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The Best Electronics Tutorial for Colpitts Oscillator

Author: Apogeeweb
Date: 1 Sep 2021
Advantages of Colpitts Oscillator

Ⅰ Introduction

A Colpitts oscillator is one of several designs for LC oscillators, which employ a combination of inductors (L) and capacitors (C) to produce an oscillation at a specific frequency. It was invented in 1918 by American engineer Edwin H. Colpitts. The voltage divider made up of two capacitors in series across the inductor serves as feedback for the active device in the Colpitts oscillator.





Ⅰ Introduction

Ⅱ What a Colpitts Oscillator Contains?

Ⅲ How the Colpitts Oscillator Works?

Ⅳ Colpitts Oscillator vs Hartley Oscillator

Ⅴ Types of Colpitts Oscillator

5.1 Common Base Colpitts Oscillator

5.2 Common Emitter Colpitts Oscillator

5.3 Buffered Colpitts Oscillator

Ⅵ Advantages of Colpitts Oscillator

Ⅶ Applications of Colpitts Oscillator

Ⅷ Conclusion

Ⅸ Frequently Asked Questions about Colpitts Oscillator

Ⅱ What a Colpitts Oscillator Contains?

The Colpitts circuit, like other LC oscillators, is made up of a gain device (such as a bipolar junction transistor, field-effect transistoroperational amplifier, or vacuum tube) with its output connected to its input in a feedback loop containing a parallel LC circuit (tuned circuit) that serves as a bandpass filter to set the oscillation frequency. The amplifier's input and output impedances will be different, and these must be linked into the LC circuit without overdamping it.


Ⅲ How the Colpitts Oscillator Works?

The Colpitts oscillator is commonly used in RF applications, with a frequency range of 20KHz to 300MHz. The capacitive voltage divider configuration in the tank circuit serves as the feedback source in the Colpitts oscillator, and this arrangement provides superior frequency stability than the Hartley oscillator, which uses an inductive voltage divider system for feedback. The circuit diagram of a typical transistor-based Colpitts oscillator is shown below.


Colpitts oscillator


The resistors R1 and R2 in the circuit schematic provide a voltage divider biasing for the transistor. The transistor's collector current is limited by the resistor R4. The input DC decoupling capacitor is Cin, and the output decoupling capacitor is Cout. The emitter resistor, Re, is used to ensure thermal stability. The emitter by-pass capacitor is denoted by Ce. The emitter by-pass capacitor's job is to keep the amplified AC signals from crossing Re. If the emitter by-pass capacitor is missing, the amplified AC signal will drop across Re, causing the transistor's DC biasing conditions to change, resulting in lower gain. The tank circuit is made up of capacitors C1, C2, and inductor L1.


Tank circuit in a Colpitts oscillator


When the power source is turned on, the capacitors C1 and C2 begin to charge. They start discharging through the inductor L1 when they are completely charged. The electrostatic energy stored in the capacitors is transmitted to the inductor as magnetic flux when the capacitors are fully drained. The inductor begins to discharge and the capacitors are re-charged. Oscillation is caused by energy being transferred back and forth between capacitors and inductors. The voltage across C2 is in phase opposite that of C1, and the voltage across C2 is sent back to the transistor. The enhanced feedback signal at the transistor's base emerges across the collector and emitter.


The transistor compensates for the energy lost in the tank circuit, maintaining the oscillations. One 180° phase shift is produced by the tank circuit, and the other 180° phase shift is produced by the transistor. That means the input and output are in phase, and positive feedback requires to keep oscillations going for long periods. The equation below can be used to calculate the frequency of the Colpitts oscillator's oscillations.


Where L is the inductance of the tank circuit's inductor and C is the effective capacitance of the tank circuit's capacitors. The effective capacitance of the serial combination C= (C1C2)/(C1+C2) if C1 and C2 are independent capacitances. The Colpitts oscillator can be made variable by utilizing ganged variable capacitors in place of C1 and C2.


Ⅳ Colpitts Oscillator vs Hartley Oscillator

The Colpitts oscillator is extremely similar to the Hartley oscillator, however they are constructed differently. The Colpitts oscillator employs a single inductor in parallel with two capacitors in series, whereas the Hartley oscillator utilizes the exact opposite, one single capacitor in parallel with two inductors in series. In high-frequency operation, the Colpitts oscillator is more stable than the Hartley oscillator.


Colpitts Oscillator vs Hartley Oscillator


In high-frequency operation, the Colpitts oscillator is an ideal choice. It can generate output frequencies in the Megahertz and Kilohertz ranges.


Ⅴ Types of Colpitts Oscillator

5.1 Common Base Colpitts Oscillator

A typical Colpitts oscillator design is shown below. The Colpitts LC tank circuit operates similarly to the Hartley oscillator, however it only has a single inductor and two capacitors. Instead of the tapped inductor used in the Hartley, the capacitors create a single 'tapped' capacitor. The total capacitance in series (CTOT) of the two capacitors (connected in series) is calculated as follows:


common base Colpitts oscillator


The total capacitance required for the tank circuit to achieve parallel resonance at the specified frequency is given. The oscillation frequency is calculated using the same formula as the Hartley oscillator.


However, in this case, the number C is the sum of the values C2 and C3 in order (CTOT).


C2 and C3's values are chosen so that their ratio delivers the required proportion of feedback signal. The ratio of voltages across two capacitors in series, on the other hand, is inversely proportional to the ratio of their values, implying that the smaller capacitor has a higher signal voltage across it. The fundamental advantage of the Colpitts design is that the single inductor in the tuned circuit eliminates any mutual inductance between two coils, where the alternating magnetic field generated up around one inductor drives a current into the inductor of the other coil. This alters the resonance frequency of the tuned circuit by changing the total inductance of the coils.


