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OPERATIONAL AMPLIFIER WITH FAST SATURATION RECOVERY

IP.com Disclosure Number: IPCOM000005547D
Original Publication Date: 1984-Apr-01
Included in the Prior Art Database: 2001-Oct-15
Document File: 1 page(s) / 58K

Publishing Venue

Motorola

Related People

M. Guillien: AUTHOR [+2]

Abstract

The capacitor used in most operational amplifiers for frequency compensation brings usually an unnecessary delay to the recovery of linear operation from a saturation ouput state. This increases the total harmonic distortion for a sine wave or causes an error in the duty cycle for a square wave.

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(& MOTOROLA Technical Developments Volume4 April 1984

OPERATIONAL AMPLIFIER WITH FAST SATURATION RECOVERY

by M. Guillien and C. Vanhecke

   The capacitor used in most operational amplifiers for frequency compensation brings usually an unnecessary delay to the recovery of linear operation from a saturation ouput state. This increases the total harmonic distortion for a sine wave or causes an error in the duty cycle for a square wave.

   The purpose of this paper is to show a very simple way to overcome the aforementioned drawback. Let's look at a conventional operational amplifier. In a typical operational amplifier, when the input differential voltage forces the output to its maximum possible value, say Vcc, Q2 will saturate, thus switching 04 and 05 off. Hence the voltage at node A will drop to almost zero volts (OV).

   At that point, the voltage across the capacitor is Vcc. To recover linear operation, e.g. before the output voltage can decrease, the voltage at node A must be 2VBE. The capacitor voltage is then Vcc .2VBE. The charge difference is given by the current loin a time delay.

T = CX2VBE

10

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The figure shows a way to dramatically decrease that undesired delay. The principal is to let node A voltage decrease only by the necessary amount for the output voltage swing to be maintained.

The voltage across diode connected transistor QO equals VBE of Q3 + VBE of 01 VBE of QS.

When the operational amplifier is in the linear mode or if the output is due to be low, the...