Circuits and Systems, 2013, 4, 11-15
http://dx.doi.org/10.4236/cs.2013.41003 Published Online January 2013 (http://www.scirp.org/journal/cs)
A New Design Technique of CMOS Current Feed Back
Operational Amplifier (CFOA)
Hassan Jassim
Department of Electrical Engineering, College of Engineering, Babylon University, Babylon, Iraq
Email: hssn_jasim@yahoo.com
Received September 14, 2012; revised October 29, 2012; accepted November 6, 2012
ABSTRACT
A new design technique employing CMOS Current Feedback Operational Amplifier (CFOA) is presented. This design
approach applies CFA OTA as input stage cascaded with class AB cross-coupled buffer stage. The performance pa-
rameters of CMOS CFOA such as bandwidth, slew rate, settling time are extensively improved compared with conven-
tional CFOA. These parameters are very important in high frequency applications that use CMOS CFOA as an active
building block such as A/D converters, and active filters. Also the DC input offset voltage and harmonic distortion (HD)
are very low values compared with the conventional CMOS CFOA are obtained. P-Spice simulation results using 0.35
µm MI-ETEC CMOS process parameters shows considerable improvement over existing CMOS CFOA simulated
model. Some of the performance parameters for example are DC gain of 67.2 dB, open-loop gain bandwidth product of
104 MHz, slew rate (SR+) of +91.3 V/µS, THD of −67 dB and DC input offset voltage of −0.2 mV.
Keywords: Synthesis CFA OTA and CMOS CFOA; Cross Coupled Buffer Stage; High Performance CFOA; Low
Input Offset Voltage CFOA; Low Distortion CFOA
1. Introduction
The role of analog integrated circuits in modem elec-
tronic systems remains important, even though digital
circuits dominate the market for VLSI solutions. Analog
systems have always played an essential role in interface-
ing digital electronics to the real world in applications
such as analog signal processing and conditioning, in-
dustrial process, motion control and biomedical meas-
urements [1]. However, the conventional CMOS CFOA
design is still facing certain problems, first, the offset
voltage on the current feedback can not be made zero.
CFOA usually adopts an analog buffer as the input stage.
As a result, the non-inverting input has very high im-
pedance, while the inverting input has very low imped-
ance. Hence, the CFOAs offset is higher than folded
cascade voltage amplifier (VFA) Design. Second, the
constant bandwidth feature of the CFOA is only ap-
proximate if the inverting input impedance is not small
enough [2,3]. The low-input offset voltage is considered
as an important aspect of the performance of an amplifier
especially when signals are in the range of few hundred
micro volts [4]. Several CMOS realizations for the
CFOA have been reported in the literature [5-12]. The
design still suffers from many drawbacks such as high
distortion, high noise, high consumption of power and
complex circuitry. The CFOA has been always seen as
an extension of the CCII, therefore, the design approach
was cascade with CCII+ with a voltage follower to real-
ize a complete circuit. The obtained bandwidth was al-
ways the degraded version of CCII+.
The current feedback operational amplifier (CFOA), a
two-port (four-terminal) network. The CFOA could be
realized by using second generation current conveyor
CCII+ cascaded with a voltage follower [13].
This paper describes an alternative approach to CMOS
CFOA design which provides symmetrical high imped-
ances (infinite for DC) inputs together with high per-
formance parameters in high frequency operation. This
design approach applies CFA OTA as input stage cas-
caded with class AB cross-coupled buffer as output stage.
The symmetrical input stage of CFA OTA will reduce
the DC offset voltage of CMOS CFOA with improve-
ment of high frequency parameters. Moreover, class AB
cross coupled buffer stage provide high current drive
capability. P-Spice simulation results confirm the theo-
retical calculations.
2. Theoretical Background of CFB OTA
The two output terminals are not seen as one port each,
but as four independent terminals that can have different
impedance levels. As a consequence, hybrid stages ap-
pear, namely a H input stage and a H output stage. The H
input stage, which has become well known through the
CFB opamp, can also be understood as an extended input
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