June 1969 Radio-Electronics
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Electronics,
published 1930-1988. All copyrights hereby acknowledged.
The introduction of
transistors (FET's) into the electronics world was a major benefit to designers needing
lower power consumption and perhaps more importantly, high input impedances for active
circuits. The two most fundamentally distinct type of FET's are the metal-oxide-semiconductor
field-effect transistor (MOSFET) and the junction field-effect transistor (JFET). Both FET types are
voltage-controlled devices and do not require a bias current (hence the high input impedance)
like a bipolar junction transistor (BJT)
does. Neither FET type has a PN junction. A JFET uses a high resistance semiconductor
channel region between the source and drain with an ohmic contact to the gate, whereas
the MOSFET has a insulative oxide layer between the gate and the channel. Check out the
links to each transistor types for more theoretical detail. I don't have Part 1 yet,
but hope to be able to buy the May 1969 issue soon.
One of the most immediate uses of JFET's was in multimeters to achieve a high input
impedance that imposes very little impact on the circuit being measured due to the voltage
division effect. My first JFET multimeter was a Microanta (Radio Shack) model 22-208
purchased in 1978. Its $59.95 price in 1978 is equivalent to about $225 in today's money
BBLS Inflation Calculator). Looking at the 1979 Radio Shack catalog
page, compare the input impedance of the el cheapo ($8.95) model 22-027 at 1,000 ohms/volt
with the higher end ($49.95) model 22-207 at 100,000 ohms/volt. That means when a reading
of 1 volt is displayed on the scale, the effective internal resistance of the meter is
1,000 ohms (1 kΩ) and 100,000 ohms (100 kΩ), respectively. If the circuit being
measured (device under test, aka DUT) has an output impedance of, say, 10,000 ohms (10 kΩ),
then the reading is equivalent to what would be indicated if an infinite impedance meter
is used to measure the voltage when a 1 kΩ or 100 kΩ resistor, respectively,
is placed in parallel with the DUT's 10 kΩ. In the former case, the altered
output impedance being measure then becomes 10 kΩ in parallel with 1 kΩ (abbreviated
as 10 kΩ || 1 kΩ), or 909 Ω (a 90.9% reduction). In the later case, the
altered output impedance being measure then becomes 10 kΩ in parallel with 100 kΩ
(10 kΩ || 100 kΩ), or 9.09 kΩ (a 9.09% reduction). The 10 MΩ JFET
meter (10 kΩ || 10 MΩ) creates a reduction of only 0.1%. Proof of the preceding
is left as an exercise for the reader.
JFET's - Part 2
Build these two useful field-effect transistor
projects - Part 2
by Ray Clifton
Last month in the first of two articles we looked at the theory and operating characteristics
of a few junction field-effect transistors (JFET's) costing under $2.50. Now you should
be ready to experiment with and build some practical devices using JFET's.
Design a Mike Preamp
You'll need an assortment of 1/2-watt resistors, a few 25-volt electrolytics and some
coupling capacitors. Choose part values around those shown in Figs. 2 and 3. For ease
of breadboard experimenting, I recommend a few perforated boards with push-in terminals.
A bias box is needed, since bias is dynamically determined. A simple arrangement using
two 9-volt transistor-radio batteries across a 1-megohrn linear-taper pot, with a switch
to save the batteries, is shown in Fig. 1.
You'll also need a power supply capable of 20 volts and not more than 5 mA-batteries
will do nicely. To test and design the circuit, you will need an audio generator. (A
single frequency of between 400 and 2500 Hz is all you need, and output voltage can be
quite low - 50 mV rms.) An oscilloscope is essential to observe waveforms, but need not
have high-frequency response, since you'll be observing only the frequency mentioned
above. A high-impedance voltmeter (vacuum tube or transistor) is essential, and a milliammeter
is nice, but not essential.
Simple perf-board mounting of two 9-volt batteries, switch and 1-megohm
pot provides a variable bias source. The circuit for this setup below.
An easy but useful project is a simple preamplifier for a crystal or ceramic microphone.
Fig. 2 shows the circuit, and here's how component values were selected and the first
Assume a microphone output of about 30 mV rms; this calls for a fairly low-level input
JFET. Among those listed in Table I (last month's issue), the D1102 isn't a bad choice.
It has a low gate-to-source cutoff voltage (VP) with reasonable transadmittance
(yfs) and drain-to-source current (IDSS). The E102 and 2N5033 would
also be good choices. (Even better small-signal JFET's are available, but they cost more
Like vacuum tubes, most JFET's work well with a gate resistor of 470,000 ohms or 1
megohm. Remember that the gate circuit is high-impedance and normally doesn't draw current.
Thus, R1 in the input stage is 1 megohm.
