Transistor as an oscillator

Transistor as an oscillator

An electronic oscillator basically converts dc energy into ac energy of high frequency ranging from a few Hz to several MHz. Hence, it is a source of alternating current or voltage. Unlike an amplifier, oscillator does not require any external signal source.

Basically, there are two types of oscillators: Sinusoidal and non-sinusoidal. Sinusoidal oscillators generate oscillations in the form of sine waves at constant amplitude and frequency as shown in Figure 9.37(a). Whereas non-sinusoidal oscillators generate complex non-sinusoidal waveforms like  Square-wave, Triangular-wave or Sawtooth- wave as shown in Figure 9.36(b).

Sinusoidal oscillations can be of two types: Damped and undamped. If the amplitude of the electrical oscillations decreases with time due to energy loss, it is called damped oscillations as shown in Figure 9.38(a). On the other hand, the amplitude of the electrical oscillations remains constant with time in undamped oscillations as shown in Figure 9.38(b).

Transistor Oscillator

An oscillator circuit consists of a tank circuit, an amplifier and a feedback circuit as shown in Figure 9.39. The tank circuit generates electrical oscillations and acts as the ac input source to the transistor amplifier. Amplifier amplifies the input ac signal. The feedback circuit provides a portion of the output to the tank circuit to sustain the oscillations without energy loss. Hence, an oscillator does not require an external input signal. The output is said to be self-sustained.

Amplifier

The transistor amplifier circuit is already explained in section {9.4.5}.

Feedback network

The circuit used to feedback a portion of the output to the input is called the feedback network. If the portion of the output fed to the input is in phase with the input, then the magnitude of the input signal increases. It is necessary for sustained oscillations.

Tank circuit

The LC tank circuit consists of an inductance and a capacitor connected in parallel as shown in Figure 9.39. Whenever energy is supplied to the tank circuit from a DC source, the energy is stored in inductor and capacitor alternatively. This produces electrical oscillations of definite frequency. (Refer section 4.9.1, Volume 1 of XII std. Physics text book)

But in practical oscillator circuits there will be loss of energy across resistors, inductor coils and capacitors. A small amount of energy is used up in overcoming these losses during every cycle of charging and discharging of the capacitor. Due to this, the amplitude of the oscillations decreases gradually. Hence, the tank circuit produces damped electrical oscillations. Therefore, in order to produce undamped oscillations, a positive feedback is provided from the output circuit to the input circuit.

The frequency of oscillations is determined by the values of L and C using the equation.

Barkhausen conditions for sustained oscillations

The following condition called Barkhausen conditions should be satisfied for sustained oscillations in the oscillator.

• The loop phase shift must be 00 or integral multiples of 2π.

• The loop gain must be unity. |Aβ| =1

Here, A→Voltage gain of the amplifier,

 Î² →feedback ratio; (fraction of the output that is fed back to the input)

There are different types of oscillator circuits based on the different types of tank circuits. Examples: Hartley oscillator, Colpitt’s oscillator, Phase shift oscillator, and Crystal oscillator.

Applications of oscillators

• to generate a periodic sinusoidal or non sinusoidal wave forms

• to generate RF carriers

• to generate audio tones

• to generate clock signal in digital circuits

• as sweep circuits in TV sets and CRO

EXAMPLE 9. 9

Calculate the range of the variable capacitor that is to be used in a tuned-collector oscillator which has a fixed inductance of 150 μH. The frequency band is from 500 kHz to 1500 kHz.