Page 93 - Electrician - TT (Volume 2)

ELECTRICIAN - CITS

In case of oscillators, if the loop gain kAv is made positive, i.e. by feeding back signal which is in-phase with the
input signal, then there will be an output signal even though there is no external input signal. In other words, an
amplifier is modified to be an oscillator by positive feedback such that it supplies its own input signal.
Example

An amplifier has a voltage gain of 40 without feedback. Determine the voltage gains when positive feedback of
the following amounts is applied.
i k = 0.01

ii k = 0.02
iii k = 0.025
Solution
A 40 40
i Avf = V = = = 66.7
1 – kA V 1 – 0.01 x 40 0.6
A 40 40
ii Avf = V = = = 200
1 – kA V 1 – 0.02 x 40 0.2

iii Avf =
In (iii) the gain of the amplifier become infinite when the loop gain kAv = +1. This is known as the critical value
of the loop gain kAv. It is important to note that the output voltage cannot be infinite. Instead the amplifier will
start working as an oscillator without the need of any separate input. If the feedback path contains a frequency
selective network, the requirement of kAv = 1 can be met at only one particular frequency, such that, the output
of the oscillator will be a sinusoidal signal of a particular frequency. Such oscillators are known as sine wave
oscillators.
There are 3 types of oscillators.
1 Hartley oscillator
2 Colpitts oscillator
3 Crystal oscillator
Out of three Hartley oscillator only discussed.
Hartley oscillator: One of the simplest of sinusoidal oscillators is the Hartley oscillator shown in Figs 2a and 2b.
As in Fig 2a is a series fed Hartley oscillator. This circuit is similar to the tickler coil oscillator, but the tickler circuit
coil L1 is physically connected to L, and is hence a part of L (like an auto-transformer). This oscillator is called
series-fed because, the high frequency oscillations generated and the dc paths are the same, just as they would
be in a series circuit. Series fed Hartley oscillators are not preferred due to their poor stability of oscillations.
Fig 2b is parallel fed Hartley oscillator commonly used in radio receivers. Parallel fed Hartley oscillators are
known for their high stability of oscillations.
The circuit at Fig 2b is actually an amplifier with positive (regenerative) feedback to have sustained oscillations.
The capacitor C2 and inductor L2 form the path for RF current in the collector to ground circuit.
RF current through L2 induces a voltage in L1 in proper phase and amplitude to sustain oscillations.
The position of the tap at the junction of L1 and L2 determines how much signal is fed back to the base circuit.
The capacitor C and the inductors L1 + L2 forms the resonant tank circuit of the oscillator which determines
the frequency of oscillations. Capacitor C can be made variable capacitor for tuning the oscillator to different
frequencies. C1 and R1 form the RC circuit which develops the bias voltage at the base.
The RF choke at the collector keeps the high frequency ac signal out of the Vcc supply. In cheaper oscillator
circuits the RF choke is omitted and is replaced by a resistor.
Resistor R2 connected in the emitter provides dc stabilization. R2 is by-passed by C3 to prevent ac degeneration.
The Hartley oscillator coil has three connections. These are usually coded on the coil. If they are not, it is
generally possible to identify them by a resistance check. The resistance between the taps T and P as in
Fig 3, is small compared with the resistance between T and G., If the coil connections are not made properly, the
oscillator will not work.

80

CITS : Power - Electrician & Wireman - Lesson 60-69