If you are just entering the field of electronics, the concepts
presented in this 45-year-old article for basic field effect
transistors are still relevant. Significant improvements has
been made since then, but the fundamentals stand. One of the
most useful items in this article is Table 1, which compares
and contrasts vacuum tubes, bipolar junction transistors, and
field effect transistors.
Field-Effect Transistor Circuits
By Joseph H. Wujek, Jr. and Max E. McGee
A grouping of six simple, low-cost circuits that
illustrate many of the principles of FET operation.
One of the most important new semiconductor devices is the
field-effect transistor (FET). This article describes six low-cost
circuits which may be built to demonstrate the important properties
of FET's. The U-110 and/or U-112 p-channel FET's are used in
the circuits discussed and are relatively low priced. Siliconix
has offered the U-110 and U-112 together as a package for $2.75.
The U-110 may be had alone for $1.00 under this offer. The industrial-type
FET's, U-146 and U-147, are slightly higher in price. The bipolar
transistors used are General Electric epoxy devices which sell
for $0.50 to $1.00 each.
General Properties of FET's
the similarities among vacuum tubes, transistors, and FET's
are given in Fig. 1. We must recognize the inherent differences
which exist among vacuum tubes, transistors, and FET's and the
table serves only as an aid in pointing out bias polarities.
Fig. 1. Tabular comparison of tubes, transistors,
The FET resembles the vacuum tube in that the impedance looking
into the gate is very high and can be on the order of hundreds
of megohms. Also, the FET is a low-noise device, better than
bipolar transistors and competitive with vacuum tubes. On the
other hand, FET's resemble transistors in the leakage currents
which flow between their electrodes when the device is cut-off.
The Source Follower
The source-follower circuit is analogous to the vacuum-tube
cathode-follower or transistor emitter-follower. We might expect
similar behavior from these circuits and such is the case. We
thus have high input impedance, relatively low output impedance,
and a voltage gain that can be made very close to unity.
Fig. 2 shows a simple source-follower circuit and the bandpass
characteristics obtained with two different FET devices. The
2-megohm resistor establishes the gate bias and is similar to
the grid-leak resistor used in tube work. However, this resistor
must be made small enough so that increased leakage current
between the gate and source will not drastically change the
bias. For the U-110 and the U-112, leakage between gate and
source at room temperature is on the order of 5 nanoamps (5
x 10-9 amp), so a 1or 2-megohm resistor is adequate.
Fig. 2. A source-follower circuit along with
At elevated temperatures the increase in leakage current
would dictate that a smaller resistor be used so as to reduce
changes in bias with leakage current. It is possible to bias
FET's so that very small temperature drift results.
The common-source circuit is analogous to the common-emitter
transistor and common-cathode vacuum-tube circuits. Again, properties
of this circuit are similar to the transistor and tube counterparts.
Input and output impedances are intermediate in value and a
voltage gain greater than unity may be realized.
Fig. 3 shows a common-source circuit and the bandpass plot
obtained by using either the U-110 or U-112 FET.
Fig. 3. Common-source FET amplifier circuit
along with response.
The very high input impedance of the FET enables us to build
the simple Miller oscillator of Fig. 4. The high impedance of
the gate circuit results in light loading of the crystal. The
LC combination in the drain circuit is tuned to resonate slightly
below the parallel resonance of the crystal. For the type of
devices considered in this article, the upper limit of frequency
operation is only a few megahertz. For crystals other than the
512-kHz unit shown, the LC combination must be changed accordingly.
Fig. 4. Miller oscillator circuit.
The output of the oscillator will not tolerate much loading,
but the source-follower circuit can be used as a driver to provide
low output impedance without loading the oscillator stage excessively.
With differences in FET types and layout details, some modification
of the LC network may also be required. For the circuit we tested,
"clean" oscillations were observed for the four FET types indicated
on the figure without retuning the circuit, and with the supply
voltage varying from 6 to 22 volts.
A circuit which performs like an improved source-follower
or emitter-follower is shown in Fig. 5. The FET again provides
very high input impedance, while the transistor output provides
low output impedance. Unlike the source-follower or emitter-follower,
this circuit can be built to have a voltage gain greater than
unity. This is accomplished by a resistor in the feedback path
as shown in Fig. 5A (lower right).
Fig. 5B gives the bandpass characteristics when used with
a voltage gain of unity and with voltage gain greater than unity.
The bandwidth is dependent upon the impedance of the driving
source. When driven by a 600-ohm test oscillator, the upper
3-dB point is 2 MHz. Bandwidth decreases as the driving source
impedance increases. At low frequencies the amplifier input
impedance is about 100 megohms and the output impedance is less
than 2000 ohms.
Fig. 5. FET/Transistor pair has gain and
high input impedance.
Fig. 6 shows a stretcher which senses the peak amplitude
of a pulse and holds this voltage level for a time much longer
than the width of the pulse. The diagram includes a push-button
to provide the pulse, but of course the pulse could be coupled
in from a suitable external source.
Fig. 6. Pulse-stretcher circuit with FET
Transistors Q1 and Q3 provide impedance transformation and
isolate the FET from both the source and the load. When the
input pulse appears, the capacitor is charged through Q1 and
the diode. After the input pulse terminates, Q1 is cut-off and
the diode is back-biased. The input impedance of Q2 is very
high so that the charge leaks off the capacitor mainly by leakage
current through the diode and the capacitor. The FET (Q2) then
presents the d.c. level to Q3 which acts as an output driver.
Fig. 6 also gives the duration of the output obtained with four
different FET's. (Note that the FET is connected in reverse
in order to make the drain negative.)
The time constant can be increased by using an FET having
a very low gate leakage and by selecting a diode and a capacitor
with very low leakage. By using these more expensive components,
FET stretcher circuits with output pulse times as long as 30
hours have been built. The circuit can be used as a peak-amplitude
detector or to obtain a required time delay. Reset is accomplished
by either allowing the output to decay or by shorting the capacitor
The FET can also be used as a linear gate or electronic switch
as shown in Fig. 7. The resistance between source and drain
with the switch "closed" is approximately 1/gm. With
the switch "open", only a small leakage current flows between
source and drain. This type of circuit can also be used as an
Fig. 7. A linear gating or amplitude modulator
We have presented six simple, low-cost circuits that illustrate
many of the principles of FET operation. These circuits are
designed to furnish an understanding of the devices and to stimulate
thinking toward other applications.
The authors wish to acknowledge the cooperation of Mr. Charles
MacDonald of Siliconix, Inc. and Mr. Al Kenrick of General Electric
May 1967 Electronics World
of Contents] People old and young enjoy waxing nostalgic about
and learning some of the history of early electronics. Electronics World
was published from May 1959 through December 1971. All copyrights are hereby acknowledged.
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