February 1967 Electronics World
People old and young
enjoy waxing nostalgic about and learning some of the history of early electronics. Electronics World
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The tetrode transistor is more commonly known today as a dual
base transistor or a dual gate FET.
The New Tetrode Transistor
By 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
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. 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.
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
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
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
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 neutralization.
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 FET.
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.
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.
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.
The 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 levels.
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 to 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.
Fig. 4. Circuit configurations for tetrode
transistors. (A) FET 45-MHz i.f. amplifier shows how a.g.c.
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.
(B) A crystal-controlled FM oscillator.
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 signals.
(C) Simplified autodyne converter uses both
gates of FET transistor.
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 successfully developed.