Test: Signal Generators & Waveform Shaping Circuit - Electronics and Communication Engineering (ECE) MCQ

Hysteresis is desirable in a Schmitt trigger because it prevents noise from causing false triggering.

Ail oscillators do not require positive feedback for their operation because if the positive resistance of the LC tank circuit is cancelled by introducing the right amount of negative resistance across the tank circuit, then the steady oscillation can be maintained. Some of the devices which possess negative resistance as mentioned above are dynatron, transitron, UJT and tunnel diode. Hence, assertion is not correct.

The phase change may change Aβ to negative. This can happen only when the total change in the phase angle of  becomes 180⁰ . The  required phase change in one stage i.e   should be 60⁰  or,  

 ∴ Here, ω_c is the frequency at which the phase angle changes the sign. The value of Aβ at this frequency is

Both assertion and reason are individually correct statements. The correct reason for the assertion is that the resonant frequency of the crystal is temperature dependent, therefore it is necessary to enclose the crystal in a temperature controlled oven to achieve the required frequeny stability.

The closed-iooD aain with positive feedback is

If the magnitude of loop gain i.e. |βA| - 1, then A_f = ∞. Hence, the gain of the amplifier with positive feedback is infinite and the amplifier gives an a.c. output without any a.c. input signal. Thus, the amplifier acts as an oscillator. Regenerative or positive feedback is utilized in oscillators.
Hence, both assertion and reason are correct but reason is not the correct explanation of assertion.

The operating frequency of a Wien-bridge oscillator is given by

For sustained oscillations, the minimum vaiue of current gain or forward current gain is

For the given tank circuit,

The frequency of Hartley oscillator is given by