March 1946 Radio-Craft
[Table of Contents]
Wax nostalgic about and learn from the history of early electronics.
See articles from Radio-Craft,
published 1929 - 1953. All copyrights are hereby acknowledged.
Solid state equivalent of vacuum tube diode
Often when posting this kind of circuit design and/or analysis article from a
vintage electronics magazine like Radio-Craft I point out that except for
biasing voltage and resistor values, most can conceptually be applied to solid state
circuits by substituting semiconductor junction diodes for tube diodes and field
effect transistors for tube triodes, tetrodes, and pentodes. In some cases solid
state diodes can be directly substituted if the voltage, current, and power handling
is sufficient. Many examples of using diodes as voltage limiting devices for the
purpose of signal rectification, for leveling, and for protection against an overvoltage
condition. Signal detection is another common use for diodes, but that is not covered
here. As a visual aid, I created the graphic above to show how you can replace the
vacuum tube with a semiconductor diode.
Limiting Circuits Are Important in Radar, FM and All Pulsing Circuits
Fig. 1 - Positive limiting with series diode.
Fig. 2 - Similar to above, negative limiting.
Fig. 3 - Series positive limiting ground.
Fig. 4 - Series negative limiting below ground.
Fig. 5 - Positive limiter with diode.
Fig. 6 - Negative limiting version of Fig. 5.
By Jordan McQuay
Amplitude of waves is of great importance in radar, television and electronics
circuits. Control of wave amplitude is the function of limiters, sometimes known
Limiter removes one extremity or the other, or both extremities, of any form
of input wave. In some applications the signal level may be reduced by a limiter,
so the output never exceeds a certain datum. Or, only peaks above a certain reference
voltage may be allowed to pass the limiter stage.
Limiting action can be accomplished by varistor's, or similar devices whose resistance
decreases with rising voltage. Selenium and copper-oxide rectifiers also provide
some limiting effects. But such devices are difficult to control, lack precise definition,
and are none too dependable for exacting work in electronics.
Most effective means of precision limiting is by vacuum tubes. Diodes, triodes,
and even pentodes are used for such amplitude control - connected in a variety of
circuits, depending upon the type of control action and discrimination required.
For instance, a limiter may function as a protective device: limiting all amplitudes
to 50 volts peak - since higher voltages might damage some later stage. Or, a limiter
may be required to pass only those parts of an input pulse that exceed 30 volts
- thus creating impulses above a certain reference level. A limiter can convert
an input sine wave into a square or rectangular wave. A peaked wave can be converted
into a steep-front pulse by removing positive or negative extremities.
Thus, a limiter may be used as a wave-shaping circuit, in addition to its amplitude
Limiting circuits are classified according to five types: (1) Series diode limiters,
(2) Parallel diode limiters, (3) Double diode limiters, (4) Triodes, tetrodes, and
pentodes with grid limiting, and (5) Overdriven amplifiers. Inclusion of overdriven
amplifiers in the class of limiting circuits is entirely accurate, even though such
devices are occasionally referred to as square-wave generators.
The Diode Principle
Conduction takes place in a diode vacuum tube only when the plate is. positive
with respect to the cathode (or the cathode is negative with respect to the plate).
Under such conditions, electrons pass from the cathode only in the direction of
This unilateral characteristic of the ordinary diode makes it ideal for limiting
If the cathode is grounded, the plate must be positive with respect to ground
for conduction to take place. If a fixed positive voltage is applied to the cathode,
the diode will not pass current until the plate has risen above an equal positive
voltage. In much the same manner, if the cathode is biased below ground potential,
the diode will conduct only when the plate is above the negative value of the cathode
Current begins to flow in the diode circuit when the plate is first made positive
with respect to the cathode. As the plate becomes more positive, the flow of current
increases rapidly and the internal resistance of the diode diminishes to a few hundred
Use of the diode as a limiter permits control of the threshold of action over
wide limits of voltage.
The output wave will be a direct function of the input signal for any type of
diode limiter, and, neglecting minor circuit losses, a unity transfer of energy
can be expected.
The same wave-train of varying amplitude is used to illustrate the function of
all of the following limiter circuits. But the limiter action is independent of
the form or shape of the input wave.
Simplest diode arrangement is a series limiter circuit in which the tube functions
much as a rectifier or polarity switch.
When the input signal Es is applied to the series diode circuit (Fig.
1), the tube cannot conduct during positive alternations, because the cathode is
positive with respect to the plate. In addition, the resistor R is large enough
to prevent any small current flow. Therefore, during positive portions of the input
signal there is no output voltage.
