May 1958 Radio-Electronics
Wax nostalgic about and learn from the history of early electronics. See articles
published 1929 - 1948. All copyrights hereby acknowledged.
You and I know them as 'varactor diodes,' but originally the semiconductor
junctions whose reverse bias determines its capacitance was called
the 'Varicap.' The new and wondrous semiconductor craze was in full
swing by 1958. Scientists, engineers, and hobbyists were burning
the midnight oil (to use a popular phrase
of the day) performing experiments and designing circuits
to replace vacuum tubes and manual controls with transistors and
other electrically variable semiconductors. The Varicap had the
ability to tune receiver and transmitter oscillators and filters
without the need for high tube bias voltages and large mechanically
variable multi-plate capacitors. This article from Radio-Electronics
says early Varicaps cost $4.50 apiece ($36.91 in 2014 dollars per
BLS Inflation Calculator), so they were not cheap by any account.
However, their cost was justified by reducing circuit complication (a
mechanically variable capacitor and possibly a vacuum tube),
improving reliability (no moving parts to
wear out, vibration immunity, resistance to environmental factors),
and the prestige of claiming to be a modern 'electronically tuned'
Using the Varicap
By Rufus P. Turner
The capacitance of this startling little semiconductor,
the size of a 1/4-watt resistor, varies with he voltage
applied to it.
The capacitance of a reverse-biased semiconductor junction varies
with the reverse dc voltage-capacitance decreases as voltage is
increased. This effect has been noted in both diodes and power rectifiers.
In selenium rectifier plates, for example, the capacitance is comparatively
large, often reaching 0.25 μ.f or more. Attempts to use this
voltage-sensitive capacitance have been thwarted by the comparatively
low reverse resistance of the junction - rather high currents are
needed to effect the capacitance changes, and the Q is too low for
most practical applications. Also, both the capacitance and reverse
resistance are extremely temperature-sensitive in ordinary diodes
An important milestone was reached with development of the silicon
junction diode. This semiconductor device has capacitance in small
but useful amounts readily varied by the reverse bias. And the reverse
resistance of the silicon p-n junction is so high (often 10,000
megohms at -1 volt) that almost no current at all is required to
do the job. Essentially a high-Q component, the silicon junction
is noted for the stability of its capacitance over a wide temperature
range. In research laboratories during the past 2 years, the voltage-sensitive
capacitance of the silicon p-n junction has been used in experimental
voltage-operated tuning, frequency modulators, automatic frequency
control, capacitor type amplifiers, tunable filters and numerous
sensitive remote-control devices. Workers who had been intrigued
by the earlier dielectric amplifier (using voltage-sensitive ceramic
capacitors), only to be frustrated by their severe temperature drift,
have had their interest re-stimulated by the silicon junction.
Now, a useful new semiconductor component, the Varicap (see Radio-Electronics,
January, 1958, page 45) has become commercially available. This
simple, two-terminal, p-n junction device, designed for use as a
voltage-variable capacitor, opens new vistas for the simplification
of many electronic circuits. The number of applications to old circuits
and the possibilities for new circuits will be limited only by the
experimenter's imagination and ingenuity. No larger than most 1/4-watt
resistors and resembling a miniature crystal diode, the Varicap
will do the work of a reactance modulator tube or of a variable
capacitor, both of which are many times its size.
A miniature tuning capacitor and reactance tube dwarf
Varicap which may replace them.
AFC circuit which you can add to FM receiver fits on
2 1/8 x 2 3/8-inch Phenolic board.
Fig. 1 - Schematic symbol and equivalent circuit of the
Fig. 2 - How Varicap capacitance varies with bias voltage.
Electrical characteristics of the Varicap
Fig. 3 - Test setup to demonstrate Varicap performance.
Fig. 1 shows the schematic symbol and equivalent circuit of the
Varicap. The markings in Fig. 1-a indicate the dc bias voltage polarity.
The positive end of the unit is marked with a painted black band.
