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Gain and Noise Optimization Method for FET Input Amplifiers

IP.com Disclosure Number: IPCOM000110118D
Original Publication Date: 1992-Oct-01
Included in the Prior Art Database: 2005-Mar-25
Document File: 3 page(s) / 92K

Publishing Venue

IBM

Related People

Christopher, RJ: AUTHOR [+2]

Abstract

A method is disclosed that permits optimization of a class of cascode amplifiers for both voltage gain and, simultaneously, input sensitivity. The class of cascode amplifiers addressed are those using a field-effect transistor (FET) as the input device and a bipolar transistor as the upper device. The optimization is accomplished with minimal effect on amplifier characteristics, such as bandwidth, input or output impedance, and stability.

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Gain and Noise Optimization Method for FET Input Amplifiers

       A method is disclosed that permits optimization of a
class of cascode amplifiers for both voltage gain and,
simultaneously, input sensitivity.  The class of cascode amplifiers
addressed are those using a field-effect transistor (FET) as the
input device and a bipolar transistor as the upper device.  The
optimization is accomplished with minimal effect on amplifier
characteristics, such as bandwidth, input or output impedance, and
stability.

      The voltage gain of an FET device (as well as the cascode
circuit discussed here) is the product of the FET transconductance,
gm, and the load resistance, RL.  The transconductance of the FET
varies as the square root of the quiescent drain current, while the
channel noise of the FET varies inversely with the fourth root of the
quiescent drain current.

      The primary disadvantage in using an FET as the input device in
a cascode circuit is the inherently low transconductance and
increased device noise experienced when operated at moderate or low
drain-to-source bias currents.  The noise characteristics of the FET
device will normally dominate the noise characteristics of the
preamplifier.  Thus, the sensitivity of the preamplifier is
ultimately determined by the channel noise of the input FET device.

      Due to the nature of the dependence of transconductance and
channel noise on quiescent drain current, one typically trades input
sensitivity for voltage gain.  The sensitivity of the preamplifier
can be improved by increasing the quiescent drain current through the
FET.  However, increasing the drain-to-source bias current through
the FET, without other changes to the circuit, typically forces a
reduction in the value of the load resistor to maintain a constant
output operati...