The drain-load resistor is also borrowed from vacuum-tube design. The greater the
drain resistor, the higher the voltage gain (AV) of the stage. But distortion
also increases with greater load resistance, and after a certain point the increase in
AV is negligible. For the D1102 at Q1, 100,000 ohms is a reasonable compromise
Coupling capacitors C1 and C3 determine frequency response and, for 1-megohm gate
resistors, 0.1 μF provides best low frequency response. You can use 0.05 or even 0.01
with little noticeable loss of low frequencies. If you want only speech-quality response,
use 0.005 μF.
For source resistor R3, temporarily tack-solder in about 4700 ohms, and put the bias
box in series (positive lead to source, negative lead to ground). This allows you to
vary bias until you find the optimum value. Source-bypass capacitor C2 can be any value
from about 10 to 50 μF, at 25 or 50 volts. It's simply a brute-force audio bypass
around the source-bias resistor.
Setting Bias Dynamically
Couple 30 mV of rms audio into C1 and the gate circuit. Set the bias box at 5 volts
or more, and tie a power supply to R2, as shown in Fig. 2. The supply should be 20 volts, ±2
volts. Hang a scope across the output. (You don't need a load resistor yet, for the output
is high-impedance and the scope loads the circuit.
Note: Be very careful with JFET's. Use heat-sink pliers on the leads when soldering,
and beware of transients, which can zap an FET in a millisecond. Normally a JFET shows
resistance between drain and source, and vice versa. Between gate and either drain or
source, it shows diode action (high resistance in one direction, low in the other). When
zapped, the JFET loses its diode action from gate to channel, and shows similar resistance
in both directions.
With all systems go, decrease source-to-ground bias below 4 volts and watch the sine
wave coming out of the drain. (Use a fairly decent amplifier and speaker to monitor that
sine wave.) Juggle the bias around and observe the scope.
Rear-mounted components for mike preamp.
This perf-board arrangement lets you measure voltages and waveforms
easily. Since component values vary, parts such as source-resistor R3 can be tack-soldered
until the correct R4 value is determined.
Fig. 2 - Circuit for a JFET mike (microphone) preamp, which operates
with 30 mV input from crystal or ceramic mikes. Overall voltage gain of the circuit is
The JFET's listed in last month's Table I are inexpensive because their parameters
aren't tightly specified. Thus the D1102 you use may have an IDSS of anywhere
from 0.2 to 1.0 mA; the bias you end up using may differ from mine. I found 0.5 volt
a safe compromise, and at this bias value my D1102 drew about 100 μA of drainsource
current. By Ohm's law, 5000 ohms is required to cause a drop of 0.5 volt with 100 μA
flowing. I chose 4700 ohms as the nearest 10% value (R3). Drain current then became 115 μA,
which had no ill effect on the stage operation.
The second stage - also common-source - can be designed the same as the first, but
you must insert a volume control in the gate circuit or you'll overdrive Q2. More than
about 70 mV rms into the gate circuit causes clipping in the output. The pot also allows
you to increase or decrease gain to compensate for different microphones and distance
between talker and mike. Fig. 2 shows values I obtained for the Q2 stage with an MPF105,
which makes a fairly good medium-level amplifier. Other suitable devices are the 2N3819,
2N5163, MPF153 and TIS34.
Output stage 03 is a simple source follower to provide fairly low-impedance output,
which makes coupling to following amplifiers less critical than would a high-impedance
drain follower. I used a 2N4304, but a D1201 would also be suitable. Both have a fairly
high VP of 10 volts, which you want in a source-follower output stage, to
handle wide inputs.
The source follower is designed almost like the preceding common-source stages. The
major difference is that load resistor R9 is in the source circuit, rather than in the
drain. This being the case, and since it's desirable to have lower-impedance output,
R9 is made 47,000 ohms instead of 100,000. This provides less voltage gain, but here
you are working with a few volts, so gain is less important. When determining bias in
this circuit, insert the bias box in series with R8 and R9, starting with about 4700
ohms for source resistor R8. Remember that the gain of a source follower is never greater
By the way, use decoupling capacitor C8 even if you use a battery supply. It prevents
undesirable inter-stage coupling.
A Simple Audio Oscillator
If you do any experimenting with audio circuits, a small oscillator is handy to have
around. The circuit of Fig. 3 is the basic design for a phase-shift oscillator using
JFET's. Design is fairly straightforward, and is built around the first stage. If you
are willing to spend more money, you can buy a JFET with low VP and high yfs
(the 2N4338, for instance, which costs $4.95) and build the oscillator with only two
stages. (In this case, omit Q2 and Q3, and tie C1 directly to the drain of Q1.) Unfortunately,
none of the devices listed in Table I have both low VP and high yfs
and none will sustain oscillation from drain to gate. Therefore two more stages are required
to provide sufficient feedback.
Here's how to design the circuit. Build stage Q1 first, temporarily using the values
shown in Fig. 3 for feedback/phase-shift components C1, C2, C3 and R1, R2 and R3. (Do
not connect R15 yet.) You can use 100,000 ohms for drain load R4 (I used 82,000 simply
because I had run out of 100K's).