Fig. 1-b shows the equivalent circuit. Capacitance C varies approximately
as 1/√V, where V is the reverse bias voltage, and is practically
constant (for any given value of V) from -65°C to 150°C.
Both capacitance and the series resistance Rs) are substantially
independent of the operating frequency. The maximum frequency at
which the equivalent circuit remains as shown in Fig. 1-b is 500
Varicaps are available in six capacitances, as shown in the chart.
These capacitances are obtained with a dc bias of -4 volts. Capacitance
tolerance is ± 20%. A Varicap costs about $4.50.
Fig. 2 shows the capacitance variation with reverse dc bias voltage.
This curve applies to all types of Varicaps, regardless of their
nominal capacitance, and shows that each has 100% of the rated capacitance
when the bias is -4 volts. Only a few millimicroamperes of current
flow when bias is applied to the Varicap. Thus, virtually no power
is required to vary the capacitance of this very-high-resistance
Since the bias signal may be either steady or fluctuating, a
variety of control signals may be employed. The frequency range
extends from dc to more than 500 mc.
In any application, the total voltage applied to the Varicap
(that is, the dc bias voltage plus the signal-voltage peak when
there is an alternating component) must not exceed the unit's maximum
operating voltage. Also, since the Varicap is a diode operated in
the reverse direction, the dc bias voltage should not be set so
low that the signal-voltage peak will swing operation into the forward,
or conducting, region.
The Varicap actually utilizes the capacitance of the p-n junction
to do its job. Why capacitance exists in the junction won't be rehashed
here. Junction capacitance in semiconductor devices is familiar
to the reader; collector capacitance, for example, is well known
for its role in limiting the high-frequency response of transistors.1,2
In a conventional capacitor, a small leakage current flows through
the dielectric because it is not a perfect insulator. The higher
the insulation resistance, the lower this current. In a mica capacitor
in good condition, dielectric resistance may be 100,000 megohms
or more, and the leakage current at low dc voltages so tiny that
it can be ignored completely. In a tubular paper capacitor, the
dielectric resistance may be as low as 1,000 megohms; therefore
leakage current is much higher than in a mica unit. The leakage
current is highest of all in an electrolytic capacitor; it may be
an appreciable part of a milliampere. Because of leakage current,
the equivalent circuit of a capacitor shows a leakage resistance
in parallel with the capacitor plates.
The leakage resistance is so high in a mica capacitor that its
shunting effect is negligible. The dielectric between the plates
approaching a perfect insulator, there is (to all practical intents
and purposes) no appreciable leakage path between the plates, and
the parallel resistance may be erased from the equivalent circuit.
Similarly, in the Varicap, the leakage resistance is extremely high
(in the order of tens of thousands of megohms) since the reverse-biased
silicon p-n junction passes only a few thousandths of a microampere.
As in the mica capacitor, the parallel resistance may be ignored
and the junction considered a capacitance, since its reactance
is many orders of magnitude lower than the shunting resistance.
The situation is much the same as having a very good dielectric
between the "plates" of the junction. This capacitance varies, as
explained earlier, with the impressed dc reverse voltage.
A conventional capacitor also has a series resistance component
Rs. At high frequencies, the magnitude of this resistance
is due to the resistance of plates, leads and various in-phase components
of current. The Q of the capacitor is affected by this series resistance.
The Varicap also has an Rs component. It is shown in
Fig. 1-b and is specified for each type in the chart. The Q of the
Varicap (but not its capacitance) similarly depends upon this series
component. As mentioned earlier, however, this series resistance
component is independent of frequency up to 500 mc.
It is well to reflect that other semiconductor junctions, such
as germanium diodes and selenium rectifiers, pass much higher reverse
(leakage) currents. These leakages are not only higher than those
of high-quality silicon junctions but increase markedly with increased
reverse voltage. In these units, because the leakage resistance
is often of the same order of magnitude or even lower than the capacitive
reactance, the useful change produced by changing the voltage on
these devices is not exclusively a capacitance change but rather
a change in the impedance of the equivalent R-C circuit. In this
respect, the selenium rectifier somewhat resembles the electrolytic
capacitor with its high leakage. Quite to the contrary, the extremely
high leakage resistance of the silicon p-n junction and its useful
capacitance identify it as a high-quality capacitor.
