In an amplifier, an externally applied ac input voltage is amplified at the output. An oscillator is a device, which produces an output of a desired frequency without any external input voltage.

This can be achieved if a part of the output of a transistor is fed back into the input circuit. If the feedback voltage is in the correct phase, oscillations of a transistor oscillator. The oscillator circuit has two principal sections.

1. Amplifying Section 
This section is just a common-emitter amplifier having a high voltage gain.

2. Feedback Section 
It comprises a network of LC oscillatory circuits to provide the necessary positive feedback in the correct phase.

A sinusoidal input is given which appears as an amplified signal in the output. This means that an external input is necessary to sustain ac signal in the output for an amplifier.

Figure- Common-emitter transistor oscillator  

In an oscillator, we get ac output without any external input signal. In other words, the output in an oscillator is self-sustained.

To attain this, an amplifier can be used. A portion of the output power is returned back to the input in phase with the starting power. Inductive coupling or LC or RC networks can achieve the feedback.

For understanding the oscillator action, we consider the circuit as shown in the given figure, in which the feedback is accomplished by inductive coupling from one coil winding T₁ to another coil winding T₂.

The two coils T₂ and T₁ are wound on the same core and hence are inductively coupled through their mutual inductance. The input signal fed to the input circuit of the amplifier gets amplified at the output. Some portion of the output signal is fed to the feedback section, which introduces a 180° phase shift in the signal. Exactly how this is achieved in the feedback section is beyond the level of the present discussion. The signal is then fed back to the input circuit. Thus the transistor supplies its own input and starts oscillating at a frequency, which is determined by the values of the capacitances and inductances used in the feedback section.

Suppose switch S₁ is put on to apply proper bias for the first time. Obviously, a surge of collector current flows in the transistor. This current flows through the coil T₂ where terminals are numbered 3 and 4. This current does not reach full amplitude instantaneously but increases from X to Y. The inductive coupling between coil T₂ and coil T₁ now causes a current to flow in the emitter circuit the ‘feedback’ from input to output). As a result of this positive feedback, this current also increases from X’ to Y’.

The current in T₂ connected in the collector circuit acquires the value Y when the transistor becomes saturated. This means that maximum collector current is flowing and can increase no further. Since there is no further change in collector current, the magnetic field around T₂ ceases to grow. As soon as the field becomes static, there will be no further feedback from T₂ to T₁. Without continued feedback, the emitter current begins to fall. Consequently, collector current decreases from Y towards Z. However, a decrease of collector current causes the magnetic field to decay around the coil T₂. Thus, T₁ is now seeing a decaying field in T₂. This causes a further decrease in the emitter current till it reaches Z’ when the transistor is cut-off. This means that both I_E and I_C cease to flow.

Therefore, the transistor has reverted back to its original state. The whole process now repeats itself. That is, the transistor is driven to saturation, then to cut-off, and then backs to saturation. The time for change from saturation to cut-off and back is determined by the constants of the tank circuit or tuned circuit. The resonance frequency n of this tuned circuit determines the frequency at which the oscillator will oscillate.



The tank or tuned circuit is connected in the collector side, hence, it is known as tuned collector oscillator. If the tuned circuit is on the base side, it will be known as tuned base oscillator. There are many other types of tank circuits, say RC or feedback circuits giving different types of oscillators like Colpitt’s oscillator, Hartley oscillator, RC-oscillator.

In an amplifier, an externally applied ac input voltage is amplified at the output. An oscillator is a device, which produces an output of a desired frequency without any external input voltage.

This can be achieved if a part of the output of a transistor is fed back into the input circuit. If the feedback voltage is in the correct phase, oscillations of a transistor oscillator. The oscillator circuit has two principal sections.

1. Amplifying Section 
This section is just a common-emitter amplifier having a high voltage gain.

2. Feedback Section 
It comprises a network of LC oscillatory circuits to provide the necessary positive feedback in the correct phase.

A sinusoidal input is given which appears as an amplified signal in the output. This means that an external input is necessary to sustain ac signal in the output for an amplifier.

Figure- Common-emitter transistor oscillator  

In an oscillator, we get ac output without any external input signal. In other words, the output in an oscillator is self-sustained.

To attain this, an amplifier can be used. A portion of the output power is returned back to the input in phase with the starting power. Inductive coupling or LC or RC networks can achieve the feedback.

For understanding the oscillator action, we consider the circuit as shown in the given figure, in which the feedback is accomplished by inductive coupling from one coil winding T₁ to another coil winding T₂.

The two coils T₂ and T₁ are wound on the same core and hence are inductively coupled through their mutual inductance. The input signal fed to the input circuit of the amplifier gets amplified at the output. Some portion of the output signal is fed to the feedback section, which introduces a 180° phase shift in the signal. Exactly how this is achieved in the feedback section is beyond the level of the present discussion. The signal is then fed back to the input circuit. Thus the transistor supplies its own input and starts oscillating at a frequency, which is determined by the values of the capacitances and inductances used in the feedback section.

Suppose switch S₁ is put on to apply proper bias for the first time. Obviously, a surge of collector current flows in the transistor. This current flows through the coil T₂ where terminals are numbered 3 and 4. This current does not reach full amplitude instantaneously but increases from X to Y. The inductive coupling between coil T₂ and coil T₁ now causes a current to flow in the emitter circuit the ‘feedback’ from input to output). As a result of this positive feedback, this current also increases from X’ to Y’.

The current in T₂ connected in the collector circuit acquires the value Y when the transistor becomes saturated. This means that maximum collector current is flowing and can increase no further. Since there is no further change in collector current, the magnetic field around T₂ ceases to grow. As soon as the field becomes static, there will be no further feedback from T₂ to T₁. Without continued feedback, the emitter current begins to fall. Consequently, collector current decreases from Y towards Z. However, a decrease of collector current causes the magnetic field to decay around the coil T₂. Thus, T₁ is now seeing a decaying field in T₂. This causes a further decrease in the emitter current till it reaches Z’ when the transistor is cut-off. This means that both I_E and I_C cease to flow.

Therefore, the transistor has reverted back to its original state. The whole process now repeats itself. That is, the transistor is driven to saturation, then to cut-off, and then backs to saturation. The time for change from saturation to cut-off and back is determined by the constants of the tank circuit or tuned circuit. The resonance frequency n of this tuned circuit determines the frequency at which the oscillator will oscillate.



The tank or tuned circuit is connected in the collector side, hence, it is known as tuned collector oscillator. If the tuned circuit is on the base side, it will be known as tuned base oscillator. There are many other types of tank circuits, say RC or feedback circuits giving different types of oscillators like Colpitt’s oscillator, Hartley oscillator, RC-