRR bandwidth.
from the scaling of VLSI technology, etc. But real world In this paper we proposed a simple and high effective cell
signals are analog; hence mixed-mode ICs are becoming which can be added to the circuits like folded cascode structure
progressively dominant i.e. System On Chip (SOC). In other and it cause improvement in CMRR and PSRR; whereas it
words Mixed-mode signal processing attracts increasing preserves CMRR bandwidths. This structure is composed of
attention since it simplifies design, enables compactness and only four transistors and increases power consumption slightly.
reduces cost. However signal interference from the digital to
the analog part remains a serious problem to overcome [1]; this II. PROPOSED HIGH PERFORMANCE COMMON MODE
is due to the fact that in a SOC, both analog and digital parts of
DEVIATING CELL
circuit have the same substrate. In these circuits, analog parts
must be resistant to the power supply interference coming from Fig .1 shows the conceptual schematic of the proposed
digital part. These variations which are caused by transient common mode deviating cell. The main idea is using a circuit
currents of digital circuits can cause undesired effects on power in parallel with signal path which has infinitive input resistance
supply rails or analog circuits’ inputs as common mode. Hence to the differential currents and zero to the commons. By using
both CMRR and PSRR play important roles in analog circuits. this idea we can deviates common mode currents and prevent
On the other hand, technology scaling imposes power supply to them from going to the output. On the other hand differential
be lowered. Decreasing power supply imposes some currents cannot flow in this block because of infinite resistance
restrictions on design procedure and harms common-mode to these signals ideally. Fig.2 shows transistor level
rejection ratio (CMRR) and power supply rejection ratio implementing of this idea. This structure is composed of n-type
(PSRR). Hence, for such circuits differential building blocks transistors M1, M2, p-type transistors Md1, Md2 and current
are accepted as a good solution. Therefore it is desired to source of Ib. transistors Md1, Md2, and current source of Ib
978-1-4244-8971-8/10$26.00 c 2010 IEEE

2. which form the differential pair, and transistors M1 and M2 From (1) and (2) we can obtain common mode input
make the deviation path for the common mode signals. impedance Rind and differential input impedance Rinc of the
Averaging property of differential pair is used to control gates proposed circuit as follows.
of M1 and M2. When a differential signal is applied to the
inputs of differential pair, its source voltage remains constant;
but when its input signal is common, the circuit acts as a ⎛ 1 ⎞
voltage follower and the source voltage follows its inputs. Rinc = ⎜ ro1,2 ⎟
⎜ α g m1,2 ⎟
Using this property of differential pair we can control gate ⎝ ⎠
voltages of M1 and M2 so that, when the common signals are Rind = ro1,2
applied to the inputs of the proposed cell, voltage following (3)
property of differential pair aids a) transistors M1 and M2 to act
as a diode and sink common mode currents; b) input Where gm1,2 and ro1,2 are transconductance and output
impedance of Md1 and Md2 to increase because their VGS resistance of M1 (or M2) respectively.
remains constant. On the other hand when a differential signal
is applied to the cell inputs, gate voltage of transistors M1 and
M2 doesn’t change and these transistors show no action to this
type of signals; this means resistivity of circuit to the
differential signals is high significantly.
∞ 0r rd =
rd = rc = c = 0 ∞
Figure 2. Transistor implementation of proposed common mode alienates
cell
Figure 1. A conceptual schematic of proposed common mode alienates cell
Any deviation from ideal case leads to a decrease in
CMRR. Here we used a simple current mirror to provide tail
current of Ib. If considering small signal condition, Voutcm can
be obtained from (1):
voutcm =
( )
g md 1,2 rod 1,2 2rotail vincm
= (1 − α ) vincm
(
1 + g md 1,2 rod 1,2 2rotail ) (1)
(a)
where Voutcm, Vincm, gmd1,2, rod1,2,rotail are common mode
output voltage, common mode input voltage, transconductance
of Md1 (or Md2 ), output resistance of Md1 (or Md2 ), output
resistance of tail current source respectively, and α is
1
attenuation factor we defined here as α =
1 + g md 1, 2 ( rod 1, 2 2 rotail )
where for differential mode input voltage Vindm and differential
output voltage Voutdm , equal zero.
(b)
voutdm = 0
(2) Figure 3. (a) Conventional current buffer. (b) Proposed High CMRR current
buffer.

3. III. APPLICATIONS IV. SIMULATION RESULTS
The proposed structure is very simple, power efficient, and HSPICE simulation were performed using TSMC 0.18µm
has low transistor number. This structure can be applied to the CMOS technology at 1.8 V power supply. The transistors W/L
prevalent structures such as current buffers, folded cascode are shown in table .1. Two circuits are simulated at the same
structures and cause significant improvement in CMRR and condition to make comparison. Fig .5 shows frequency
PSRR in these structures. Here we applied our proposed circuit response of conventional and proposed current buffer.
to the current buffer and folded cascode OTA and compared Differential and common mode input currents are applied to
the CMRR of them both. Fig .3 (a) shows a simple current the both simple and proposed current buffer from in+ and in-
buffer; differential input currents are applied to the Iin1 and Iin2 and output currents are taken from Io1 and Io2. The figure
as shown in the fig .3 (a) and the output taken from Io1 and Io2. exhibits about 12 dB CMRR for proposed current buffer this is
Applying proposed cell to the simple current buffer, the significant enhancement in CMRR value compared to its value
modified current buffer is obtained (See fig .3 (b)). For simple in simple one.
current buffer both differential and common mode input
impedances are obtained from rin=1/gmc where for proposed
current buffer we can obtain input impedance to the differential
and common mode signals as rind=1/gmc and rinc =α/gmc||roc
where gmc and roc are transconductance and output impedance
of Mc1 (or Mc2) respectively.
A conventional folded cascode input stage is shown in fig
.4 (a). Modified version of this is shown in fig .4 (b)
(proposed). Differential and common mode input impedance
for proposed OTA are the same as proposed buffer.
Figure 5. CMRR of the proposed and conventional current buffer
Fig .6 shows the frequency response of the conventional
and proposed folded cascode OTA. Differential and common
mode input currents are applied to the both simple and
proposed current buffer from in+ and in- and output currents
are taken from Io1 and Io2.
As shown in fig .6, the proposed circuit increases (up to 12
dB) the CMRR whereas it preserves CMRR bandwidth.
Preserving CMRR bandwidth is very interesting feature of this
circuit which is not accessible in other similar works.
(a)
Figure 6. CMRR of the proposed and conventional folded cascode OTA
Monte Carlo analysis is performed by 3% variation on
transistors aspect ratio to simulate fabrication condition. Fig .7
shows Mont Carlo analysis for both simple and proposed
(b) current buffer. Comparison of Mont Carlo analysis for simple
and proposed folded cascode OTA is shown in fig .8. As
Figure 4. Conventional folded cascode OTA. (b) Proposed High CMRR shown, CMRR of proposed circuits increased at least 10 dB in
folded cascode OTA.
comparison with the conventional structures.

