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A PUSH-PULL TRANSCONDUCTANCE AMPLIFIER WITH EXTENDED POWER SUPPLY AND COMMON MODE RANGE

IP.com Disclosure Number: IPCOM000005796D
Original Publication Date: 1989-Aug-01
Included in the Prior Art Database: 2001-Nov-07
Document File: 2 page(s) / 112K

Publishing Venue

Motorola

Related People

Matthew A. Rybicki: AUTHOR

Abstract

A fully differential class - A transconductance amplifier that maintains push-pull operation at its output over an increased common mode input range is described. Its improved slew rate, due to its push-pull nature, is ideal for driving heavy capacitive loads. This amplifier maintains its push-pull output feature over a wider power supply voltage range than other class-A push-pull architectures. Its gain and bandwidth are comparable to that of a single stage class-A folded cascade amplifier built in the same process and in the same area.

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@ MOZOROLA

Technical Developments Volume 9 August 1989

A PUSH-PULL TRANSCONDUCTANCE AMPLIFIER WITH EXTENDED POWER SUPPLY AND COMMON MODE RANGE

by Matthew A. Rybicki

ABSTRACT

   A fully differential class - A transconductance amplifier that maintains push-pull operation at its output over an increased common mode input range is described. Its improved slew rate, due to its push-pull nature, is ideal for driving heavy capacitive loads. This amplifier maintains its push-pull output feature over a wider power supply voltage range than other class-A push-pull architectures. Its gain and bandwidth are comparable to that of a single stage class-A folded cascade amplifier built in the same process and in the same area.

INTRODUCTION

   As more analog and digital circuitry arecombined on one chip, the analog circuitry is being forced to operate at the lower 5V f 10% digital supplies (soon maybe 3.3V + 10%). This low supply operation reduces the com- mon mode input voltage range as well as the dynamic range at the outputs of the operational amplifiers. With the wide spread use of switched capacitor circuits, a push-pull transconductance amplifer would offer improved slewing and thus faster settling times. The amplifier discussed below offers a solution to all of the above men- tioned problems.

AMPLIFIER DESIGN

   The amplifier presented in the attached figure is a two stage push-pull transconductance amplifier with an improved common mode input range. The first stage or input stage is a differential transconductance stage made up of the current source to Vdd and the two p-channels Ml and M2. The first stage produces a difference current out of the drains of Ml and M2 equal to the product of, the difference between VINN and VINP (vin) and the transconductance (gml) of this input pair (ie: vin gml). This difference current is then passed to the second stage.

   The second stage accepts the difference current (vin gml) from the input stage and "folds" them up through n-channel gate transistors M3 and M4 to current mirror pairs M51M7 and M61M8. This difference current is mir- rored and then sourced through common gate devices M9 and Ml0 which act to increase the amplifier's output impedance. As the difference currents are pushed through the small signal source impedances of devices M3 and M4 (rs3 = l/gm3), a voltage is generated that is equal to vin gml llgm3. Since devices Ml3 and Ml4 have a transconductance that is equal to gm3, then the drain currents in devices Ml...