The New Tetrode Transistor
February 1967 Electronics World
tetrode transistor is more commonly known today as a dual base transistor
or a dual gate FET.
February 1967 Electronics World
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The New Tetrode TransistorBy Joseph Tartas
By adding another base connection
to a conventional triode transistor, two control elements are provided.
This unique advantage offers a great deal of circuit simplicity in formerly
complex semiconductor circuits.
The tetrode transistor
has been around for almost fifteen years but unfortunately has not attracted
circuit designers as much as the more familiar triode transistor. Because
of the current interest in the FET (field-effect transistor), investigation
is being conducted into the unique advantages of this extra-element
transistor and the circuit simplicity it affords.
Fig. 1. (A) A tetrode transistor has two base connections. (B) and
(C) Versions of the transistor equivalent circuit.
Fig. 2. How base 2 applies a.g.c. to device. (A) With base 2 negative,
base 1 controls electron flow. (B) With base 2 at zero volts, gain
is about unity. (C) With base 2 positive, base 1 has very little
influence on the electron flow.
Fig. 3. (A) Circuit using a single positive supply source. (B) Similar
circuit uses dual voltage supply to provide grounded base 1 circuit.
This simplifies r.f. grounding.
Fig. 4. Circuit configurations for tetrode transistors. (A) FET
45-MHz i.f. amplifier shows how a.g.c. is applied.
(B) A crystal-controlled FM oscillator.
(C) Simplified autodyne converter uses both gates of FET transistor.
Mechanically, the tetrode transistor is built as shown in Fig. 1A.
It greatly resembles the conventional triode transistor, with the exception
of a second base connection.
In this case, the exact equivalent
circuit is difficult to illustrate. However, one manufacturer presents
it as an ideal transistor with the base-to-base resistance as an external
component as shown in Fig. 1B, which could be better represented as
illustrated in Fig. 1C. It is important to the working concept of the
tetrode that the equivalent circuit representation show three facts.
(1) The base-to-base resistance is a fixed ohmic resistance (except
for larger than normal base currents). (2) The resistor divider action
in the equivalent circuit is for a.c. only. (3) Most significant is
the fact that the emitter and collector currents remain substantially
constant for the entire range of minimum to maximum signal gain. While
the latter is not true for the FET, the matter of the amount of signal
modulation still holds true in essence.
To further clarify this,
we must examine the base layer of the bipolar tetrode in relation to
the signal path and emitter-collector current. Fig. 2 demonstrates the
way in which tetrode action is obtained and the means by which the base
2 voltage controls the signal gain of the transistor while the collector
current remains substantially constant.
In Fig. 2A, the arrows
indicate the flow of electrons (in the case of the n-p-n tetrode) near
the base 1 area of the base layer due to repulsion by the negatively
charged base 2 end. This configuration represents the circuit under
maximum gain conditions, with base 1 grounded for d.c. (common base)
and the emitter grounded for a.c. (common emitter). Because of the resistance
between the base connections, the base layer will have a maximum negative
charge at the base 2 end, with the charge decreasing toward the base
1 end, so that in the narrowly confined area of base 1, the charge is
all positive relative to both emitter and base 2. With such a charge,
the emitted electrons are repelled by the negative base 2 charge and
attracted by the positive base 1 charge. Under these conditions, all
of the collector current (which is essentially all of the emitter current)
flows through the base 1 region and is modulated by the incoming signal.
As the negative base 2 voltage is decreased (toward zero voltage),
less of the electrons are repelled by base 2, and at zero volts the
electrons flow randomly across the entire emitter-base-collector junctions
shown in Fig. 2B. When this occurs, only a small amount of current flows
near base 1 and hence only a small amount of signal modulation appears
at the collector. Such a condition might represent a gain of unity or
If the base 2 voltage is allowed to reverse polarity and
become positive by a small amount (relative to base 1) as shown in Fig.
2C, then the emitted electrons are repelled by the base 1 charge and
essentially all of the current flows in the base 2 region. This state
is minimum gain and actually represents attenuation of the incoming
signal by 20 dB or more.
It is interesting to note that the
linearity of the base-to-base resistance is dependent to a large degree
on the amount of emitter current as well as the level of the base-to-base
current. Because the tetrode transistor is intended for small-signal
applications only (and hence a small emitter current), the base-to-base
resistance is normally used within its linear characteristics, and it
is just these characteristics that lend themselves nicely to r.f. and
i.f. circuitry. For the transistor, there is practically no change in
collector current and hardly any change in base current; therefore,
there is essentially no change in input capacitance or loading. Because
of this, the response of the amplifier does not shift or skew as the
stage gain is radically reduced. The Miller effect, commonly encountered
in vacuum tubes, is thus eliminated.
