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OPERATIONAL TRANSCONDUCTANCE

AMPLIFIER

Under the Guidance of: Mr. Manish Kumar

Submitted by:

ANUPAM SHARMA 8102151

GARIMA BHATIA 8102160

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OPERATIONAL TRANSCONDUCTANCE AMPLIFIER:

An OTA is a voltage controlled current source, more specifically the term “operational” comes

from the fact that it takes the diff erence of two voltages as the input for the current conversion.

The ideal transfer characteristic is therefore

IOut = gm(VIn+ − VIn−)

Fig1: Circuit Diagram of OTA

In the circuit, following observations were made-

i)  There are three current mirrors in the circuit- M1 and M3, M5 and M7, and M4 and

M8. This implies that for perfectly matched MOSFETs current flowing through M3

will be equal to the current flowing through M1, and likewise.

ii)  The drain current through M2 and M6 will be decided by their respective Input (Gate)

Voltages. If both Vg are same, the same drain current will flow through M2 and M6.

iii)  The same drain current will flow through M3 and M4, which mirrors the current

through M1 and M2. Similarly, M7 and M8 will mirror drain current through M5 and

M6.

iv)  Since M4 and M8 act as a current mirror, M4 will force the Drain Current in M8 to be

the same as that through M4. But, M8 also has the same amount of current flowing

through it as through M5. Because, a single branch cannot have two different currents

flowing through it, the difference in the currents will be observed at the output.

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Applying KCL at the output node:

Iout=Id2-Id1

Where Id2=Current flowing through M6

Id1=Current flowing through M2

Steps:

i)  A common-source amplifier using current based MOSFET model [1] for given GBW

and CL was designed.

ii)  Resistance used in the amplifier was replaced with a PMOS load.

iii)  Current mirrors were designed.

iv)  OTA was designed.

References:

[1] R. L. Oliveira Pinto, A. I. A. Cunha, M. C. Schneider and C. Galup- Montoro, “An Amplifier

Design Methodology Derived From A Mosfet Current-Based Model”.

[2] A. I. A. Cunha,O.C. Gouveia-Filholl, M. C. Schneider and C. Galup-Montoro, “A Current-Based

Model For The Mos Transistor” 

[3] Behzad Razavi, Design of Analog CMOS Integrated Circuits.

[4] Sung-Mo (Steve) Kang and Yusuf Leblebici, CMOS Digital Integrated Circuits Analysis and

Design.