Working principle of Colpitt's oscillator
This is the article on working principle of Colpitt's oscillator. Here I am explaining the working principle of Colpitt's oscillator along with circuit diagram, wave-forms and necessary mathematical derivations in detail.
Almost all oscillators and circuits with sine-wave generators use an LC resonant circuit. Only a few RC oscillators such as the RC phase shifter generator , the generator Wien-Robinson and certain crystal oscillators come from without additional inductances. A resonant circuit circuit to the oscillator, when the losses occurring in the circuit to be compensated by an amplifier so that the ring or loop gain has the value 1. The overall phase shift must be at 0 ° or 360 ° at the resonant frequency.
A part of the resonant circuit voltage must be in phase fed back to the amplifier. These can be divided into two capacitors in the resonant circuit capacitance. This circuit is referred to as three-point capacitive connection. Is based on the center of mass, then the two voltages are of opposite phase to each other across the capacitors. The following figure shows the signal conditions at the resonant circuit with capacitive three-point circuit.
An oscillator with capacitive three-point circuit after its inventor Edwin Henry Colpitts called Colpitts oscillator. This oscillator can be operated in a common emitter amplifier including a transistor, the base and collector circuit.Colpitts oscillator in the emitter circuit
In the emitter circuit between the output control signal and a phase shift of 180 °. It is canceled by the capacitive circuit with three-point ground reference between the two capacitors. The resonant circuit is located in the collector circuit. The DC voltage passes through an inductor sufficiently high inductance to the amplifier. It is necessary to repeal the ground reference of the signal to the operating voltage. An ohmic resistance is less suitable. For a good decoupling its value should be large, thereby reducing the resonant circuit quality. The inductor decouples the signal due to their high AC resistance to the DC voltage and has virtually no influence.
The phase voltage of the resonant circuit passes through a coupling capacitor to the base. It separates the base from the electrically resonant circuit, which otherwise would lie directly on the operating voltage. The working point is adjusted by base voltage divider or only with a base resistor. The emitter resistor stabilizes the operating point and determines the signal amplification through current feedback. The output signal can be taken directly on the resonant circuit or by means of transformer coupling to the resonant circuit coil. An output transformer provides good impedance matching to the next stage and reduced quality losses that arise when a load of the oscillator circuit.
The illustrated circuit worked fine with a software simulation using ideal components. She swings after the start of the simulation to safe and remains stable without clipping.Colpitts oscillator circuit in base
The choke coil is eliminated and thus the resonant circuit is higher than the operating voltage of signal-to ground. A capacitor to ground is supportive. A phase rotation is not necessary since the base circuit amplifies without phase rotation. The feedback signal to the emitter is decoupled by the resonant circuit capacitors galvanically isolated from the supply voltage. The base is connected via a high-capacity capacitor for the signal to ground. The operating point is set with a base voltage divider and base bias resistor and the emitter resistor.
In the simulation program, the function of this circuit could be checked. The oscillation occurs shortly after the start of the simulation. Then the oscillator frequency remained stable without clipping.
Instead of the base-emitter circuit should primarily be used in oscillators. Here, the fed back frequency signal is passed through the capacitor to earth base. The base-emitter input circuit of the transistor is thus free of parasitic frequencies. They can lead to disruptions in a previously connected stage. If the oscillator in the intermediate frequency tuning circuit of the antenna of a receiver, as the basic circuit in the base can not penetrate into the antenna circuit and the oscillator frequency is radiated from there.Colpitts oscillator in the collector circuit
To operate a Colpitts oscillator including an amplifier in the collector circuit is possible, but is very rarely used. Its voltage gain is almost one, and there is no phase rotation. Thus, despite the ever-existing losses in the oscillating circuit, the oscillator condition is satisfied with k · V 0 = 1, one uses the split resonant circuit capacity from a kind of voltage transformation. The entire resonant circuit voltage is fed back via a sufficiently large coupling capacitance to the base. The proportion is smaller than the emitter resistance of the output signal.
This circuit worked fine with the calculated dimension. Circuit swings after the start of the simulation securely. The output remained stable and largely undistorted.
Colpitts oscillators are well suited for high fixed frequencies. To make the oscillator circuits shown in the tunable brings problems. If the frequency varies with a variable capacitor, so the coupling factor, and thus the ring gain changes. Tunable Colpitts oscillators for very high frequencies in the VHF and VHF range between 80 ... 300) MHz are realized with a variable capacitor. At first glance, these circuits are not immediately recognizable as a three-point capacitive circuits. They take advantage of the internal switching capacity of several picofarads of the transistor.
In earlier times there were in the car radio variometer vote. Is a tuning coil, in which the inductance is varied by the position of a ferrite core and a second inner coil. With it the Colpitts oscillator is easily tunable. For mechanically stressed resonant circuits, the vote is not a variable capacitor plate. The vibrations do not allow constant capacitance values and hence no proper tuning. In contrast, if a ferrite core is moved along the taut rope through the bobbin so the inductance changes. To set the vertical vibrations remain ineffective.
This is a very thoughtful, educational analysis of the Colpitts oscillator!