Since the total base 1
current is the sum of the base 1-emitter current and the base-to-base
current, there is very little change in bias with an alteration in base
2 control voltage.
Because of the isolation provided by the
second base, the need for neutralization is greatly reduced in the tetrode.
Since the nature of feedback is analogous in both tubes and transistors,
similar methods of compensation are possible in either case. However,
for tetrode transistor circuitry, it is only in rare cases that the
maximum potentialities of a tetrode circuit can be improved solely through
The tetrode is useful only in small-signal applications, in most
cases the application is at r.f. frequencies where convenience of the
second base may be used to best advantage. Examples include a.g.c.-controlled
i.f.. stages; converters where the second base becomes a separate injection
element; r.f. signal generators with the output level controlled through
the second base; and either an r.f. video attenuator, when used as a
variable impedance directly across a line.
While potential applications
are unlimited, the familiar transmitter and receiver circuitry are easily
adaptable to use of the tetrode, whether the transistor is bipolar or
At the present time, the only bipolar tetrodes available
are the 3N34 and 3N35, both manufactured by Texas Instruments. However,
the more recently introduced FET tetrodes are beginning to appear in
greater numbers each month. Nearly all the leading transistor (and also
tube) manufacturers have tetrode FET's in their current transistor listings,
There is also evidence that some companies are planning to produce pentode
transistors by similar techniques.
Without going into a discussion
of the field effect vs holes or electrons, it is interesting to find
that the dual-gate FET (or tetrode), although different in physical
construction, still obtains a variable gain control by a method similar
to that of the bipolar tetrode. The FET, however, unlike the bipolar
transistor, has a variable drain current (equivalent to the collector
current) with a change in a.g.c., just as in a vacuum tube. The big
difference is that there is little or no change in input capacitance
with a.g.c., nor is there any significant change in input loading, in
spite of the current change,
The FET is operated in a manner that is similar to that
of both the vacuum tube and the bipolar grown-junction tetrode, except
for the aforementioned differences. The bias may be derived from a divider
where feasible, or a grounded-base (or gate) circuit may be used where
preferable, as shown in Fig. 3. While the single supply of Fig.
3A is similar to that of the vacuum-tube circuit using a.g.c., it must
be remembered that the separate base element of the transistor is used
for that purpose. For high-frequency applications, it may be more desirable
to return input circuits directly to ground, as shown in Fig. 3B, supplying
negative bias to the emitter. This configuration is described as common
base for d.c. and common emitter for r.f. (through the emitter bypass
capacitance). The resistor-capacitor network in base 2 lead is used
only as a filter.
a.g.c. voltage is usually derived from the detector circuit of a receiver,
although in other types of equipment, it may be more elegant. Whatever
the source, its prime purpose is to keep the output level of the stage
or stages at a constant level over an extremely wide range of input
The bipolar transistor literature originally recommended
a constant-current source for the second base, but it was found that
the voltage source for the required base current could be controlled
with a less complicated circuit, with more standard results from unit
FET tetrodes require only a voltage source, with
maximum gain occurring at zero volts on the second gate, at least for
those elements used for a.g.c. action. All in all, the basic ideas used
in either transistor or vacuum-tube a.g.c. are quite similar. The only
precaution involved in the a.g.c. voltage is that it should be kept
within the recommended limit, as breakdowns occur beyond this limit
that can permanently damage the transistor.
Fig. 4 shows a number of typical applications of the tetrode
transistor. Fig. 4A, a 45-MHz amplifier using an FET, shows a gain of
20 dB and an approximate a.g.c. range of 40 dB for a change of zero
to 6 volts on gate 2. This gate may be returned to a fixed voltage source
of 6 volts through a potentiometer for manual gain, or it may be returned
to ground or to base 1 for a fixed gain with no control.
The crystal-controlled oscillator of Fig. 4B is frequency-modulated
by applying a reverse a.c. current to base 2, thus causing a characteristic
change in the output capacitance that shunts the crystal and tuned circuit.
In a similar application with a v.f.o. (non-crystal controlled),
a 250-kHz swing was achieved for a center frequency of 750 kHz for a
base 2 current of 150 to 200 μA.
An old favorite, the
autodyne converter, has been modernized in Fig. 4C by using the two
gates of an FET as separate injection elements for input and local oscillator
Because of the control action exerted by the
introduction of the second base element, there is almost no limit to
the variety of possible applications for tetrode transistors. Automatic
level controlled generators, variable load impedances, a.f.c. circuits,
and switch able r.f. amplifiers are only a few of those that have been