Oscillator
Definition: A device that converts a direct current or an uncontrolled alternating current into an alternating signal of a specific frequency is called an oscillator.
Feedback
Let the voltage amplifier shown in figure have a gain of A. So, if the input voltage is VS and the output voltage is Vo,
Now, a feedback circuit is added between the points P and Q of this amplifier circuit. The potentials at points P and Q are controlled so that a portion of the output voltage Vo (say, βVo) is returned to the input through the feedback circuit. This phenomenon is called feedback. β is called the feedback ratio; Obviously, β < 1.
In this case, the effective input voltage of the amplifier circuit, Vi = Vs + βVo
That is, the output voltage,
Vo = AVi = A(Vs + βVo) = AVs + AβVo
or, Vo – AβVo = AVs or, Vo(1 – Aβ) = AVs
Thus, the effective amplification of the feedback-coupled amplifier circuit,
In general, the inherent gain A of the amplifier is called the open loop gain and the effective gain Af due to feedback is called the closed loop gain. The term Aβ is called the loop gain.
Negative feedback
If the loop expansion Aβ is real and negative, then (1 – Aβ) > 1 and Af < A according to equation number (2). Such feedback results in a relatively low value of the effective gain Af relative to the amplifier’s magnification A; This is called negative feedback. However, despite the gain reduction, negative feedback in amplifier circuits is common for a few special advantages: 1. It is possible to set the gain to a constant value, 2. Distortion of the output signal concerning the input signal can be eliminated, 3. It is possible to brush out the internal noises of the amplifier, 4. Effective bandwidth increases etc.
Positive feedback: Barkhausen criterion
If the loop gain Aβ is real, positive, and less than 1, then (1 – Aβ) < 1 and Af > A. This results in a higher value of effective gain Af relative to the inherent gain A of the amplifier. This is called positive feedback.
In general, amplifiers and feedback circuits use reactive elements such as inductors or capacitors. As a result, both A and β become complex expressions rather than real expressions i.e., these expressions contain a phase factor in addition to the value.
Let us assume that the components of an amplifier and positive feedback circuit are chosen such that the following condition is satisfied:
In this case, from equation number (2), Af → ∞ i.e., the effective gain of the amplifier is infinite. As a result, an output signal with a specific frequency is generated at the output even if there is no external input signal. This mechanism is called an oscillator. The condition mentioned in equation number (3) is called the Barkhausen criterion of oscillation. This condition means that the value of the loop gain in the circuit, |AΒ| = 1, and the phase shift due to a full feedback cycle is zero (or, any integer multiple of 2π). Keeping the components used in the circuit unchanged, condition (3) holds for only one particular frequency (say, f0).
Only for this frequency does the value and phase of the feedback voltage equal the value and phase of the input voltage so that the feedback voltage effectively behaves equivalent to the input signal. An external input signal is no longer required to obtain the output signal.
Thus, an oscillator can produce an output signal of a specific frequency without any external input signal. This is why the oscillator can be called a self-sustaining device.
‘Origin of output without input’—This statement applies only to signals. Considering the law of conservation, it can be seen that to obtain a constant alternating voltage or alternating current of a certain frequency at the output, a source of power supply must be connected to the input. Generally, a DC source or an AC source of unregulated frequency is used for this purpose.
Classification of oscillators
Depending on the type of active mechanism by which the vibration is generated, the oscillators are divided into two categories: 1. Feedback oscillator, and 2. Negative resistance oscillator. Again, depending on the range of frequency generated from an oscillator, the oscillator can be classified as an audio frequency oscillator, radio frequency oscillator, etc.
In the case of sinusoidal oscillators, it is also common to name LC oscillator, RC oscillator, crystal oscillator, etc. according to the circuit used to determine the frequency.
LC feedback oscillator
An LC circuit (L is an inductor and C is a capacitor) is used as the feedback circuit for the feedback amplifier shown in figure.
As can be seen from the analysis, such a circuit produces a pulsating alternating current or voltage of a constant frequency. The value of this active frequency is,
Now the DC source applied to the input can be considered. No matter how stable the DC value from such a source is, there is always some fluctuation or fluctuation mixed with the feedback operation; These are called noise. The fluctuation of each noise can be analyzed as a sum of many different frequency sinusoidal waves. Each such sinusoidal wave is amplified through amplifier A and reaches point Q at the output terminal. Then the LC feedback circuit brings only the frequency f0 described in equation (4) to point P, the corresponding wave is again amplified by amplifier A. The LC circuit behaves as a rejector circuit for all frequencies other than f0, resulting in no feedback for waves at other frequencies. Thus, only the wave of frequency f0 undergoes repeated feedback and amplification and eventually reaches a steady state at the output end. All other frequencies at the output are negligible; In other words, the LC feedback oscillator generates a transient wave with constant frequency f0. A frequency-determining LC circuit is called a tank circuit. Note that the noise at the input end is the source of the fixed-frequency output wave.
If the value of the components of the LC circuit in the figure is changed, the value of f0 is also changed according to equation number (4). As a result, by controlling the quality of these components, it is possible to produce transitional waves of other frequencies from the oscillator. In particular, the LC feedback oscillator becomes a variable frequency oscillator by continuously varying the capacitance value of capacitor C. An oscillator used to produce more complex waveforms such as square waves, triangular waves, etc. instead of sine waves is commonly called a multivibrator.
Designing of an oscillator using transistor amplifier
An example of how an n-p-n transistor is used as an oscillator is shown in Fig.
This oscillator is a frequency-directional tank circuit consisting of a capacitor C and a pair of coils L and L’ (mutual inductance = M). It is this junction that acts as feedback from the collector output to the base input. As we know, a common emitter (CE) junction transistor produces a 180° phase difference between input and output. Now, the elements of the tank circuit are selected such that the feedback again produces a 180° phase difference. As a result, the input voltage and the feedback voltage are in the same phase.
This oscillator is called the tuned collector oscillator. Typically, this type of oscillator is used to obtain high-frequency (1 MHz scale) transient output. Incidentally, several other types of oscillators made of transistors are also used in various applications.