TUNNEL DIODE, TUNNEL-DIODE OSCILLATORS

mechanical action called TUNNELING. 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.

A tunnel diode, biased at the center point of the negative- resistance range 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 super heterodyne 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.


If the diode is biased at VDD the incremental input conductance presented to the passive resonant circuit is negative and can compensate for the positive losses of the inductance, capacitance, leads, etc. Such losses are modeled by a single conductance G=1/R.

Shifting the axis to the Q-point, the new voltage variable is v'=V-VDD. The original diode I-V characteristic is described functionally as I=f1(V)

The translated diode-current variable is i'=I-I0=I-f1(DD)

The circuit equation in the original variables is