Oscillators Circuit Diagrams

Voltage Controlled Colpitts Oscillator Circuit Diagram

The Voltage Controlled Colpitts Oscillator Circuit Diagram refers to an electronic circuit that generates periodic oscillating electronic signals, such as sine waves, square waves, or other waveforms. Oscillators are categorized based on their output frequency, and electronic oscillators, known as voltage controlled oscillators, have their oscillation frequency controlled by input voltage. These voltage controlled oscillators can be classified into two main types: Linear Oscillators and Nonlinear Oscillators.

Voltage Controlled Colpitts Oscillator Circuit Diagram

Voltage Controlled Colpitts Oscillator Circuit Diagram

Nonlinear oscillators are employed to generate output waveforms that are non-sinusoidal in nature. On the other hand, linear oscillators are designed to produce sinusoidal output waveforms and are further divided into various types, including Feedback oscillators, Negative resistance oscillators, Colpitts oscillators, Hartley oscillators, Armstrong oscillators, Phase shift oscillators, Clapp oscillators, Delay line oscillators, Pierce oscillators, Wien bridge oscillators, Robinson oscillators, and more. This article specifically focuses on one type of linear oscillator circuit, namely the Colpitts Oscillator.

Colpitts Oscillator

An oscillator is essentially an amplifier that employs positive feedback to transform a DC input signal into an AC output waveform. It achieves this by generating a variable frequency drive and shaping the output waveform, whether it be a sine wave or square wave, using positive feedback instead of an input signal. Oscillators that incorporate inductors (L) and capacitors (C) in their circuits are referred to as LC oscillators, representing a specific type of linear oscillator.

Colpitts Oscillator

Various methods can be employed to design LC oscillators, with the Hartley oscillator and Colpitts oscillator being notable examples. Among these, the Colpitts Oscillator, developed and named after the American Engineer Edwin H. Colpitts in 1918, is the more commonly used design.

Colpitts Oscillator Theory

The Colpitts Oscillator comprises a tank circuit, an LC resonance subcircuit formed by two series capacitors connected in parallel with an inductor. The frequency of oscillations is determined by the values of these capacitors and the inductor in the tank circuit.

This oscillator closely resembles the Hartley oscillator in all aspects, making it the electrical dual of the Hartley design. It is specifically engineered for generating high-frequency sinusoidal oscillations within the radio frequency range, typically spanning from 10 KHz to 300 MHz. The primary distinction between these two oscillators lies in their component configuration: the Colpitts oscillator employs tapped capacitance, whereas the Hartley oscillator utilizes tapped inductance.

Colpitts Oscillator Circuit

Almost all oscillator circuits generating sinusoidal waveforms incorporate the LC resonant circuit, except for a handful of electronic circuits like RC oscillators, Wien-Robinson oscillators, and a few crystal oscillators that do not necessitate additional inductors for this function.

Circuit Diagram of Colpitts Oscillator

This oscillator can be implemented using gain devices such as Bipolar Junction Transistors (BJTs), operational amplifiers, and Field Effect Transistors (FETs), similar to other LC oscillators. Capacitors C1 and C2 create a potential divider, and the tapped capacitance in the tank circuit serves as the feedback source, offering enhanced frequency stability compared to the Hartley oscillator, which employs tapped inductance for feedback.

The resistor in the circuit stabilizes it against temperature variations. Capacitor Ce, connected parallel to Re, acts as a bypass capacitor, providing a low-reactance path for the amplified AC signal. Resistors R1 and R2 form a voltage divider in the circuit, supplying bias to the transistor. The circuit features an RC coupled amplifier with a common emitter configuration transistor. The coupling capacitor Cout prevents DC flow by establishing an AC path from the collector to the tank circuit.

Colpitts Oscillator Working

Upon activating the power supply, capacitors C1 and C2 depicted in the circuit above initiate charging. Once these capacitors reach full charge, they discharge through inductor L1 within the circuit, resulting in damped harmonic oscillations in the tank circuit.

Tank Circuit with Capacitors and Inductors

Consequently, the oscillatory current in the tank circuit generates an AC voltage across C1 and C2. As these capacitors discharge completely, the electrostatic energy stored in them transforms into magnetic flux, charging the inductor. Conversely, when the inductor discharges, the capacitors recharge, continuing this energy exchange cycle. This process of charging and discharging capacitors and inductor sustains oscillations, and their frequency is determined by the tank circuit’s resonant frequency, comprising the inductor and capacitors. This tank circuit serves as an energy reservoir due to the frequent energy exchange between the inductor and capacitors within the LC network forming the tank circuit.

The Barkhausen criterion ensures continuous undamped oscillations. To achieve sustained oscillations, the total phase shift must be either 360 degrees or 0 degrees. In the given circuit, since capacitors C1 and C2 are center tapped and grounded, the voltage across capacitor C2 (feedback voltage) is 180 degrees out of phase with the voltage across capacitor C1 (output voltage). The common emitter transistor introduces a 180-degree phase shift between the input and output voltage. Consequently, according to the Barkhausen criterion, undamped continuous oscillations are achieved.

The resonant frequency is given by


Where ƒr is the resonant frequency

C is the equivalent capacitance of series combination of C1 and C2 of the tank circuit

It is given as


L1 represents the self-inductance of the coil.

Applications of Colpitts Oscillator

  • It is used for generation of sinusoidal output signals with very high frequencies.
  • The Colpitts oscillator using the SAW device can be used as the different type of sensors such as a temperature sensor. As the device used in this circuit is highly sensitive to perturbations, it senses directly from its surface.
  • It is frequently used for the applications in which very wide range of frequencies is involved.
  • Used for applications in which undamped and continuous oscillations are desired for functioning.
  • This oscillator is preferred in situations where it is intended to withstand high and low temperatures frequently.
  • The combination of this oscillator with some devices (instead of tank circuit) can be used to achieve great temperature stability and high frequency.
  • It is used for the development of mobile and radio communications.
  • It has many applications used for commercial purposes.

Hence, this article discusses in brief about the Colpitts oscillator, theory, working and applications of Colpitts oscillator along with its tank circuit are used in free electronic project kits. For more information regarding the Colpitts oscillator, please post your queries by commenting below.


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