How Feedback Circuits Enable Oscillations in Transistor Amplifiers
Electronic devices function on a tank circuit to enable the sharing of information. Generally, an amplifier with a sinusoidal input attains an amplified output signal. In a feedback amplifier and transistor oscillator, the oscillator generates an amplified output signal without any intake of input signals. The working of an oscillator is a repetitive process with the amplified input and output resulting in feedback with persistent operations. This ensures the transmission of information signals back and forth in an electronic device without any interval.
This infers a single input lead with endless outputs based on the feedback and frequency regulated, the external signal delivers an alternating current which is self-sustainable.
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Types of Feedback Amplifiers
In the working of an oscillator, feedback refers to the ability of the output signal to return to the input. There are 2 types of feedback amplifiers:
Positive Feedback Amplifier
As shown in the image, Vin is the input signal sourced through the transistor to Vout is the output, further, the succeeding network formed with the sourcing of Vout back to Vin is positive feedback indicated with Vf in the figure. These positive feedback amplifiers are utilized for oscillations.
Negative Feedback Amplifier
On the contrary, a negative feedback amplifier indicates the incapacity of the (Vout) output signal to return to the (Vin) input signal.
Types of Transistor Oscillators and Functions
Let's quickly take a look at some types of transistor oscillators and how they function.
Working of Colpitts oscillator
A variation of an oscillator tank circuit formed with 2 capacitors and 1 inductor. Its connectivity can be achieved in series allowing the inductor to be placed in a parallel position to the capacitors.
The working of the Colpitt oscillator was invented in 1918 and named after scientist Edwin Colpitts. As compared to the working principle of the Hartley oscillator, it stimulates preferable frequency stability.
Working Principle of Hartley Oscillator
Hartley Oscillator is a tank circuit composing 2 inductors and one capacitor. The inductors are linked in a combining series whereas the capacitor is positioned parallel to the series of inductors. It was invented in the year 1915 and named after an American scientist Ralph Hartley. It generally operates with frequencies ranging from 20 kHz to 20MHz.
Working of Wien Bridge Oscillator
A Wein bridge oscillator is a bridge circuit formed with 4 resistors and 2 capacitors. It produces sine waves with largely ranging frequencies. The working of the Wien bridge oscillator was formulated and named after Max Wein in the year 1891 for measuring impedances.
Now that we have analyzed and understood the functioning of feedback amplifier and transistor oscillator, we've comprehended the functioning of varied oscillations like the working of Colpitts oscillator, working principle of Hartley oscillator, and working of Wien bridge oscillator.
One of the factors that remain constant is the positive feedback gained to achieve repetitive and long-term processing of signals.
Clapps Oscillator.
The Clapps oscillator consists of three capacitors and an inductor that is already set to the oscillator frequency. The Clapps oscillator is also known as the Gouriet oscillator. This is named after its founder James Klinton Clapp. Although, it is said that these kinds of oscillators were built by several independent persons. One among them was Gouriet. The Clapps oscillator has excellent frequency stability.
Robinson Oscillator
The Robinson oscillator is a further development of the already existing marginal oscillator. It is therefore often referred to as marginal Robinson oscillator. A British physics scientist named Neville Robinson is behind the invention of this oscillator.
Dynatron Oscillator
The dynatron oscillator was invented by a scientist named Albert Hull in 1918.
Like the Clapps oscillator, the dynatron oscillator also has better frequency stability. The dynatron oscillator can oscillate between a huge range of frequencies and this can be counted as one of its many advantages.
Phase Shift Oscillator
A phase shift oscillator is a combination of inverting amplifiers, resistors and capacitors. These oscillators are also called auto oscillators as they are mainly used for audio frequencies.
Pierce Oscillator
The Pierce oscillator is derived from the existing Colpitts oscillator. The name of the oscillator is kept after its inventor George. W. Pierce.
The components of the pierce oscillator are one resistor, two capacitors, and a quartz crystal. All the digital clocks are run by a Pierce oscillator. It is a quartz oscillator.
Optoelectronic Oscillator
The optoelectronic oscillator is also known as OEO and is based on the concept of transforming light energy into microwave signals. The optoelectronic oscillator is known to have stability as well as a high-quality factor, among various other factors that aren't generally found in basic electronic oscillators. This type of oscillator also has photonic components and is known to operate at high speed. It is an optoelectronic circuit known to modulate optical continuous wave signals.
Armstrong oscillator
The Armstrong oscillator was invented by a US engineer named Edwin Armstrong and other Australian engineers Alexander Meissner. However, both of them have invented it independently, so the oscillator is also named the Meissner oscillator. The feature that makes it unique is the tickle coil that is used, hence sometimes it's also called a tickler oscillator. This electronic oscillator uses a combination of a capacitor and an inductor to produce oscillations.