5.2 Common Emitter Colpitts Oscillator

The Colpitts analog of the Common Emitter Hartley Oscillator is shown below.



   common emitter Colpitts oscillator


It employs a common emitter amplifier, and because the tuned (tank) circuit tapping point is connected to the ground in this design, the tank circuit generates anti-phase waves at the top and bottom of L2, ensuring proper phase relationships for positive feedback between collector and base. The feedback is delivered to the base via C1, which also functions as a DC block, preventing the greater voltage on L1 from causing the base bias voltage to be thrown off.


The supply rail (+Vcc) is connected to the tank circuit (L2, C2, and C3) through L1. Because the DC supply is significantly decoupled by huge capacitors in the DC Power supply, if the tank circuit were connected directly to the supply, there would be no anti-phase AC signal present at the top of the tank circuit. As a result, between the tuned circuit and the supply, an RF choke (L1) with a high impedance at the oscillation frequency is provided. This permits the development of a signal voltage across L1 for feedback purposes.


Automatic class C bias is utilized, with the emitter only partially disconnected by a small amount of C5 to provide the previously mentioned "slide bias."


The Colpitts oscillator, like the Hartley, can produce an excellent sine wave shape and has the added benefit of improved stability at very high frequencies. It's easy to spot because it's always got a "tapped capacitor" on it.


The fact that any load placed on the output by circuits that the output is supplying essentially inserts a dampening resistance across the tank circuit complicates the design of a sine wave oscillator. This can have an adverse influence on both the wave shape and frequency stability of the oscillator waveform, as well as lowering the amplitude of the oscillator output by lowering the Q factor of the tuned tank circuit.


5.3 Buffered Colpitts Oscillator

As demonstrated below, feeding the oscillator output into an emitter follower buffer amplifier is a standard technique.


buffered Colpitts oscillator


TR1's load impedance has been changed to the RF choke, and the tank circuit is now isolated from TR1 by two DC blocking capacitors, C1 and C4. As a result, instead of a tuned amplifier, this variant of the Colpitts oscillator uses a tuned feedback channel. The emitter follower stage (R4, TR2 and R5) has a very high input impedance, which has no effect on the oscillator, and a very low output impedance, which allows it to drive loads with impedances as low as a few tens of ohms.


Variations in supply voltage can also affect the frequency stability of oscillators. When good frequency stability is required, it is typical to use a stabilized power supply. Extra decoupling capacitors may be required for oscillator supplies to reduce undesired 'noise.' Automatic class C bias, which is given in this circuit by only partially disconnecting the emitter of TR1 by C5, is generally used to achieve stable amplitude.


Ⅵ Advantages of Colpitts Oscillator

  • The Colpitts oscillator may produce very high-frequency sinusoidal pulses.

  • It can tolerate extreme heat and cold.

  • There is a lot of frequency stability.

  • Both variable capacitors can be used to change the frequency.

  • A small number of components is all that is required.

  • Over a certain frequency range, the output amplitude remains constant.

  • The Colpitts oscillator was created to address the shortcomings of the Hartley oscillator and is known to have no unique flaws. As a result, a Colpitts oscillator has a wide range of uses.


Ⅶ Applications of Colpitts Oscillator

  • The Colpitts oscillator is mostly employed for fixed frequency generation due to the challenges in achieving a smooth variation of inductor and capacitor.

  • The Colpitts oscillator is most commonly found in mobile phones and other radio frequency-controlled communications devices.

  • The Colpitts oscillator is a great choice for high-frequency oscillation. Colpitts Oscillator is used in high-frequency oscillator-based systems.

  • Colpitts Oscillator is utilized in a few applications where continuous and undamped oscillation is required as well as thermal stability.

  • For applications that require a broad range of frequencies with minimal noise.

  • Colpitts oscillator is used in a variety of SAW-based sensors.

  • The Colpitts oscillator is used in a variety of metal detectors.

  • A Colpitts oscillator is used in frequency modulation radio frequency transmitters.

  • It has a wide range of uses in both military and commercial items.

  • Signal masking-related chaotic circuits are also required in microwave applications Colpitts oscillator in various frequency ranges.


Ⅷ Conclusion

To summarise, the Colpitts Oscillator consists of a parallel LC resonator tank circuit whose feedback is achieved by way of a capacitive divider. The Colpitts oscillator exists in several forms like most oscillator circuits, and the most common form is the transistor circuit. The tank sub-center circuit's tap is made at the junction of a "capacitive voltage divider" network, which feeds a fraction of the output signal back to the transistor's emitter. The 180o phase shift produced by the two capacitors in series is inverted by another 180o to produce the requisite positive feedback. The resonance frequency of the tank circuit determines the oscillation frequency, which is a purer sine-wave voltage.


Ⅸ Frequently Asked Questions about Colpitts Oscillator

1.What is the use of Colpitts oscillator?

It is used for generation of sinusoidal output signals with very high frequencies. The Colpitts oscillator using SAW device can be used as the different type of sensors such as temperature sensor. As the device used in this circuit is highly sensitive to perturbations, it senses directly from its surface.


2.What is the basic principle of oscillator?

There are many types of electronic oscillators, but they all operate according to the same basic principle: an oscillator always employs a sensitive amplifier whose output is fed back to the input in phase. Thus, the signal regenerates and sustains itself. This is known as positive feedback.


3.What is meant by Colpitts oscillator?

A Colpitts oscillator, invented in 1918 by American engineer Edwin H. Colpitts, is one of a number of designs for LC oscillators, electronic oscillators that use a combination of inductors (L) and capacitors (C) to produce an oscillation at a certain frequency.


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