Colpitts Oscitor

The Colpitts Oscillator design uses two center-end capacitors in series with a parallel inductor to create the resonance tank circuit that produces sinusoidal oscillations.

In many ways, the Colpitts oscillator is the opposite of the Hartley Oscillator we examined in the previous lesson. Just like the Hartley oscillator, the tuned tank circuit consists of an LC resonance sub-circuit that connects between the collector and the base of a single-stage transistor amplifier that produces a sinusoidal output waveform.

The basic configuration of the Colpitts Oscillator is similar to that of the Hartley Oscillator, but the difference this time is that the center stroke of the tank sub-circuit is now at the junction point of a "capacitive voltage divider" network instead of a gradual autotransformer type inductor.

Colpitts Oscillator

The Colpitts oscilator uses a capacitive voltage dividing network as a feedback source. Two capacitors, C1 and C2, are placed in a single common inductor L, as shown. C1, C2 and L then create the tank circuit for oscillations with the following condition: XC1 + XC2 = XL

The advantage of this type of capacitive circuit configuration is that the frequency stability of the oscelator is improved with a simpler design with less core and mutual induction within the tank circuit.

As with the Hartley oscillator, the Colpitts oscillator uses a single-stage bipolar transistor amplifier as the gain element that produces a sinusoidal output.

Colpitts Oscillator
Basic Colpitts Oscillator Circuit

The transistor's emitter terminal is effectively connected to the connection of the two capacitors C1 and C2, which are connected serially and act as a simple voltage divider. When the power supply is applied for the first time, the C1 and C2 capacitors charge and then discharge through the L coil. Oscillations throughout the capacitors are applied to the base-transmitter connection and appear reinforced in the collector output.

Resistances, R1 and R2 normally provide normal stabilizer DC polarization for the transistor, while additional capacitors act as DC blocking bypass capacitors. A high reassurance (ideally open circuit) at the oscillation frequency (εr) and a radio frequency coil (RFC) are used in the collector circuit to provide low resistance in DC to help initiate oscillations.

The required external phase shift is achieved similar to that of the Hartley oscillator circuit with the positive feedback required for continuous non-dampened oscillations. The amount of feedback is determined by the ratio of C1 to C2. These two capacitances are usually combined to provide a fixed amount of feedback, so that when one is set, the other automatically follows the other value.

The frequency of oscillations for a Colpitts oscillator is determined by the resonance frequency of the LC tank circuit and given as follows:

Colpitts Oscillator

Where: CTis the capacitor of serially connected C1 and C2, and the following is calculated:

Colpitts Oscillator

The configuration of the transistor amplifier is a Common Emitter Amplifier with an output signal of 180o-phase according to the input signal. The additional 180o phase shifts required for oscillation are achieved by connecting the two capacitors to each other in series but in parallel with the inductive coil, and the total phase shift of the circuit is zero or 360o.

The amount of feedback depends on the values C1 and C2. We can see that the voltage on C1 is the same as the oscillator output voltage Vout, and the voltage on C2 is the oscillator feedback voltage. Then the voltage in C1 will be much greater than in C2.

Therefore, by changing the values of the C1 and C2 capacitors, we can adjust the amount of feedback voltage returned to the tank circuit. However, a large amount of feedback can cause the output sinus wave to deteriorate, while small amounts of feedback may not allow the circuit to be released.

The amount of feedback then generated by the Colpitts oscitor is based on the capacitance ratio of C1 and C2 and is what governs the stimulation of the oscilator. This rate is called the "feedback rate" and is simply given as follows:

Colpitts Oscillator

Feedback Fraction

Colpitts Oscitor Question Example 1

A Colpitts Oscillator circuit with two capacitors of 24nF and 240nF respectively is connected in parallel with a 10mH inductor. Determine the oscillation frequency of the circuit, the feedback fraction and draw the circuit.

The oscillation frequency of a Colpitts Oscillator is given as follows:

Colpitts Oscillator

Since the Colpitts circuit consists of two capacitors connected in series, the total capacitance is as follows:

Colpitts Oscillator

The inducor's inductive is given in 10mH, followed by the oscillation frequency:

Colpitts Oscillator

The frequency of oscillations for the Colpitts Oscillator is therefore 10.8kHz, and the feedback fraction is given as follows:

Colpitts Oscillator

Colpitts Oscillator Circuit

Colpitts Oscillator

Colpitts Oscitor Using Op-amp

Just like in the previous Hartley Oscillator, in addition to using a bipolar connection transistor (BJT) as the active stage of oscillators, we can also make a transactional amplifier (op-amp). The operation of an Op-amp Colpitts Oscillator is exactly the same as the transistor version with the operating frequency calculated in the same way.

Colpitts Oscillator
Colpitts Oscillator Op-amp Circuit

As a reversing amplifier configuration, note that the R2/R1 ratio adjusts the amplifier gain. A minimum gain of 2.9 is required to initiate oscillations. Resistance R3 provides the necessary feedback to the LC tank circuit.

The advantages of the Colpitts Oscillator over Hartley oscillators are that the Colpitts oscillator produces a purer sinusoidal waveform due to the low impedance pathways of capacitors at high frequencies. In addition, due to these capacitive reassurance properties, the FET-based Colpitts oscilator can operate at very high frequencies. Of course, any op-amp or FET used as an upgrade device should be able to operate at the required high frequencies.


To summarize, the Colpitts Oscillator consists of a parallel LC resonator tank circuit, whose feedback is obtained through a capacitive divider. Like most oscillator circuits, the Colpitts oscillator is found in several forms, the most common form of which is similar to the transistor circuit above.

The center stroke of the tank sub-circuit is carried out at the junction of a "capacitive voltage divider" network to feed part of the output signal back to the transistor's absorber. Two serially connected capacitors produce a phase shift of 180o, which is reversed by another 180o toproduce the necessary positive feedback. The oscillation frequency, which is a purer sine wave voltage, is determined by the resonance frequency of the tank circuit.

In the next lesson about oscillators, we will look at RC Oscillators that use resistors and capacitors as tank circuits to produce a sinusoidal waveform.