Solved Example
In an Oscillator Working Principle, What is Positive Feedback?
Answer- Positive feedback adopted in the working of an oscillator stimulates an output frequency without the implementation of any input. Positive feedback boosts the output signal by charging a quicker and higher signal in the direction of the input. It functions in a loop permitting continuous and undamped oscillations. Following the principle 'more produces more' it is utilized in procedures like fruit ripening and contractions in childbirth. Amplification in an oscillator only administers with positive feedback as it feeds the output signal back to the input to model it to be in phase, further the feedback and input enhance the amplifier.
Did You Know?
A classic based on feedback control theories published in 1868 was the first-ever written theory relating to feedback by James Clerk Maxwell as a popular paper named 'On governors'.
The verbal application of the word 'feedback' was implemented in the US in the 1860s, however, the official usage of the word 'feedback' as a noun was witnessed in the year 1909 by Nobel laureate Karl Ferdinand Braun referring to bonding of the elements in an electronic circuit.
FAQs on Feedback Amplifier Transistor Oscillator: Complete Guide
1. What is an electronic oscillator and what is its primary function?
An electronic oscillator is a circuit that produces a continuous, repetitive, alternating waveform (like a sine wave or square wave) without any external input signal. Its primary function is to convert direct current (DC) from a power supply into an alternating current (AC) signal of a specific frequency. These are fundamental components in devices like radios, clocks, and computers.
2. How does a simple transistor amplifier become an oscillator?
A transistor amplifier can be converted into an oscillator by feeding a portion of its output signal back to its input. This process is called feedback. For oscillation to occur, the feedback must be positive (regenerative), meaning the feedback signal must be in phase with the original input signal, thereby reinforcing it and leading to sustained oscillations.
3. What is the Barkhausen criterion for sustained oscillations?
The Barkhausen criterion defines the two essential conditions that a circuit must meet to operate as an oscillator and produce sustained, stable oscillations. As per the 2025-26 NCERT syllabus, these conditions are:
The total phase shift around the feedback loop must be 0° or an integer multiple of 360°.
The magnitude of the loop gain (the product of the amplifier's gain 'A' and the feedback factor 'β') must be equal to or slightly greater than one (i.e., |Aβ| ≥ 1).
4. What is the fundamental difference between an amplifier and an oscillator?
The fundamental difference lies in their input signal and feedback type. An amplifier requires an external AC input signal, which it strengthens using negative feedback for stability. In contrast, an oscillator is a self-starting circuit that generates its own AC signal without any external AC input, using positive feedback to sustain the oscillations.
5. Why is a transistor a crucial component in a feedback oscillator circuit?
A transistor is the active component in a feedback oscillator. Its primary role is to function as an amplifier. The circuit's passive components (like resistors, capacitors, and inductors in the tank circuit) naturally lose energy with each cycle. The transistor takes DC power and provides the necessary amplification (gain) to the feedback signal, compensating for these energy losses and ensuring the oscillations do not die out.
6. What are the main types of transistor feedback oscillators based on their frequency-determining components?
Transistor feedback oscillators are primarily classified based on the components used in their feedback (or tank) circuit. The main types include:
LC Oscillators: Use inductors (L) and capacitors (C) to determine the frequency. Common examples are the Hartley Oscillator (using a tapped inductor) and the Colpitts Oscillator (using a tapped capacitor).
RC Oscillators: Use resistors (R) and capacitors (C) for frequency control. A common example is the RC Phase-Shift Oscillator.
7. What are some common real-world applications of transistor oscillators?
Transistor oscillators are essential in a vast range of electronic devices. Some key applications include:
Radio and TV Broadcasting: For generating the high-frequency carrier waves for transmitters and local oscillators in receivers.
Digital Systems: As clock generators in computers, microprocessors, and digital watches to synchronise operations.
Signal Generators: Used in laboratories and workshops to produce signals for testing other electronic circuits.
Induction Heating: High-power oscillators are used to generate high-frequency currents for heating metals.
8. If positive feedback creates oscillations, what is the purpose of negative feedback in electronics?
While positive feedback creates instability to generate oscillations, negative feedback is used to create stability and control in electronic circuits, particularly in amplifiers. Its primary purposes are to:
Reduce distortion and noise.
Increase the bandwidth of the amplifier.
Stabilize the amplifier's voltage gain, making it independent of transistor parameters or temperature variations.
9. What is a relaxation oscillator and how is its output different from an LC oscillator?
A relaxation oscillator is a type of oscillator that produces a non-sinusoidal repetitive waveform, such as a square wave, triangular wave, or sawtooth wave. It works on the principle of repeatedly charging and discharging a capacitor through a resistor. This is different from a typical LC oscillator (like Hartley or Colpitts), which is designed to produce a smooth, purely sinusoidal waveform by the resonant exchange of energy between an inductor and a capacitor.
