Negative Resistance oscillator

• When the operating point enters the region of positive resistance, the amplitude of oscillation is limited. 

• To obtain the maximum output, the quiescent point must be accurately located at the center of the negative resistance region.The frequency of oscillation is given by,

                                                F = 1/2 root LC

•  A tunnel diode has a characteristic with a negative resistance region between voltages of approximately 0.1 and 0.3 V and can be used as an oscillator at frequencies up to 100 0Hz.

 BARKHAUSEN CRITERION:

•     The essential condition for maintaining oscillations are:

•     | βA | = 1, i.e. the magnitude of loop gain must be unity.

•     The total phase shift around the closed loop is zero or 360 degrees.

PRACTICAL CONSIDERATIONS:

•     The condition that  | βA | = 1 gives a single and precise value of Aβ which should be set through out the operation of the oscillator circuit.

•     But in practice, as transistor characteristics and performance of other circuit components change with time, | βA |  will become greater or less than unity.

•  Hence, in all practical circuits | βA |  should be set greater than unity so that the amplitude of oscillation will continue to increase without limit but such an increase in amplitude is limited by the onset of the nonlinearity of operation in the active devices associated with the amplifier is shown below.

•  In this circuit, Βa is larger than unity for positive feedback. This onset of Non linearity is an essential feature of all practical oscillators.