2-9
Of the numerous oscillator circuit types, three of the more common ones, the Pierce, the Colpitts and the Clapp, consist of the same circuit except that the rf ground points are at different locations.  The Butler and modified Butler are also similar to each other; in each, the emitter current is the crystal current.  The gate oscillator is a Pierce-type that uses a logic gate plus a resistor in place of the transistor in the Pierce oscillator.  (Some gate oscillators use more than one gate).
Pierce
Colpitts
Clapp
Gate
Modified Butler
Butler
b
c
Î
b
c
Î
b
c
Î
b
c
Î
b
c
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Oscillator Circuit Types
     The choice of oscillator circuit type depends on factors such as the desired frequency stability, input voltage and power, output power and waveform, tunability, design complexity, cost and the crystal unit's characteristics.
     In the Pierce family, the ground point location has a profound effect on the performance. The Pierce configuration is generally superior to the others, e.g., with respect to the effects of stray reactances and biasing resistors, which appear mostly across the capacitors in the circuit rather than the crystal unit.  It is one of the most widely used circuits for high stability oscillators. In the Colpitts configuration, a larger part of the strays appears across the crystal,  and the biasing resistors are also across the crystal, which can degrade performance.  The Clapp is seldom used because, since the collector is tied directly to the crystal, it is difficult to apply a dc voltage to the collector without introducing losses or spurious oscillations.  (See the references for more details.)
     The Pierce family usually operates at "parallel resonance" (see "Resonator Frequency vs. Reactance" in Chapt. 3), although it can be designed to operate at series resonance by connecting an inductor in series with the crystal.  The Butler family usually operates at (or near) series resonance.  The Pierce can be designed to operate with the crystal current above or below the emitter current.
     Gate oscillators are common in digital systems when high stability is not a major consideration.


A. Benjaminson, "Computer-Aided Design of Crystal Oscillators," U. S. Army R & D Technical Report DELET-TR-84- 0386-F, August 1985, AD-B096820;  "Advanced Crystal Oscillator Design,"  U. S. Army R & D Technical Report SLCET-TR-85-0445-F, January 1988, AD-B121288;  "Advanced Crystal Oscillator Design,"  U. S. Army R & D Technical Report SLCET-TR-88-0804-1, February 1989, AD-B134514; "Advanced Crystal Oscillator Design,"  U. S. Army R & D   Technical Report SLCET-TR-88-0804-F, December 1991, AD-B163808.

J. P. Buchanan, Handbook of Piezoelectric Crystals for Radio Equipment Designers, WADC Technical Report 56-156, October 1956 (692 pages), available from NTIS, AD 110448.