Colpitt oscillator using transistor | Design and Simulation (in TINA-TI)
An oscillator is used to produce electronic signal with oscillating periods. Eg: Sine wave, square wave etc.. Oscillators are broadly classified into two – linear oscillators and non-linear oscillators. As the name implies, linear oscillators are used to produce linear or sinusiodal waveforms. Colpitts Oscillator is an electronic oscillator which uses an inductor and capacitors to form an LC oscillator circuit. Colpitts oscillator was invented by American scientist Edwin Colpitts in 1918. It is another type of sinusoidal LC oscillator and is basically a harmonic oscillator, which has a lot of applications. The Colpitts oscillator can be realized using valves, transistors, FETs or op-amp. It is much similar to the Hartley oscillator except the addition of tank circuit. In Colpitts oscillator the tank circuit consists of two capacitors in series and an inductor connected in parallel to the serial combination. The frequency of the oscillations are determined by the value of the capacitors and inductor in the tank circuit. Thus the main difference between a Colpitts Oscillator and a Hartley Oscillator is that the former uses tapped capacitance, while the latter uses tapped inductance.
Colpitts oscillator is generally used in RF applications and the typical operating range is 20KHz to 300MHz. In Colpitts oscillator, the capacitive voltage divider setup in the tank circuit works as the feed back source and this arrangement gives better frequency stability when compared to the Hartley oscillator which uses an inductive voltage divider setup for feedback. The energy lost in the tank circuit is compensated by the transistor and the oscillations are sustained. The tank circuit produces 180° phase shift and the transistor itself produces another 180° phase shift. That means the input and output are in phase and it is a necessary condition of positive feedback for maintaining sustained oscillations.