ISSN:2229-6093
Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066
Operational Transconductance Amplifier for Low Frequency
Application
Abhay Pratap Singh*, Sunil Kr. Pandey*, Manish Kumar*
*(Dept. of Electronics and Communication Engineering
Jaypee Institute of Information Technology, A-10, Sector-62, Noida-201307, India
Email: apsingh007@gmail.com, sunilpandeyiert@gmail.com, manish.kumar@jiit.ac.in )
Abstract
This paper presents the design of CMOS operational
transconductance amplifiers (OTAs) with very small
transconductance (of order of nano ampere per volt),
which uses in very low frequency continuous time filters.
This design uses current division technique to reduce the
transconductance of OTA. This design is simulated in
SPICE tool using 0.25µm technology model file.
Keywords - Low Frequency, Low Transconductance, OTA,
SPICE
1. Introduction
Low frequency circuits have a very important role in
systems for biomedical, telemetry, real time speech
recognition and infield of neural networks [1]-[3]. Thus,
there is a strong motivation for developing integrated
solution for circuits that are capable of operating at very low
frequency. The design of these circuits is not an easy task
especially for integrated circuit (IC) implementation. We
know that time constant of operational transconductance
amplifiers-capacitor (OTA-C) filter is determine by the ratio
of load capacitor to the OTA small signal transconductance.
For an OTA-C filter implementation such low frequency
implies large capacitance and very low transconductance
[4]-[5]. Thus there are two different techniques to solve this
problem. One is to design an OTA with very low
transconductance and is to realization of very large
capacitance on a chip. Due to silicon area limitation,
practical capacitances are limited to be below 50pF. Hence
for implementation of 10Hz pole, transconductance of
3nA/V are required. In this paper, current division technique
is uses for implementation of very large time constant.
Using this technique we design a low transconductance
OTA which can be used in low frequency application.
Fig 1: Ideal model of OTA
Fig 2: Equivalent circuit of OTA
The output current I0 of the ideal OTA can be expressed by
equation
I 0  gm (VP  VN )
where gm, the transconductance can be expressed in terms of
bias current (Ibias), charge (q), Boltzmann constant (K) and
temperature (T) in Kelvin, as follows:
2. Operational Transconductance Amplifier
(OTA)
OTA is a voltage controlled current source, its takes the
difference of the two voltages as the input for the current
conversion. There is an additional input for a current to
control the amplifier's transconductance.
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gm 
KT
2qI bias
Since the output of an OTA is derived as the current, the
output impedance of the OTA is very high (ideally infinity).
Since gm of the OTA is dependent on the Ibias current, the
output characteristics of the OTA may be controlled
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ISSN:2229-6093
Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066
externally by the bias current (Ibias). It adds new dimension
to design and applications of OTA circuit.
Fig. 4 shows the source degeneration technique. Here
the effective GM is given by
3. Techniques for decreasing transconductance
GM  gm _ M1, 2 1 gm _ M1, 2 R
3.1 Current division
In this technique, the current generated by the voltage to
current converter is further reduces by using current mirror
with large division factor. For moderate transistor
dimension, the transconductance of a differential pair can be
found as small as 10-7 to 10-9 A/V. Hence, using current
division technique we can find filters in range of few Hz.
The main cost of current division is that the large amount of
silicon area.
Here we see that the effective transconductance is
reduced by the factor 1+gmR.
4. Low transconductance OTA
Fig 5: OTA with current division technique []
Fig 3: Current division technique
The effective GM is given by
GM 
gm _ MC
M 1
Where MC is the composite transistor (before splitting)
The small-signal current are split by the ratio of their size
(by factor M). Thus, the effective transconductance is
reduced by factor M+1 compared to the before current
division. [6]
The overall schematic of the OTA obtained by a
combination of both the above-mentioned technique is
shown in Fig. 5. Here transistors M14 and M15 are biased in
linear region so it acts as source degeneration registers.
Transistors MM1, MM2 acts as current divider with M1and
M2.
Small signal analysis gives the overall gain GM as


g m _ M 1, 2

GM  
M  1g m _ M1, 2
1

g 0 _ M14








Where
M
3.2 Source degeneration
g m _ MM1, 2
g m _ M1
and
g0 _ M14  nCox
WM14
LM14
2 I SS L16
nCoxW16
5. Simulation results
This design is simulated in SPICE tool using 0.25µm
technology model file. The W/L ratios of different
transistors are given below.
Fig 4: Source degeneration technique
Calculated transconductance of circuit is Gm= 9.865 nA/V
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ISSN:2229-6093
Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066
----------------------------------------------------------------Transistor
W/L (micrometer)
M1, M2
MM1, MM2
M3, M17, M18
M4, M11
M5, M6, M7, M8
M10, M12
M15, M16, M17
0.5/120
0.5/1
0.5/60
0.5/50
0.5/300
0.5/150
0.5/1
Table1. Transistors W/L ratios used in OTA
6. Conclusion
A low transconductance OTA is described in this paper
using current division and source degeneration
technique. Circuit is designed with minimum W/L
ratio. This circuit is designed for low frequency
application. SPICE simulation of the circuit confirms
the theoretical conclusions.
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