What is a Colpitts Oscillator?
A colpitts oscillator is a one of the type of LC oscillator. The colpitts oscillator is a combination of inductor and capacitors to produce an oscillating frequency. In colpitts oscillator the feedback are taken from a voltage divider made of two capacitors in series across the inductor but in Hartley oscillator the feedback is taken out from voltage divider made of two inductors in series across the capacitor.
A colpitts oscillator is invented by American engineer Edwin H. Colpitts, in 1918. It is use to generate radio frequency. The colpotts oscillator is exact opposite of the Hartley oscillator as we look in previous tutorial.
Colpitts Tank Circuit
The basic configuration of collpitts is same as Hartley oscillator but the difference this time is that the tank circuit is made of junction of two “capacitive voltage divider” network instead of two inductive voltages (tapped autotransformer) in the Hartley oscillator.
The feedback circuit uses a capacitive voltage divider network. Two capacitors, C1 and C2 are in series and these series capacitors are parallel with inductor. The C1, C2 and L create a tank circuit. The condition for oscillations being: XC1 + XC2 = XL, is same as for the Hartley oscillator circuit.
The main advantage of this type of capacitive circuit is that with less mutual inductance and self inductance within the tank circuit, the frequency stability of this oscillator is improve with a more simple design.
Colpitts Oscillator Circuit
The circuit diagram of colpitts oscillator are shown below, the circuit contains common emitter amplifier circuit and tank circuit. Resistors R1 and R2 is a voltage divide to bias amplifier. RC is a collector resistance and RE in emitter resistance. The capacitors Ci and Co are input and output capacitors for blocking the DC current and CE is a bypass capacitor, it bypass the alternating current. The one end of capacitors is joining together and other end of C1 is connected to collector of transistor via C0 and C2 is connected to base of transistor. the feedback output are taken out through this path.
Working of Colpitts Oscillators
Now let us discuss the working of colpitts oscillator. Firstly, switch ON the power supply, transistor starts conduction. The collector current Ic increases due to which the capacitors C1 and C2 get charged. When acquiring the maximum charge feasible these capacitors are start discharge through inductor.
During this process electrostatics energy store in capacitor gets converts in to magnetic flux. This is store in inductor L. When inductor is fully charge it start discharging through capacitor. And again capacitor will start charging. Likewise, the charging and discharging of tank circuit element will continue the oscillations.
From the figure the output is present is across C1 and thus is in-phase with the tank circuit’s voltage and makes-up for the energy lost by re-supplying it.
On the other hand, the voltage feedback to the transistor is obtained across the capacitor C2, which means the feedback signal is out-of-phase with the voltage at the transistor by 180o.
The voltage across C1 and C2 are opposite in polarity as the point where they join is grounded.
Further the signal are provided 180 degree phase shift by transistor. The result of total phase shift is 0 degree or 360 degree. Which is satisfied the Barkhausen principle.
Colpitts Oscillator Frequency
The frequency of colpitts oscillator is determined by the tank LC circuit. The formula of oscillating frequency is given as:
where CT is the total capacitance of C1 and C2 connected in series and is given as:
The transistor use common emitter configuration with the output is 180 degree phase shift on input. Another 180 degree phase shift provide by feedback circuit which is two capacitors are in series with the inductor. The resultant phase shift is 0 degree of 360 degree.
The amount of feedback is depends on the value of C1 and C2. The voltage across the C1 is same as the oscillator’s output voltage and voltage across C2 is the oscillator feedback voltage. Then the voltage across C2 will be much lesser than that across C1.
Therefore the changing the value of capacitors we can adjust the amount of feedback voltage returned to the tank circuit. However the large amount of feedback may cause of distorted sinusoidal wave, while small amount of feedback may not allow to oscillator circuit.
The amount of feedback of colpitts oscillator is depends on capacitors C1 and C2. The ratio of C1 and C2 is called feedback fraction and it is given as:
Colpitts Oscillator Op-amp Circuit
In non inverting amplifier configuration the ratio of R2/ R1 sets the amplifiers gain. The minimum gain required of oscillation is 2.9. In the figure the resistor R3 is provide the feedback path to the LC tank circuit.
The advantage of colpitts oscillator over the Hartley oscillator is that the colpitts oscillator produces a more pure sinusoidal waveform due to the low impedance paths of the capacitors at high frequencies. Also due these capacitive reactance properties the FET based Colpitts oscillator can operate at very high frequencies. Of course any operational and field effect transistor is use as a amplifying device must be able to operate at the required high frequencies.
Colpitts Oscillator Summary
The colpitts oscillator consist of two capacitors are connected in series and this series capacitors are in parallel with inductor. The midpoint of series is connected to ground and one end of capacitor C1 is connected to output amplifier and C2 end of capacitor provide feedback to input of amplifier.
The common emitter configuration provides 180 degree phase shift and another phase shift provides by feedback circuit. Hence totoal phase shift is 0 degree or 360 degree. The pure sine wave is determined by the LC tank circuit.
The advantages of Colpitts oscillator are given below −
- It is best advantage for generation sinusoidal signals with very high frequencies.
- Frequency can be varied by capacitors.
- Less number of components is required.
- It can operate high and low temperatures.
- The output amplitude is constant over a fixed frequency range.
- It has high frequency stability.
The drawback of Hartley oscillator is removing by using colpitts oscillator. The colpitts oscillator provides constant amplitude over fixed frequency range.
The applications of colpitts oscillator are given as:
- It is use for High frequency sine wave generator.
- It is used as R.F. Oscillator.
- It is best for temperature sensor with some associated circuitry.
- In Mobile applications.
- It is use in radio receivers.
- It is use in many consumer applications.