Oscillators are fundamental components in RF circuits. There are many types of oscillators such as Hartley oscillator, Colpitts oscillator, Wien Bridge oscillator, Phase Shift oscillator, crystal oscillator and so on. Among them the most stable is the crystal oscillator. There are many circuit configuration that can be construct crystal oscillator. Here we will illustrate Colpitts oscillator configuration for crystal oscillator design. In Colpitts oscillator a capacitive divider along with inductor in the feedback circuit. Now if Colpitts configuration is used in crystal oscillator circuit, the inductor is replaced with the quartz crystal. So it means the crystal will acts as an inductor.
Circuit Diagram
The following shows the circuit diagram of Crystal Oscillator with Colpitts configuration.
A practical oscillator system consist of two sub-system- an amplifier and a feedback circuit. Here the BJT BC547 transistor along with its voltage divider bias forms the amplifier. The C1 and C2 capacitive divider circuit along with the crystal X1 forms the feedback circuit. These are indicated in the circuit drawing below.
Circuit Operation
The amplifier circuit is biased in the active region using voltage divider biasing method. The feedback circuit resonant frequency is determined by the crystal frequency and the capacitive divider circuit. Due to noise such as thermal noise in the system, there is some initial signal input into the feedback circuit resonant at a specific frequency. Here the resonant frequency was made to 11.25KHz. The output from the feedback circuit is fed back into the amplifier circuit via the input coupling capacitor Cc1. The small signal is amplified at the BJT collector. This amplified signal is fed into the feedback circuit via the output coupling capacitor Cc2. And this process repeats again and again and circuit goes into oscillation. The capacitor Cb is bypass capacitor that shorts the ac signal to the ground. The coupling capacitors and bypass capacitor are used to isolate the amplifier circuit from the rest of the circuit so that the desired operating point does not drift.
The BJT amplifier is biased using voltage divider circuit. Basically, the resistors R1 and R2 are used to fix the base voltage(which must be higher than base to emitter voltage which is around 0.7V. The resistors RE and RC are chosen according to the required operating point or the bias point. The bias point refers to the selected value of the collector to emitter voltage(Vce) and the collector current(Ic). These two are the output quantities of a BJT amplifier. The tutorial how to bias a BJT using voltage divider biasing method explains how to calculate the resistors and the capacitors value.
The feedback circuit is constructed using the two capacitors C1 and C2 and the crystal oscillator. Usually the crystal is a quartz crystal type. The frequency of oscillation is determined by the frequency of the crystal. The online oscillator calculator page can be used to determine frequency of oscillation for Colpitts oscillator and other types of oscillators.
Animation & Simulation Video
The following video shows animated circuit simulation which shows how the crystal oscillator works. It shows how the signal flows in the circuit, particularly via the feedback circuit.
This oscillator can be build on a breadboard and tested with PC oscilloscope. See Practical Colpitts Oscillator Breadboard to learn how to do this.