One of the first applications that comes to mind is use of the
Varicap as a voltage-variable tuning capacitor in an L-C circuit.
Fig. 3 shows the author's test setup to demonstrate this effect
and to check the tuning range for one set of operating conditions.
In this arrangement, C2 is a type V56 Varicap which serves as
the tuning capacitor of the L-C circuit, L2-C2. Capacitor C1 blocks
direct current flow from the coil. This capacitance is very large
with respect to C2. Adjustable dc bias is supplied by a battery
through potentiometer R2. The bias level is indicated by the dc
voltmeter. Isolating resistor R1 blocks rf flow into the dc circuit
but introduces no appreciable dc voltage drop because of the negligible
direct current flowing through the Varicap. An rf choke can be used
in place of R1. The rf vtvm acts as a high-impedance resonance indicator.
The test signal is supplied by a conventional rf signal generator
link-coupled to the L-C circuit through coil L1. Coil L2 has been
wound for resonance with C2 in the vicinity of 2 mc.
Near zero dc voltage, the Varicap has its highest capacitance
(nominally greater than 100 μμf) and the L-C circuit therefore
is tuned to its lowest frequency. At -9 volts, the capacitance is
low (approximately 39 μμf) and the circuit is tuned to its
highest frequency. To keep within operating ratings of the Varicap,
the dc voltage should not be set to less than 1, nor the rf voltage,
indicated by the vtvm, to more than 0.5 volt rms.
To demonstrate the effectiveness of the Varicap as a voltage-variable
tuning capacitor: (1) Set the dc voltage to -1. (2) Tune the rf
signal generator for resonance, as indicated by peak deflection
of the vtvm. Set the generator output control to hold this deflection
to 0.5-volt rms. (3) Record the generator frequency as f1. (4) Set
the dc voltage to -9, noting that the vtvm deflection falls indicating
detuning of the circuit. (5) Retune the generator to locate the
new, higher resonant frequency and record this as f2, The tuning
range afforded by the 8-volt bias change is equal to f2 - f1.
In the test setup shown in Fig.3, the circuit was tuned from
1400 kc at -1 volt to 2250 kc at -9 volts, a tuning range of 850
kc. A wide frequency band may be covered with the same capacitance
change if L2's inductance is made smaller to increase the operating
frequency. In some applications of this principle, it will be desirable
to use the Varicap as a voltage-variable trimmer in parallel with
an air tuning capacitor.
Many applications of this principle suggest themselves. Examples
are the voltage tuning of rf test oscillators, local oscillators
in radio and TV receivers (especially in remote control operation),
self-excited oscillators in transmitters and absorption wave-meters.
Fig. 4 - Varicap frequency modulator circuit.
Fig. 5 - A Varicap AFC circuit for your FM receiver.
In the experimental circuit shown in Fig. 4, the Varicap (C2)
is shunted (through .01-μf blocking capacitor C3) across the
tank circuit (L-C4) of a self-excited 50-mc oscillator. An audio-frequency
voltage (af) is applied to the Varicap in series with
the 6-volt dc bias supplied by the battery. This ac fluctuates the
bias at the audio-frequency rate. The capacitance of the Varicap
accordingly fluctuates about its mean -6-volt value, frequency-modulating
the oscillator. The center frequency is determined by the setting
of the 100-μμf air capacitor (C4) and the -6-volt bias level.
Sweep width is proportional to the amplitude of Eaf
and is adjusted by varying this audio voltage. In Fig. 4, the rf
oscillator is tuned to the center frequency of 50 mc when C-4 is
set to 50 μμf, the dc bias to -6 volts and Eaf
to zero. A 0- to 4-mc sweep is obtained when Eaf is varied
from 0 to 1.5 volts rms.
