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As with vacuum tubes, the special electronics effects encountered at microwave
frequencies severely limit the usefulness of transistors in most circuit
applications. The need for small-sized microwave devices has caused extensive
research in this area. The new solid-state microwave devices are predominantly
active, two-terminal diodes, such as tunnel diodes, varactors,
transferred-electron devices, and avalanche transit-time diodes. This section
will introduce you to the basic theory of operation and some of the applications
of these relatively new solid-state devices.
The tunnel diode is a pn junction with a very high concentration of impurities
in both the p and n regions. The high concentration of impurities causes it to
exhibit the properties of a negative-resistance element over part of its range
of operation, as shown in the characteristic curve in figure 1. In other words,
the resistance to current flow through the tunnel diode increases as the applied
voltage increases over a portion of its region of operation. Outside the
negative-resistance region, the tunnel diode functions essentially the same as a
normal diode. However, the very high impurity density causes a junction
depletion region so narrow that both holes and electrons can transfer across the
pn junction by a quantum mechanical action called tunnelling.
Tunneling causes the negative-resistance action and is so fast that no
transit-time effects occur even at microwave frequencies. The lack of a
transit-time effect permits the use of tunnel diodes in a wide variety of
microwave circuits, such as amplifiers, oscillators, and switching devices.
Figure 1. Tunnel-diode characteristic curve.
TUNNEL-DIODE OSCILLATORS.
A tunnel diode, biased at the center point of the negative-resistance range
(point B in figure 1) and coupled to a
tuned circuit
or cavity, produces a very stable oscillator. The oscillation frequency is the
same as the tuned circuit or cavity frequency.
Microwave tunnel-diode oscillators are useful in applications that require
microwatts or, at most, a few milliwatts of power, such as local oscillators for
microwave superheterodyne receivers. Tunnel-diode oscillators can be
mechanically or electronically tuned over frequency ranges of about one octave
and have a top-end frequency limit of approximately 10 gigahertz.
Tunnel-diode oscillators that are designed to operate at microwave frequencies
generally use some form of
transmission line
as a tuned circuit. Suitable tuned circuits can be built from coaxial lines,
transmission lines, and waveguides.
An example of a highly stable tunnel-diode
oscillator
is shown in Figure 2. A tunnel-diode is loosely coupled to a high-Q tunable
cavity. Loose coupling is achieved by using a short,
antenna
feed probe placed off-center in the cavity. Loose coupling is used to increase
the stability of the oscillations and the output power over a wider
bandwidth.
Figure 2. Tunnel-diode oscillator.
The output power produced is in the range of a few hundred microwatts, sufficient for many microwave applications. The frequency at which the oscillator operates is determined by the physical positioning of the tuner screw in the cavity. Changing the output frequency by this method is called MECHANICAL TUNING. In addition to mechanical tuning, tunnel-diode oscillators may be tuned electronically. One method is called BIAS TUNING and involves nothing more than changing the bias voltage to change the bias point on the characteristic curve of the tunnel-diode. Another method is called VARACTOR TUNING and requires the addition of a varactor to the basic circuit.
TUNNEL-DIODE AMPLIFIERS
Low-noise, tunnel-diode amplifiers represent an important microwave application
of tunnel
diodes.
Tunnel-diode amplifiers with frequencies up to 85 gigahertz have been built in
waveguides, coaxial lines, and
transmission lines.
The low-noise generation, gain ratios of up to 30 dB, high reliability, and
light weight make these amplifiers ideal for use as the first stage of
amplification in communications and radar receivers.
Most microwave tunnel-diode amplifiers are REFLECTION-TYPE, CIRCULATOR-COUPLED AMPLIFIERS. As in oscillators, the tunnel diode is biased to the center point of its negative-resistance region, but a CIRCULATOR replaces the tuned cavity.
A circulator is a
waveguide device
that allows energy to travel in one direction only, as shown in Figure 3. The
tunnel diode in figure 2-41 is connected across a tuned-input circuit. This
arrangement normally produces feedback that causes oscillations if the feedback
is allowed to reflect back to the tuned-input circuit. The feedback is prevented
because the circulator carries all excess energy to the absorptive load (RL).
In this configuration the tunnel diode cannot oscillate, but will amplify.
Figure 3.
Tunnel-diode amplifier.
The desired frequency input signal is fed to port 1 of the circulator through a
bandpass filter. The filter serves a dual purpose as a bandwidth selector and an
impedance-matching device that improves the gain of the amplifiers. The input
energy enters port 2 of the circulator and is amplified by the tunnel diode. The
amplified energy is fed from port 2 to port 3 and on to the mixer. If any energy
is reflected from port 3, it is passed to port 4, where it is absorbed by the
matched load resistance.
TUNNEL-DIODE FREQUENCY CONVERTERS AND MIXERS
Tunnel diodes make excellent mixers and frequency converters because their
current-voltage characteristics are highly nonlinear. While other types of
frequency converters usually have a conversion power loss, tunnel-diode
converters can have a conversion power gain. A single tunnel diode can
also be designed to act as both the nonlinear element in a converter and as the
negative-resistance element in a local oscillator at the same time.
Practical tunnel-diode frequency converters usually have either a unity conversion gain or a small conversion loss. Conversion gains as high as 20 dB are possible if the tunnel diode is biased near or into the negative-resistance region.
Although high gain is useful in some applications, it presents problems in
stability. For example, the greatly increased sensitivity to variations in input
impedance can cause high-gain converters to be unstable unless they are
protected by isolation circuitry. As with tunnel-diode amplifiers, low-noise
generation is one of the more attractive characteristics of tunnel-diode
frequency converters. Low-noise generation is a primary concern in the design of
today's extremely sensitive communications and radar receivers. This is one
reason tunnel-diode circuits are finding increasingly wide application in these
fields.
Q.1.What
causes the negative-resistance property of tunnel diodes?
A1.
The tunneling action
Q.2.What
determines the frequency of a tunnel-diode oscillator?
A2.
The tuned circuit or cavity frequency.
Q.3.Why
is the tunnel diode loosely coupled to the cavity in a tunnel-diode oscillator?
A.3.To
increase the stability.
Q.4.What is the purpose of the
circulator in a tunnel-diode amplifier?
A.4.
Prevent feedback to the tuned input circuit.
Q.5.
What limits the usefulness of high-gain, tunnel-diode frequency converters?
A.5.
Stability problems.
