AN-6601 Datasheet, PDF (1/4 Pages) Fairchild Semiconductor – Low Noise JFET Amplifiers

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AN-6601
Low Noise JFET Amplifiers
Introduction
Discrete JFETs reign supreme as low noise amplifiers.
JFETs are virtually free from the problems of current noise,
popcorn noise and limited bandwidth which plague bipolar
transistors and bipolar input op amps.
Unfortunately, JFETs are awkward to use because of low
gain and the need of extensive biasing networks. However,
monolithic op amps are cheap and easy to use but suffer
from poor noise performance. By combining JFETs with an
op amp yields single and differential input amplifiers that
have the best of both worlds; low noise, high gain and ease
of use.
Figure 2. Discrete JFET and Op Amp Noise
Comparison
The main problem with JFETs is that the voltage gain is
limited by the size of the load resistance which is limited by
the power supply voltage and the FET operating current.
The voltage gain can be increased by combining the JFET (a
transconductance amplifier) with an op amp current to
voltage (I/V) amplifier, circumventing the limited load
resistor.
Figure 1. Bipolar and JFET Transistor Noise
Comparison
© 2014 Fairchild Semiconductor Corporation
Rev. 1.0 • 7/16/15
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