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.

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.

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

)(0 NPm VVgI

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:

bias

mqI

KTg

2

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

Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066

IJCTA | MAY-JUNE 2012 Available online@www.ijcta.com

1064

ISSN:2229-6093

externally by the bias current (Ibias). It adds new dimension

to design and applications of OTA circuit.

3. Techniques for decreasing transconductance

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.

Fig 3: Current division technique

The effective GM is given by

1_

M

gG CMm

M

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]

3.2 Source degeneration

Fig 4: Source degeneration technique

Fig. 4 shows the source degeneration technique. Here

the effective GM is given by

RggG MmMmM 2,12,1 __ 1

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 []

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

14

2,1

2,1

_0

_

_

11

M

Mm

Mm

M

g

gM

gG

Where

1

2,1

_

_

Mm

MMm

g

gM

and

16

16_0

2

14

14

14 WCn

LI

L

WCng

ox

SS

M

M

oxM

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.

Calculated transconductance of circuit is Gm= 9.865 nA/V

Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066

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1065

ISSN:2229-6093

-----------------------------------------------------------------

Transistor W/L (micrometer)

M1, M2 0.5/120 MM1, MM2 0.5/1 M3, M17, M18 0.5/60 M4, M11 0.5/50 M5, M6, M7, M8 0.5/300 M10, M12 0.5/150 M15, M16, M17 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.

REFERENCES

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―Built-in-self-test for analog to digital converters,‖

U.S. Patent 5 132 685, Aug. 9, 1991.

[3] P. Kinget and M. Steyaert, ―Full analog CMOS

integration of very large time constants for synaptic

transfer in neural networks,‖ Analog Integr. Circuits

Signal Proces., vol. 2, pp. 281–295, 1992.

[4] R. L. Geiger and E. Sánchez-Sinencio, ―Active filter

design using operational transconductance

amplifiers—a tutorial,‖ IEEE Circuits Devices Mag.,

no. 1, pp. 20–32, 1985.

[5] W. H. G. Deguelle, ―Limitations on the integration of

analog filters below 10 Hz,‖ in Proc. IEEE

ESSCIRC’88, 1988, pp. 131–134.

[6] P. Grade, ―Transconductance cancellation for

operational amplifiers,‖ IEEE J. Solid-State Circuits,

vol. SC-12, pp. 310–311, June 1977

[7] A.Veeravalli, E.Sánchez-Sinencio, J.Silva-Martínez,

―Transconductance amplifiers with very small

transconductances: A comparative design approach",

IEEE JSSC, vol.37, nº.6, pp.770-775, Jun.2002.

Abhay Pratap Singh et al ,Int.J.Computer Technology & Applications,Vol 3 (3), 1064-1066

IJCTA | MAY-JUNE 2012 Available online@www.ijcta.com

1066

ISSN:2229-6093