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I
voltage regulation', '.'. as well and gives
promise of application in special -purpose
trigger and switching circuits.
Nothing has appeared previously on the
direct use of the germanium diode as a
negative -resistance amplifier, although
Dowell' and Becker, Green, and Pearson'
have called attention to the application
of another semiconductor device
-the thermistor -as a negative- resistance
amplifier.
Applications
Several interesting and provocative
applications of the negative -resistance
phenomenon, and of effects in and
around the region of zero dynamic resistance,
are possible. Figures 3 to 6
illustrate some of the possible applications.
Sawtooth Oscillator
In this application, the diode occupies
the same position as a gaseous tube in
the conventional RC relaxation oscillator
circuit. See Fig. 3A. With a 1N34,
the d.c. input voltage E is adjusted to
a value between 100 and 125 volts,
depending upon the individual diode,
whereupon oscillation begins suddenly.
In a test setup of the circuit, current
I was 30 ma. The circuit constants
shown in Fig. 3A gave a frequency of
3000 cycles.
The circuit must operate into a high
impedance (100,000 ohms or more). As
the insert in Fig. 3A shows, the saw -
tooth wave is positive -going, but is not
linear. A peak voltage of 20 volts was
obtained in the test setup.
Sine -Wave Oscillator
See Figs. 3B and 4. This unit was
built originally as a tone modulator, for
intermittent use, to be included in a special
r.f. signal generator. Transformer
T, shown in Fig. 4, is a surplus "Ouncer"
transformer for coupling a 200 -
ohm microphone or line to push -pull
grids. The secondary winding (G to G)
has a measured inductance of 6 henrys.
Using a 1N54 diode, and with the d.c.
voltage E set to approximately 100
volts, fairly clean sine -wave output was
obtained at 180 cycles and 1 volt r.m.s.
open circuit. Higher output voltage
would have been obtained if the transformer
had not had a stepdown turns
ratio. Loading of the oscillator must be
kept light; otherwise high distortion
and lowered output voltage result.
Figure 3B is the circuit commonly employed
in diode sine -wave oscillators.
Air -core components are substituted for
the iron -core transformer T when r.f.
operation is desired.
Output of the sine -wave oscillator
drops rapidly at frequencies higher
than 10 kc. The author seldom has obtained
other than sporadic and feeble
operation above 100 kc., although 1 -mc.
oscillation has been reported by occasional
researchers.
Voltage Amplifier
Amplification is a curious phenomenon
exhibited by the germanium diode
when functioning as a negative resistance.
In this application, the diode is
connected in series with a load resistance
and is d.c.-biased to the negative
slope of the diode's reverse conduction
curve. An a.c. signal applied in series
with this bias causes the circuit current
to fluctuate periodically about the bias
current level as a mean value. A current
increase produces a voltage rise
across the load resistance and a voltage
fall across the diode. Under satisfactory
conditions of bias voltage level,
signal voltage amplitude, and load resistance,
the a.c. voltage across the load
resistance will be higher than the a.c.
signal voltage, thus producing voltage
amplification. The law of conservation
of energy is not violated in this case
since the d.c. bias source supplies the
"local" power.
Figure 5 shows a test circuit for
studying this effect. The adjustable d.c.
voltage source must have as low an
output impedance as is practicable and
should be automatically regulated. The
signal input transformer T must have
very low secondary resistance to avoid
introduction of appreciable resistance
in series with the diode and load resistance.
Input and output signal voltages
(E, and E respectively) are read with
a.c. vacuum tube voltmeters, and the
output signal is monitored with an oscilloscope
to check waveform. If transformer
T has a 1:1 turns ratio, the input
v.t.v.m. may be transferred to the
primary input, a more desirable point
of operation for this meter.
The d.c. bias voltage, a.c. signal voltage,
and load resistance are each made
adjustable. There are numerous combinations
of each which will operate
the circuit, but only one optimum combination
of the three will give appreciable
amplification with an individual
diode. Some diodes show good amplification
with low signal voltage levels (i.e.,
under 1 volt r.m.s.), while others require
as much as 10 volts of
t
signal with
proper adjustment of d.c. bias and load
resistance.
Using types 1N34 and 1N54 diodes,
the author has obtained voltage ampli-
2K
OUTPUT
5
WAVEFORM
CAT H
ADJUSTABLE
QC VOLTAGE
SAWTOOTH OUTPUT
Iy,d
-J 400 v
Germanium
Diode Type
E1, for
Zero Dynamic
Resistance
Overload
Represented
by E, (%)
1N34, 34A 75 25
1N35 75 50
1N38, 38A 120 20
1N39 225 12.5
1N48 85 21.4
1N51 50 25
1N52 85 21.4
1N54 75 46.5
1N54A 75 50
1N55, 55A 170 13.3
1N56, 56A 50 25
1N58, 58A 120 20
1N60 30 20
1N63 125 25
1N65 85 21.4
1N69 75 25
1N70 125 25
1N75 125 25
1N81 50 25
Table 1. Percentage overload for various diodes
when operated as negative resistances.
fications of 2 to 4 at 1000 cycles with
input signal levels between 0.5 and 5
volts. In each instance, the waveform
has been fair but stability poor, amplification
often ceasing suddenly with a
complete readjustment of the circuit
required to restore it. Furthermore,
some brand -new diodes exhibited no
amplification whatever, showing at best
a gain of 1 in the circuit.
The author has been unsuccessful in
obtaining amplification at frequencies
higher than 5000 cycles, but is continuing
experiments in this direction. It
would appear from preliminary tests
that the germanium diode, like the
thermistor, is adaptable only to low
frequency use as a negative -resistance
amplifier. Three disadvantages of the
diode amplifier appear to be: (1) the
high input signal level required, (2)
relatively low impedance input, and (3)
lack of isolation between input and
output circuits.
Lock -In Circuit
In the region of the peak back voltage
E, of a germanium diode, the reverse
current jumps suddenly from a
few microamperes to many milliamperes
as the applied reverse voltage is
increased in a small step. High current
then continues to flow, even when the
applied voltage is decreased drastically.
Fig. 3. (A) Diode sawtooth oscillator. (B) Diode sine -wave oscillator.
(A)
ADJUSTABLE
DC VOLTAGE
C AT H
yid
400 v
IRI
SUNS WAVE OUTPUT
APRIL, 1953 RADIO -ELECTRONIC ENGINEERING t