Negative resistance oscillators make use of negative resistance elements such as tetrodes, tunnel diodes, uni junction transistors etc. There are two types of negative resistance oscillators, which are commonly used for high frequency generation. These are dynatron and tunnel diode oscil­lators.

Dynatron operates in the negative resistance region of the characteristics of a tetrode, which is coupled to an L-C tank circuit.

Tunnel diode oscillator makes use of a tunnel diode for producing oscillations.

Tunnel Diode Oscillator Characteristics

Now consider a circuit shown in the figure (a) shown below where D is a tunnel diode, R is an external resistance, V is the battery voltage, V_D is voltage drop across diode and V_R is voltage drop across resistance R. The value of R is so selected as to bias the diode D in the negative resistance region AB. The working or quiescent point Q is almost at the centre of the characteristic curve AB.

When the switch S is closed, the current immediately rises to a value determined by R and the diode resistance which are in series. The applied voltage V divides across D and R according to the ratio of their resistances. However, as diode voltage V_D exceeds V_p (point A), diode is driven into the negative resistance region and its resistance starts increasing. So V_D increases further till it becomes equal to valley voltage V_v (point B). At this point further increase in V_D drives the diode into the positive resistance re­gion BC [figure (b)]. Now increase in current causes increase in V_R and de­crease in V_D, thereby bringing the diode back into the negative resistance region. This reduction in diode voltage V_D causes an increase in circuit current till point A is reached when V_D equals V_p. Thus the circuit will continue to oscillate back and forth through the negative resistance region, that is between points A and B on its characteristic. Its output across external resistance R is sinusoidal. A practical circuit of a tunnel diode oscillator in two slightly dif­ferent ways is shown in the figure shown below. Here R₂ sets the proper bias level for the diode whereas R_x in parallel with the L-C tank circuit sets proper current level for it. The capacitor C_c is the coupling capacitor. As the switch is closed, the tunnel diode is set into oscillations whose frequency is equal to the resonant frequency of the tank circuit.

Tunnel Diode Oscillator Circuit

The tunnel diode operates very fast and it is possible to make tunnel diode oscillators which operate in the microwave frequency region.