During negative alternations, however, the diode conducts normally. Polarity
of the output voltage EL developed across the resistor R is the same
as the applied voltage of the signal.
Series Diode Limiters
This circuit (Fig. 1) provides what is known as positive limiting, since a positive
portion - in this case, all of it - is limited or removed from the input.
By merely reversing the diode connections, the circuit (Fig. 2) can be used to
limit negative portions of the signal voltage. In this arrangement, the tube conducts
only during positive alternations. There is no output voltage EL during
negative cycles of the signal.
These two simple series circuits (Figs. 1 and 2) limit the input wave to the
base line or zero axis, in true rectifying manner.
Often in radar and electronics work it is necessary to provide positive or negative
limiting at some fixed value above or below ground.
When a fixed positive voltage is applied to the plate, a diode conducts at all
times except when the positive swing of the input signal to the cathode exceeds
Such a series diode limiter (Fig. 3) provides positive limiting above ground.
And the positive voltage level (above ground) depends upon the amount of fixed positive
When the input signal Es is applied to this circuit, all voltages
having a positive value greater than the bias voltage E will stop the diode from
conducting. All positive peaks will be "clipped" or limited.
Since the amount of bias determines the amount of positive-peak limiting, a negative
bias voltage E would result in extreme limiting of the positive peaks. This level
of limiting would be below ground.
By reversing the diode and bias connections, as shown in figure 4, the negative
peaks of the input signal can be limited below ground. All portions of the input
signal Es having a negative value greater; than the bias voltage E will
stop conductance of the diode.
Since the amount of limiting is determined by the bias voltage E, a positive
bias voltage would result in extreme limiting of the negative peaks, or a limiting
action above ground.
Parallel Diode Limiters
If a limiting diode is connected in parallel with the circuit load, during periods
of tube conductance unwanted energy will be short-circuited and effectively dissipated
without appearing in the output.
Such an arrangement (Fig. 5) provides positive limiting action.
Since the cathode is at ground potential, all input voltages above ground cause
the diode to conduct. When the input signal Es is applied, positive alternations
thus cause a current flow through the tube and through the series resistor R2.
As this resistor is large compared to the internal resistance of the tube, positive
portions of the input wave are dissipated and do not appear across the load resistor
Negative alternations of the input Es are unaffected by the parallel
diode, and are reproduced in the output without distortion.
By merely reversing the connections of the diode, negative limiting can be provided
Fig. 7 - Positive limiting above ground with a parallel diode
Fig. 8 - Positive limiting below ground with a parallel diode
Fig. 9 - Below-ground parallel-diode limiter.
Fig. 10 - Above-ground parallel-diode limiter.
Fig. 11 - Both positive and negative limiting can be accomplished
with a double-diode tube.
Fig. 12 - Triode positive circuit.
Fig. 13 (a ) - Plate cut-off negative limiting.
Fig. 13 (b) - Waveform from overdriven amplifier.
Fig. 14 - Waveform from overdriven amplifier.
Action of this circuit is similar to that just described. But negative portions
of the input signal are dissipated in the series resistor R2. And positive
portions are unaffected by the parallel diode.
The two circuits (Figs. 5 and 6) limit the input wave to the zero axis, or ground
potential. Since it is often desirable to limit the signal voltage to some established
positive or negative reference level, a fixed voltage can be introduced to one or
the other of the diode electrodes for this purpose.
In the parallel limiter circuit (Fig. 7), the cathode of the tube is more positive
than the plate by the amount of the fixed voltage E. Whenever positive cycles of
the input exceed the voltage E, the tube conducts and the voltage is dissipated
in the resistor R2. However, when the input does not exceed the fixed
bias voltage, the diode will not conduct. And the output will be an exact replica
of the input signal. This circuit provides positive limiting at a given volt-age
Positive limiting below ground can be achieved by merely reversing the polarity
of the bias voltage E. When the input Es is then applied to the parallel
circuit (Fig. 8), the diode conducts at all times except when negative alternations
exceed the fixed voltage E. Thus most of the input signal, is dissipated in the
series resistor R2. Output consists of a series of negative-going impulses.
Negative limiting below ground is accomplished-by the circuit shown in Fig. 9.
The bias voltage E prevents the diode from conducting during all of the positive
and most of the negative alternations. But when the negative swing of the input
exceeds the voltage E, the tube conducts and this energy is dissipated in resistor
Reversal of the voltage E permits the same circuit to provide negative limiting
above ground (Fig. 10). In this case, the diode conducts at all times except when
the positive alternations exceed in amplitude the voltage E. The output of this
parallel diode limiter is a series of positive-going impulses.
