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CMOS VARIABLE GAIN AMPLIFIER WITH AN APPROXIMATED EXPONENTIAL GAIN CONTROL

IP.com Disclosure Number: IPCOM000015860D
Original Publication Date: 2002-Jun-13
Included in the Prior Art Database: 2003-Jun-21
Document File: 9 page(s) / 44K

Publishing Venue

IBM

Abstract

AM9-97-020 CMOS VARIABLE GAIN AMPLIFIER WITH AN APPROXIMATED EXPONENTIAL GAIN CONTROL Variable gain amplifiers are widely used in communication, multimedia and storage systems for providing a signal-processing gain control in their front-end circuits. They are also essential components in circuits that include an analog-to-digital conversion where an optimized operational region is required. A typical variable gain amplifier includes an amplitude modulation amplifier, a control voltage conversion circuit with signal interconnection for providing a variable gain amplification function. FIG. 1 is a simple variable gain amplifier for generating a gain controlled amplification of a differential high frequency input signal Vin. The analog signal Vin with signal terminals Vin+ and Vin- is supplied to the amplifier with an amplitude modulation function to produce a differential output signal Vout with signal lines Vout+ and Vout-. The voltage amplification or attenuation gain G, defined as G Vout Vin, is controlled or modulated by a desired gain control signal Va generated by a voltage conversion circuit. The conversion circuit translates the control voltage Vc which is provided with respect to a reference voltage Vr, and forms an interfacing signal Va. By defining Vr as a ground reference, the overall gain G of the amplifier is a function of Vc.

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  CMOS VARIABLE GAIN AMPLIFIER WITH AN APPROXIMATED EXPONENTIAL GAIN CONTROL

AM9-97-020

CMOS VARIABLE GAIN AMPLIFIER WITH AN APPROXIMATED EXPONENTIAL
GAIN CONTROL

  Variable gain amplifiers are widely used in communication,
multimedia and storage systems for providing a
signal-processing gain control in their front-end circuits.
They are also essential components in circuits that include an
analog-to-digital conversion where an optimized operational
region is required. A typical variable gain amplifier
includes an amplitude modulation amplifier, a control voltage
conversion circuit with signal interconnection for providing a
variable gain amplification function.

  FIG. 1 is a simple variable gain amplifier for generating a
gain controlled amplification of a differential high frequency
input signal Vin. The analog signal Vin with signal terminals
Vin+ and Vin- is supplied to the amplifier with an amplitude
modulation function to produce a differential output signal
Vout with signal lines Vout+ and Vout-. The voltage
amplification or attenuation gain G, defined as G = Vout /
Vin, is controlled or modulated by a desired gain control
signal Va generated by a voltage conversion circuit. The
conversion circuit translates the control voltage Vc which is
provided with respect to a reference voltage Vr, and forms an
interfacing signal Va. By defining Vr as a ground reference,
the overall gain G of the amplifier is a function of Vc.

In an automatic gain controlled (AGC) circuit with a closed

1

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loop implementation, the gain control function is performed by
varying Vc such that a constant amplitude Vout can be
controlled when the signal amplitude of Vin is changing within
a defined range and loop bandwidth. For example, the
amplifier is able to maintain a constant amplitude of 100 mV
for Vout when Vin is changing from 10 mV to 300 mV and G is
controlled by the function values of Vc in a closed operation.
For a general description of variable gain amplifiers, please
refer to Ref. 1.

  A very desirable feature of variable gain amplifiers is to
provide an exponential gain control operation so that the
amplitude control of output signal by the control voltage in a
closed loop application is independent in the amplitude
changes of the input signal. Quantitatively, an exponential
gain control function can be expressed asF=Vout/Vin=A*
exp( B * Vc), with A and B as constants. The output signal is
then produced as Vout=A*Vi* exp( B * Vc). The change of
Vout with respect to the change of the control voltage Vc is
independent of the levels of Vin, because(MVout/MVc)=
(A*Vin*exp(B*Vc))*B= Vout * B, which is not a
function of Vin.

  Current variable gain amplifiers that are implemented in
bipolar technology use the natural exponential function of the
base-emitter characteristic of the bipolar junction devices.
However, as system hardware designs move to the all-MOS/FET
integration for higher density and lower power consumption,
this natural exponential function is no longer available.

This is beca...