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A MEANS FOR DIGITAL GAIN CONTROL OF ANALOG SIGNALS

IP.com Disclosure Number: IPCOM000005563D
Original Publication Date: 1985-Oct-01
Included in the Prior Art Database: 2001-Oct-16
Document File: 2 page(s) / 81K

Publishing Venue

Motorola

Related People

W. David Pace: AUTHOR

Abstract

Digital or switched gain control of amplifier circuits is frequently required in the design of telephones and other audio products. Modifying the gain of an op amp circuit by digitally selecting various feedback resistors produces accurate gain values and simple gain control. In MOS integrated circuit (IC) technologies the MOS transistor can operate as an analog switch to perform this function. Unfortunately, bipolar IC technologies do not offer such a simple solution. When a bipolar transistor is used as an analog switch, an offset voltage exists between the collec- tor and emitter terminals and the switched base current flows in the signal path.

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@ MOTOROLA

Technical Developments October 1985

A MEANS FOR DIGITAL GAIN CONTROL OF ANALOG SIGNALS

by W. David Pace

   Digital or switched gain control of amplifier circuits is frequently required in the design of telephones and other audio products. Modifying the gain of an op amp circuit by digitally selecting various feedback resistors produces accurate gain values and simple gain control. In MOS integrated circuit (IC) technologies the MOS transistor can operate as an analog switch to perform this function. Unfortunately, bipolar IC technologies do not offer such a simple solution. When a bipolar transistor is used as an analog switch, an offset voltage exists between the collec- tor and emitter terminals and the switched base current flows in the signal path.

   A new circuit illustrated in Figure 1 will be described that utilizes matched bipolar transistors and constant current sources to implement an analog switch for gain control. When gain reduction is desired current sources Ie, and II are activated to saturate transistor Q,. Transistor Q, is biased in the saturation region to provide a satura- tion V,, voltage to offset that of Cl,. As a result, when the base and emitter current sources are activated, feed- back resistor R, is connected from the output of the op amp to the input through two saturated transistors. If the V,, saturation voltages of Q, and Q, are closely matched, there is no dc shift in the output voltage when the gain is switched. Additional switched current sources and transistors may be added for further gain reduction.

   One application...