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BIFET Variable Gain Amplifier

IP.com Disclosure Number: IPCOM000039034D
Original Publication Date: 1987-Apr-01
Included in the Prior Art Database: 2005-Feb-01
Document File: 2 page(s) / 29K

Publishing Venue

IBM

Related People

Richetta, RA: AUTHOR [+2]

Abstract

A BIFET Variable Gain Amplifier (VGA) that works on a single +5-volt power supply is schematically shown in the drawing. The differential gain, Vout/Vin, is approximately 2Rc/Reff, where Reff is the effective resistance between the emitters of Q1 and Q2. Reff is the on resistance Ron of n-channel T1, in parallel with Ree. The VGA action of the circuit is achieved by modulating Reff by changing the current inputted at the Ig input. If the current input at Ig increases, the gate voltage of T1 decreases, and Ron increases, which in turn decreases the differential gain of the VGA. T1 is always in the triode or resistive region of operation because the maximum input swing, and therefore the maximum voltage across T1 is never greater than 300 mV.

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BIFET Variable Gain Amplifier

A BIFET Variable Gain Amplifier (VGA) that works on a single +5-volt power supply is schematically shown in the drawing. The differential gain, Vout/Vin, is approximately 2Rc/Reff, where Reff is the effective resistance between the emitters of Q1 and Q2. Reff is the on resistance Ron of n-channel T1, in parallel with Ree. The VGA action of the circuit is achieved by modulating Reff by changing the current inputted at the Ig input. If the current input at Ig increases, the gate voltage of T1 decreases, and Ron increases, which in turn decreases the differential gain of the VGA. T1 is always in the triode or resistive region of operation because the maximum input swing, and therefore the maximum voltage across T1 is never greater than 300 mV. The voltage at the gate of T1 is proportional to the Ig current and tracks the 5-volt power supply. The common mode level of Vin also tracks the 5-volt power supply so the Vgs of T1 does not vary with power supply and neither will Ron or the gain of the VGA vary. Resistor Ree causes the minimum differential gain of the VGA to be 2Rc/Ree, which occurs when the Ig current is high enough to cause T1 to turn off. The maximum gain occurs if Ig is zero and the gate of T1 goes to the 5-volt power supply, causing Ron to be minimum.

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