Use the method outlined above to determine bias. From an audio generator, feed about
1 volt rms into C1. Then provide drain-supply voltage and clamp a bias box to the source
through any value from 1000 to 4700 ohms. Hang a scope and amplifier on the drain-output
circuit and vary the bias until you get the most gain with the least distortion. Using
Ohm's law, determine the value of source resistor R5. I used 41,000 ohms with the 2N4302.
Other choices for this stage are the D1420 and E102.
The gain of the Q1 stage will be about 30 (too much output) so a pot is needed between
Q1 and Q2. Use a 1-megohm audio-taper pot, to match Q2's input. For Q2 I used an MPF106,
and by trial and error determined that 18,000 ohms was a workable value for the drain
load. Set the bias here as you did before, but don't overdrive the stage. I found about
35 mV rms from the arm of R6 was about all the stage would tolerate. Later, this control
is used to determine the cleanness of the oscillator waveform.
The Q3 stage operates as a split-load, providing one output (from the drain) for feedback
and another output (from the source) for output coupling. I used a D1422, but the 2N4304
and E100 will also work. Try a load resistor of about 47,000 ohms or less to begin with.
Then strap in the bias box and determine the source resistor. If it's less than 47,000
ohms, change R10 to match R11. I ended up with 39,000 ohms in both drain and source circuits.
By the way, you should still be using the external oscillator signal for setting up these
Rear-mounted components for audio oscillator.
To build the JFET audio oscillator, Q1 is mounted and its associated
components temporarily soldered to the push-in terminals. Best bias for Q1 and remaining
stages is determined with the bias box.
Fig. 3 - This JFET audio oscillator produces clean single-frequency
sinewave. The circuit can be made tunable by changing values of C1-C2-C3 and R1-R2-R3.
Now tie the output of Q3 to RI2 - another 1-megohm audio-taper pot. Remove the external
audio oscillator from Q1 and strap in feedback resistor R15. Try 47,000 ohms to start.
You should already have set R6 so it's not overdriving Q2. Hang the scope across R12
and fire up the circuit. If the output waveform isn't clean, change R15. (But always
turn off the power before you do!) Try less resistance, and if the waveform gets more
distorted, you are going the wrong way. Cut and try until you find the optimum value.
Don't use clip leads to try R15; the stray coupling will give you a false indication.
Tack-solder each value of R15 in place, one by one.
Once you get the oscillator itself working with a fairly clean waveform, build Q4.
I used a 2N3819 because it has a high VP and hence can accept a fairly high
gate-input signal. The MPF150 and 2N5163 would also be good choices. Transistor Q4 is
simply an isolation stage, and you can assume 100,000 ohms for drain load. Set the bias
as before, being careful not to overdrive the stage with the setting of R12. I got about
4.5 volts rms out of Q4 with a clean waveform from the oscillator.
Set R6 for purest waveform, and after you build Q4 if you are doubtful of the oscillator,
try another R15. You can vary R12 from zero output up to clipping, and you should find
several volts of output. This output is fairly high-impedance, but if you prefer low
impedance you can make Q4 a source-follower stage. You won't get as much output, but
you'll be able to use a longer output line and match low-impedance inputs.
Oscillator frequency is determined by R1, R2, R3, C1, C2 and C3, and is about 600
Hz for the values I used. The exact frequency is given by the formula f = 1/10.88 RC,
where f is the frequency in hertz, R is the resistance of R1, R2 or R3 (all equal value)
in ohms, and C is the capacitance of C1, C2 or C3 (all equal value) in farads. Note that
this formula holds only if resistors and capacitors are 1% types. I used 10% types and
came up with 600 Hz, rather than the frequency that was originally computed: 920 Hz.
If you build this circuit on a metal chassis, use a single ground-return bus insulated
from the chassis. Tie the bus to chassis at one point only - the junction of R1, R2 and
If you want to make the oscillator variable in frequency, sharpen your pencil and
compute the values of R1-R3 and C1-C3 for the frequencies you want. I suggest using a
three-gang capacitor and switching in various resistors. Use straight-line parts layout
and short leads, and put the circuit in a metal box where it won't be affected by stray
capacitance that could alter the frequency.
General Design Notes
The methods outlined above are, of course, very simple. They are meant to show you
how to use JFET's in construction projects. By experimenting with these devices, you
will become familiar with them. Then you can go on to more sophisticated designing.
Because of the spread in JFET parameters, those who design circuits for production
line use must safeguard against device-to-device variation in yfs, IDSS
and VP , They do this by using lots of feedback and sophisticated bias networks.
They also compute drain loads and bias values more exactly, to allow for temperature
and power-supply variations. After you get the feel of JFET's, you may want to collect
some specification sheets on various devices and read up on more of their characteristics.
Posted September 18, 2018