4. TABLE I. TRANSISTORS ASPECT RATIO
OTA Current Buffer
ELEMENT Proposed conventional Proposed conventional
W L W L W L W L
M1 0.36 µm 0.18 µm NA NA NA NA NA NA
M2 0.36 µm 0.18 µm NA NA NA NA NA NA
Md1 27 µm 0.18 µm NA NA NA NA NA NA
Md2 27 µm 0.18 µm NA NA NA NA NA NA
Mc1 5.4 µm 0.54 µm 5.4 µm 0.54 µm 5.4 µm 0.54 µm 5.4 µm 0.54 µm
Mc2 5.4 µm 0.54 µm 5.4 µm 0.54 µm 5.4 µm 0.54 µm 5.4 µm 0.54 µm
Mm1 3.6 µm 0.54 µm 3.6 µm 0.54 µm 3.6 µm 0.54 µm 3.6 µm 0.54 µm
Mm2 3.6 µm 0.54 µm 3.6 µm 0.54 µm 3.6 µm 0.54 µm 3.6 µm 0.54 µm
MI1 3.6 µm 0.18 µm 3.6 µm 0.18 µm NA NA NA NA
MI2 3.6 µm 0.18 µm 3.6 µm 0.18 µm NA NA NA NA
Mmp1 2.7 µm 0.54 µm 2.7 µm 0.54 µm NA NA NA NA
Mmp2 2.7 µm 0.54 µm 2.7 µm 0.54 µm NA NA NA NA
and cause CMRR to improve in these circuits. This simple and
high effective circuit uses common mode deviating technique
to improve CMRR while preserves CMRR bandwidth which is
a novel technique in order to improve CMRR. Application of
this structure on both current buffer and folded cascode
structures are shown. Simulation results in TSMC 0.18µm
CMOS technology with HSPICE are presented to demonstrate
the validity of the proposed circuit. Mont Carlo analysis is
performed for simulating fabrication condition and
corroborated the appropriate performance of the proposed
circuit.
ACKNOWLEDGMENT
This work is supported by Islamic Azad University, West
Tehran branch.
Figure 7. Monte Carlo analysis of conventional and proposed current buffer. REFERENCES
- -) Simple. - ) Proposed
[1] Shahram Minaei ,I. Cem. Go¨ knar, Oguzhan Cicekoglu, "A new
differential configuration suitable for realization of high CMRR, all-
pass/notch filters," Springer-Verlag, p. 317–326, May 2005.
[2] Allen PE, Holberg DR, CMOS analog circuit design, 2, Ed. New York:
Oxford University Press, 2002.
[3] C.-G. Yu and R. L. Geiger, "Nonideality consideration for high-
precision amplifiers—Analysis of random common-mode rejection
ratio," IEEE Trans. Circuits Syst. I, Fundam. Theory Appl., vol. 40, no.
1, p. 1–12, Jan. 1993.
[4] F. You, S. H. K. Embabi, and E. Sanchez-Sinencio, "On the
commonmode rejection ratio in low voltage operational amplifiers with
complementary N-P input pairs," IEEE Trans. Circuits Syst. II, Analog
Digit.Signal Process., vol. 44, no. 8, p. 678–683, 1997.
[5] Vadim Ivanov, Junlin Zhou, and Igor M. Filanovsky, "A 100-dB CMRR
CMOS Operational Amplifier With Single-Supply Capability," IEEE
TRANSACTIONS ON CIRCUITS AND SYSTEMS—II: EXPRESS
BRIEFS, vol. 54, no. 5, pp. 397-401, May 2005.
[6] Vadim Ivanov, Junlin Zhou, Igor Filanovsky, "A 100 dB CMRR CMOS
Figure 8. Monte Carlo analyses of conventional and proposed folded OTA. - Operational Amplifier With Single-Supply Capability," IEEE., pp. 9-12,
-) Simple. - ) Proposed 2004.
[7] Jaime Rámirez-Angulo, Sandhana Balasubramanian, Antonio J. López-
Martin, and Ramón G. Carvajal, "Low Voltage Differential Input Stage
I. CONCLUSION With Improved CMRR and True Rail-to-Rail Common Mode Input
A novel and simple structure for improving CMRR was Range," IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—II:
EXPRESS BRIEFS, vol. 55, no. 12, pp. 1229-1233, Dec. 2008
introduced. As shown, this structure can be added to the
circuitries like folded cascode amplifier, current buffers, etc.;