AN-6604 Datasheet, PDF (1/8 Pages) Fairchild Semiconductor – JFET Circuit Applications

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AN-6604
JFET Circuit Applications
Introduction
JFETs continue to be a popular choice for many analog
applications due to their low price, low noise, and input
current and broad range of operation from DC to UHF.
In Figure 1, the 2N4393 JFET was selected because of its
low I_GSS (<100 pA), very low I_D(OFF) (<100 pA) and low
pinch off voltage. Leakages of this level put the burden of
circuit performance on clean, solder-resin free PCBs, and
low leakage circuit layout.
Figure 3. JFET AC Coupled Integrator
Figure 4 is configured to reduce input capacitance. The
2N5485 is operated as a source follower with bootstrapped
gate bias resistor and drain.
Figure 1. Sample and Hold with Offset Adjustment
Figure 2 is a simple reference circuit that provides a stable
voltage reference almost totally free of supply voltage hash.
Typical power supply rejection exceeds 100 dB.
Figure 4. Ultra-High ZIN Unity Gain Amplifier
Figure 5 is a JFET cascode video amplifier that has very low
input loading and very low feedback. The 2N5485 is used
because of its low capacitance and high Y_fs. Bandwidth of
this amplifier is limited by R_L and load capacitance.
Figure 2. Low-Power Regulator Reference
Figure 3 utilizes the "u-amp" (“mu-amp”) technique to
achieve very high voltage gain. Using C1 in the circuit as a
Miller integrator, or capacitance multiplier, allows this
simple circuit to handle very long time constants.
© 2015 Fairchild Semiconductor Corporation
Rev. 1.0 • 6/19/15
www.fairchildsemi.com