Field-Effect Transistor Circuits
May 1967 Electronics World
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.
May 1967 Electronics World
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Field-Effect Transistor CircuitsBy Joseph H. Wujek, Jr. and Max E. McGee
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
General Properties of FET's
Fig. 1. Tabular comparison of tubes, transistors, and FET's.
Fig. 2. A source-follower circuit along with frequency response.
Fig. 3. Common-source FET amplifier circuit along with response.
Fig. 4. Miller oscillator circuit.
Fig. 5. FET/Transistor pair has gain and high input impedance.
Fig. 6. Pulse-stretcher circuit with FET and transistors.
Fig. 7. A linear gating or amplitude modulator circuit.
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.
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
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
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
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.
3 shows a common-source circuit and the bandpass plot obtained by using
either the U-110 or U-112 FET.
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.
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. 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.
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 to ground.
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 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