Transistor as an Oscillator

In electronics, an oscillator is a signal-generating circuit that generates an oscillating signal, which is periodic, often like a sine wave or square wave. A direct current signal through a power source is converted to an alternating current signal by use of this device. In our earlier articles, we discussed how to utilise a transistor in the form of an amplifier and the many topologies that may be used. In this section, we will discuss how we utilise a transistor in the form of an oscillator, as well as the mechanics and varieties of transistors and the principle of an oscillator.

Circuit of an Oscillator

The sections of this circuit involved in the working principle of an oscillator include: 

To provide an enhanced output over the collector side, the transistor amplifies the oscillations in the tank circuit.

Amplifier circuit: It is a circuit that amplifies the tiny sinusoidal oscillations in the bottom part of the emitter circuit to yield product in the form of amplification.

A little amount of energy is required at the tank circuit to magnify the oscillations within the amplifier. To do this, we utilised the mutual induction phenomena to return the collector circuit’s energy to the bottom circuit. With the aid of this particular circuit, it was possible to transfer energy from input to output. 

Using what we’ve learned so far, it’s safe to state that as the flux in one coil grows, the opposing flux in the other coil also increases, resulting in phase variation when the current is supplied to the other end. There are two ways in which phase variations may occur in a common emitter amplifier: from one side to another through a feedback or tickler circuit and from one side to the other via the output side to the input side itself. In this case, the feedback oscillations will be in precisely the same phase as the natural oscillations, which is why this is the case.

The Transistor’s Mechanism/Working Circuit of the Oscillator

The transistor is utilised in the form of a prevalent emitter of the circuit in oscillator circuit design, where the collector terminals and base share the emitter. The tank circuit is inserted between the emitter and base connections. A capacitor and an inductor are linked in parallel in a tank circuit, which generates oscillations inside the circuit. Base current changes due to charge and voltage fluctuations inside the circuit of the tank, which causes the base current’s forward biassing to change frequently. 

Thus, a periodic variation in the principle of an oscillator collector current is generated. LC, i.e., inductor-capacitor, oscillations are known to be sinusoidal, which means that collector and base currents fluctuate sinusoidally as an outcome of this. RL can be used to describe the output voltage if the current in the collector is changing sinusoidally. When we plot the output voltage Vout vs time, it gives a sinusoidal curve. A tank circuit requires some power to keep oscillating, but there’s none in the case of a circuit. We use a soft iron rod to link the inductor L1 and L2 in the base circuit and the collector. 

A soft rod made of iron can connect inductor L1 to  L2 through mutual induction and a portion of the collector circuit’s energy gets transferred to the inductor’s base side. Consequently, the tank circuit oscillation continues to amplify as a consequence.

Characteristics Required For Oscillation

The circuits of an oscillator must fulfil the following characteristics to maintain oscillation: There must be a phase change between 0 and 360 degrees. There must be a little bit of an increase in the loop. The transistor amplifier gain multiplied by feedback attenuation B equals the gain in the voltage within the circuit of closed feedback. Saturation will occur at both the high and low points of the waveform, which is unsuitable for a sinusoidal output. The loop gain, which is larger than one, will hasten this saturation. The oscillator will restrict the two peaks attached to the waveform if the gain in the amplifier is larger than 100.

An amplifier must be a part of the circuit of the oscillator and a part of the amplifier’s output should be sent back to the input, for it to fulfil these requirements. However, to maintain the phase, along with the voltage of excitation at the point of input, the feedback must be positive. The feedback circuit is used to compensate for the input circuit’s losses, resulting in a gain equal to or higher than one. To avoid oscillation, the transistor amplifier must have a gain of less than unity (non-sinusoidal waveforms).

Conclusion

The tank circuit develops electrical oscillations when the principle of an oscillator circuit is completed. These vibrations are made louder by the circuit. Because L and L’ are coupled in the base circuit, a portion of the amplified signal is transmitted back into the collector circuit. The oscillation’s amplitude is amplified until power dissipation in the oscillatory circuit equals power fed-back as a result of this feedback. In this stage, the oscillation amplitude remains constant. A coil linked to the external circuit may transmit the oscillations through mutual induction. The circuit’s intrinsic electrical resistance helps to dampen the oscillations. There is a drop in frequency and eventually an end to oscillation as a result of this change in amplitude.