Double Diode Limiters
Since one parallel diode can be used to limit either positive or negative extremities
of an input signal, it is apparent that two diodes can be used to limit both amplitudes
in the same circuit.
Such an arrangement is shown in Fig. 11, where positive and negative limiting
is performed by two diodes, or a double diode. Maximum amplitudes of the output
EL depends upon the values of the fixed bias voltage for each diode.
Output is developed across the load resistor R.
An examination of the output wave reveals that this circuit (Fig. 11) is a simple
means of producing a square or rectangular wave output from a sine wave input. The
circuit is occasionally used in electronics for this sole purpose.
Two series diode limiters could be connected together to perform a similar, double-limiting
function, but such an arrangement requires much more critical adjustment and is
Any triode, tetrode, or pentode can be operated as a limiter by utilizing the
cathode-grid circuit in the manner of a simple parallel diode. The circuit (Fig.
12) requires the use of a large series grid resistor R2 which acts very
much as a grid leak.
During negative cycles of the input Es no grid current flows in the
circuit. There is no voltage drop across the resistor R2 and the entire
input signal appears between the grid and cathode of the tube
When grid current flows, however, during a portion of the positive alternations
of the input, there is a voltage drop across the large grid resistor - leaving only
a small part of the positive input voltage to be applied to the tube.
The point at which grid current begins to flow is determined by the bias "E,"
developed between grid and cathode by the flow of plate current through the cathode
resistor R3. This effective bias voltage "E" establishes the limiter
Grid of the tube is normally at ground potential, and thus is negative with respect
to the cathode. Positive alternations drive the grid positive by an amount equal
to the voltage value "E," before the bias effect of the cathode resister is removed.
Any further rise in the positive input signal then results in attenuation by the
Negative portions of the input are passed by this circuit without change or limiting.
Cut-Off and Saturation
Ordinary triodes, tetrodes, and pentodes can also be operated under conditions
resulting in two other types of limiting action.
Considering the Ip-Eg characteristic curve for any triode, a central portion
of the curve will usually be almost linear-permitting distortion-free amplification
of input signals.
However, if the tube is operated at or near the end (non-linear) regions of the
curve, the output wave will be distorted.
For example, the operating point on the characteristic curve could be chosen
at such a value that the negative portions of the input signal would swing the tube
beyond cut-off. (See Fig. 13 [a].) The negative alternation would then have some
part of its extremity removed, but the positive cycle would he unaffected. This
is known as cut-off limiting in an amplifier.
If the operating point is chosen near the point of saturation on the characteristic
curve (Fig. 13 [b]), the positive portion of the wave would be distorted and limited,
while the negative part of each cycle would not be affected. This is known as saturation
limiting in an amplifier.
The chief value of these two new forms of limiting is that they may be combined
in a single amplifier to provide both positive and negative limiting with a single
All that is required is an input signal of such great amplitude that (1) it drives
the amplifier tube far into the cut-off region on negative alternations, and (2)
it drives the tube far beyond the saturation region on positive alternations (Fig.
Such a device is known as an over-driven amplifier. The combination of cut-off
and saturation limiting is used to produce a square or rectangular wave from a high-amplitude
input sine wave.
Referring to Fig. 14, assume that the normal input A1 of 50 volts
results in a linear output A2. The triode, however, is purposely operated
with an excessive input B1 of 400 volts, or higher. Then, as the grid swings positive
a condition of saturation will be reached quickly - and the output B2
will have risen to its greatest possible value. As the grid attempts to go more
positive due to the driving influence of the input signal, more and more grid current
will be drawn by the amplifier tube. The high impedance driving source will be incapable
of effecting a further increase in the grid voltage. Thus, the output wave will
remain at constant amplitude, until the signal input wave has proceeded in its cycle
and returns to the linear portion of the characteristic curve.
Once this region of linearity is reached, the output wave will fall proportionately
with a decrease in grid voltage until it reaches the cut-off point. From that point
any further decrease in grid voltage has no effect on the output wave, since it
cannot be reduced below zero. Therefore, the output will remain at zero until the
signal wave continues in its cycle and enters the region where the tube will pass
plate current again, when the entire process is repeated.
Thus, two characteristic limitations of an amplifier can be utilized to pro-duce
a steep-sided square or rectangular output wave.
Posted June 1, 2021