Bipolar Transistor Based Pierce Crystal Oscillator

 Pierce oscillator is a variant of Colpitts oscillator in which instead of inductor a quartz crystal is used. Crystal based oscillator like Pierce oscillator are stable oscillator that are used as RF oscillator in RF communication circuits. The basic working principle of Pierce crystal oscillator is similar to that of Colpitts oscillator and this was explained in details in How Pierce crystal oscillator Works. In this blog post, a variant of the basic Pierce crystal oscillator is illustrated wherein the two capacitors typically found in basic Pierce oscillator are not used. Instead the internal capacitance of bipolar transistor(BJT) are used for forming the feedback filter along with the crystal.

 The following picture shows the 2N2222 bipolar transistor based Pierce crystal oscillator constructed on a breadboard shown in the above circuit diagram.

BJT Pierce crystal oscillator

The following shows circuit diagram of the above bipolar junction transistor(BJT) based Pierce crystal oscillator which uses the internal capacitance of the transistor.

BJT internal capacitance based Pierce cystal oscillator circuit diagram

The BJT used here is 2N2222 bipolar transistor. Other transistors like 2N3904 or BC547 can also be used. The quartz crystal used here has a frequency of 16MHz. So the oscillation frequency of this Pierce oscillator has frequency of 16MHz. The feedback network is connected to the crystal resonator in such a way that it provides a 180-degree phase shift. This is achieved by connecting the crystal between the base and the collector of a bipolar junction transistor (BJT). The feedback network is connected between the emitter and the base of the transistor. The capacitor Cc1 is the coupling capacitor. The resistors R1, R2, RE are the DC biasing resistors. The capacitor CB is a emitter bypass capacitor. The inductor RFC is the radio frequency choke.

Video demonstration

The following video shows the BJT based Pierce crystal oscillator working on a breadboard.

 Such Pierce oscillator can be used in RF circuits such as in AM modulator, AM demodulator circuits, FM circuits and others where it generates local oscillator signals. Moreover it can also be used in beat frequency oscillator(BFO) circuits to generate stable signal. 


[1] 2N5550 Colpitts Oscillator

[2] Drain Feedback biased E-MOSFET Colpitts Oscillator

[3] Colpitts oscillator with LM358 and TL072 Op-Amps 


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