Page 92 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

When S1 is pressed to ON, A input of NAND gate is forced to go logic-low. Hence the output of this NAND gate is
forced to go logic-HIGH. Therefore the 'Y' output is at logic-1, so LED glows and a logic-HIGH appears at probe
tip. Also note that with HIGH at Y output, the inputs of NAND gate 2 are also at logic-HIGH and the output of
NAND gate-2 is forced to go low. As long as switch S1 is at ON position the probe tip is HIGH. When it is released
it springs back to OFF position, and the output returns to a logic-LOW condition.

Wave shapes - Oscillators

Objectives: At the end of this lesson you shall be able to
• state the working principle and gain of oscillator
• explain the RC phase-shift oscillator and frequency calculation
• state the features, gain and frequency of Hartley, colpitts and crystal oscillators
• state the working principle and frequency calculation of bistable and monostable multivibrator using CRO.

Oscillator: An oscillator is a circuit for producing voltages that vary in a regular fashion with respect to time. The
output wave forms of oscillators are repeated exactly in equal successive intervals of time as in Fig 1a and Fig 1b.
The output wave-form of an oscillator may be sinusoidal as in Fig 1a. Such oscillators are known as sine wave
oscillators or harmonic oscillators.

The output of oscillators may be square, triangular or saw-tooth wave forms as in Fig 1b. Such oscillators are
known as non-sinusoidal oscillators or relaxation oscillators.

Fig 1

It was discussed earlier that positive feedback results in converting an amplifier into an oscillator. To provide
positive feedback the feedback signal should be inphase with the input signal such that it adds up with the input
signal.
In practice, an oscillator will have no input ac signal at all, but it still generates ac signal. An oscillator will have
only a dc supply. The oscillator circuit, makes use of the noise generated in resistors at the switching on time of
dc supply and sustains the oscillations.
To build an oscillator, the following are essential;

– An amplifier
– A circuit which provides positive feedback from output to input.
The gain of an amplifier with feedback is given by,
A
V
A = 1− kA
vf
V
kA is known as the loop gain of the amplifier. In the case of the amplifiers when the sign associated with kA is
v
v
negative, the denominator has value more than 1. And, hence the value of Avt will always be less than Av (negative
feedback). But, if the value of kA is made larger, such that, it approaches unity, and, if the sign associated with
v
kA is negative then the value of the denominator decreases to less than 1, and hence, A will be larger than A .
vf
v
v
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CITS : Power - Electrician & Wireman - Lesson 60-69