To prevent exceeding the Varicap voltage ratings in this kind
of FM oscillator, the sum of the dc, peak af and peak rf voltages
must not exceed the maximum voltage shown in the chart. Also, the
dc bias must not be set so low that the sum of Eaf peak
and Erf peak will drive the Varicap into its forward,
or conducting, region, In Fig. 5, these conditions are met when
Edc = -6 volts, Eaf does not exceed 1.5 volts
rms and Erf does not exceed 3 volts. While the latter
is a relatively low rf tank voltage for tube type oscillators, it
is reasonable for high-frequency transistor oscillators with which
the Varicap frequency modulator is a natural companion.
Although the tank circuit shown in Fig, 4 has been designed for
50-mc operation, it is not imperative that this frequency be used.
The same FM scheme may be used at other center frequencies by properly
proportioning the L-C circuit. The lower the center frequency, the
lower the sweep width obtained with a given Varicap capacitance
swing, and vice versa, The transformer shown in Fig. 4 is not critical.
Any audio unit whose secondary will deliver a maximum of 1.5 volts
rms audio from a given af source is usable if it has a satisfactory
Automatic Frequency Control
The voltage-variable capacitance of the Varicap and its temperature
stability suit it for use as a simple, highly sensitive, afc device
which performs better than some reactance tube circuits. The small
size of an afc unit, containing a Varicap, four small capacitors,
four resistors and an rf choke, lets you tuck it into a receiver
with a minimum of disturbance to the set's circuitry. This should
be welcome news to hi-fi enthusiasts whose FM receivers have no
automatic frequency control.
Fig. 5 shows an afc circuit developed by Pacific Semiconductor
engineers, which I adapted for bias from the 300-volt dc supply
of an FM receiver. The photos show the complete unit ready for wiring
into the receiver. Any type of Varicap may be used. The receiver's
local oscillator is simply realigned to compensate for the shunting
capacitance introduced by the biased Varicap C2 which functions
as a frequency-controlled trimmer across the local oscillator tank.
The Varicap is referred to a dc bias of -8 volts obtained from
the receiver's 300-volt supply through the voltage divider R3, R4.
The afc dc voltage is obtained from one side of the discriminator.
For other than 300-volt supplies, the values of R3 and R4 will not
be the same as mine but must be worked out for an output of -8 volts
from the particular supply voltage of the set that you are adding
this afc circuit to.
The completed afc unit is built on a perforated phenolic board
2 3/8 inches long and 2 1/8 inches wide. The pigtails of the components
are passed through the holes in the board and interconnected underneath
to complete the wiring. Printed circuitry may be used. The four
connections to the receiver circuit are made at solder lugs mounted
along the edge of the panel. The completed unit should be mounted
as close as possible to the local oscillator so that the lead from
the tank to C1 will be short.
((Certain silicon diodes can also be used as variable capacitors
in such applications as this. In the February-March, 1958, issue
of Rectifier News, published by the International Rectifier Corp.,
El Segundo, Calif., a circuit is shown for using their 3DS1 silicon
diode as an afc control device for an FM tuner. -Editor)
Other suggested uses for the Varicap include all-electronic dc-to-dc
and dc-to-ac choppers, amplitude modulators, alignment sweep generators,
capacitor type amplifiers (both ac and dc) , ac type flip-flops,
automatic amplitude control in f oscillators, FM telemetering and
elimination of the fine-tuning control in TV receivers.
In some applications, Varicaps, like capacitors, may be operated
in parallel for increased capacitance and in series back to back
for increased voltage-handling capability.
1 William Shockley, Electrons and Holes in Semiconductors,
D. Van Nostrand Co., 1950, page 100.
2 D. C. Brown and F. Henderson. "The PN Junction
on a Variable Reactance Device for FM Production,"
Electronic Engineering, (London) November, 1957, page 556.
Posted June 11, 2014