A Pierce Oscillator is a type of oscillator that uses a crystal instead of a parallel-resonant circuit [LC circuit]. A pierce oscillator also uses the voltage developed from a tap between two capacitors in the tank circuit, as shown to the right. range. Both of the Pierce Oscillators use a PNP transistor as the amplifier. Basically any PNP transis
tor may be used as long as it exhibits gain at the desired frequency of operation. Two common general purpose PNP transistors include the 2N3904 PNP Transistor and 2N3906 PNP Transistor. Both of these transistors offer frequency operation over 100MHz. The important criteria is that the transistor provides amplification in the frequency range of oscillation. Both of these circuits are very similar. The DC voltage is supplied by Vcc, set to be in range of the transistor selected. The 2N3904, for example, has a maximum Collector-Base voltage of 60 volts and Collector-Emitter voltage of 40 volts. The DC bias currents are set by a number of resistors. Resistor Re sets the emitter current. The collector current is set by resistor Rc. The Base voltage is set by the voltage divider formed by resistors Rf and Rb and is equal to Vcc x Rb/(Rb + Rf)].