Colpitts oscillator

The Colpitts circuit, also called Colpitts oscillator, an electronic circuit for generating a periodic alternating voltage ( sine wave ). The oscillation frequency can be determined by the dimensions of the components. The Colpitts circuit is characterized by its high insensitivity in the dimensioning and reliability in operation. There are several variations of the Colpitts circuit, for example, the Clapp, Seiler and the Vačkář circuit.

History

The Colpitts oscillator was logged in 1918 by Edwin H. Colpitts patent. When an electron tube amplifier ( triode) was used in the anode base circuit. 7 is cathode, 9 grid and 8 anode of the tube. The resonant circuit composed of the variable capacitors 13, 16 and the primary winding 20 of the transformer 22, the operating voltage of the battery 11 is supplied through the RF choke 12, which prevents the high frequency voltage is short-circuited. ( Compounds 23 and 24 are needed in the description of variants). On the winding 23, the vibration generated is removed.

The frequency-determining network consists of an RC element (ra -C13 ) and a series resonant circuit (C16 -L20 ). ra is not visible, differential output resistance of the anode base circuit ( approximately 300 ohms). C13 is dimensioned such that the cutoff frequency of the RC element is substantially smaller than the oscillation frequency.

Transistor circuit

The resonant circuit in a Colpitts oscillator circuit consists of the two capacitors C1, C2 and the inductor L1. The amplifier Q1 operates in the gate circuit and rotates the phase between input and output at 0 °. The high frequency voltage at the drain (output) is divided by the capacitive voltage divider C1, C2 and is fed to the source ( input). For maintaining the vibration at least one of a gain is required. For a clean output signal has no harmonics, the gain should be set so that the oscillator starts to oscillate safely. C3, the output of the oscillator is extracted.

The frequency determining network is here a parallel resonant circuit ( L1 -C1 -C2), which is fed via the non-visible, the differential output resistance R of the gate circuit. One recognizes the resonant circuit, considering that the DC voltage source BAT represents a short circuit for AC voltages and thus the upper end of the coil is grounded AC coupled (in practice, the battery is additionally bridged by a capacitor ). The non-visible, differential input resistance Re of the gate circuit loads the resonant circuit. Since this resistance is only to a small part of the voltage resonant circuit voltage due to the capacitive voltage divider, the damping is considerably smaller.

Vote

For a tuning oscillator in superheterodyne receiver of the Colpitts oscillator is well suited when the inductance is varied ( variometer vote). It is less suitable, if C1 and C2 are used as a tuning capacitor, characterized because the division ratio, and hence the loop gain is changed and the tuning range is limited by the other capacitor. One can, however, C1 and C2 make much smaller than necessary and turn the tuning capacitor in parallel. Alternatives for capacitive tuning, the Hartley circuit or the Clapp circuit.

Frequency of the vibration generated

The frequency generated is determined by the inductance of the coil and the series arrangement of the capacitances of the capacitors C1 and C2 is determined ( Thomson resonance formula ):

The additional capacity of the remaining components reduce this calculated frequency.