Oscillator circuit types and circuit models

Oscillator circuit types and circuit models

Among the many oscillator circuits, three commonly used are the Pierce oscillator, the Colpitts oscillator,
and the Clapp oscillator. These oscillator circuits are identical except for the location of the RF ground point.
The Butler and modified Butler oscillator circuits are also similar to each other, and the emitter current in each circuit is the crystal current
. The gate oscillator is a Pierce type that uses a logic gate and adds
a resistor in place of the transistor of the Pierce oscillator. (Some gate oscillators use more than one gate.)

The choice of oscillator circuit type depends on the required frequency, stability, input voltage and power, output
power and waveform, adjustability, design complexity, cost, and crystal device characteristics.
In the Pierce series, the location of the ground point has a great influence on performance. The Pierce circuit connection is
generally better than the other circuits with respect to parasitic reactance and bias resistance because they are mostly connected across the circuit capacitors rather than
across the crystal device. It is one of the most widely used circuits for high-stability oscillators. In the Colpitts circuit connection,
a large part of the parasitic capacitance appears across the crystal, and the bias resistor is also connected across the crystal, which can degrade performance. The Krapp
circuit connection is rarely used because the collector is connected directly to the crystal, which makes it difficult to apply a DC voltage to the collector
without introducing losses or parasitic oscillations (for more details, see the references).
Although the Pierce series can be made to operate in series resonance by placing an inductor in series with the crystal, it
is usually operated in "parallel resonance" (see Section 3 on the relationship between resistance frequency and reactance). The Butler series usually operates in (or close to) series resonance, and the Pierce series can be designed to operate
with crystal currents above or below the emitter current . Gate oscillators are common circuits in digital systems when high stability is not a primary concern.