An N-channel JFET has a low bias current when its gate is
biased negative to the source. However, this requires either
that the gate voltage be biased negative with respect to the source
voltage or that source voltage be biased positive with respect to
the gate voltage. For ac amplifier designs, gate biasing can be made
self-biased by using an RC network at the source to hold the positive
voltage for a period longer than the time period of the input
pulse/frequency source.
In the design described here, the JFET source is constantly
biased. Hence, its frequency response is excellent from dc to 10
MHz. This improvement is due to stable diode biasing. Diode
biasing exhibits only a 2-mV/°C drop in voltage with increasing
temperature from 0°C to 100°C. This is excellent stability.
In this example of a pulsed amplifier design, the signal source has
been biased by a 1-GO resistance to ground, and the JFET source
is biased using a fast diode, a 1N4148, at a source-drain current
of about 5 mA. The gate is maintained at ground potential so that
no bias voltage is injected into the input dc circuit. This scheme
makes the gate remain at zero voltage unless a signal to be amplified
changes it.
This technique is illustrated in the figure, where the input
impedance of the circuit is maintained at 1 GO, unless it’s shunted
by the impedance of the signal source. The JFET’s output current
is kept significantly high—at 5 mA—to let the JFET drain voltage
remain at its mid-operating point, about 5 V. This enables the
JFET to work for both positive and negative input signals.
In this circuit, a J309 or IFN152 JFET is the most suitable
device, with an input bias current of about 100 pA. The amplifier
delivers a good signal response to about 10 MHz. A 2N4117A
JFET would reduce the input bias current but also reduce the highfrequency
response considerably.
This circuit makes an excellent input-stage amplifier for oscilloscopes,
where input impedance isn’t greater than 1 MO. The
amplifier improves the input stage to 1 GO. The circuit is most
suitable for piezoelectric detectors, photodiode-based radiation
detectors, and low-capacitance sensors with a very small charge in
the range of 10 fC to 1 pC.