IEEE Circuits and Systems Magazine - Q4 2020 - 23

Colpitts oscillator can achieve higher oscillator frequencies, higher quality factor
and lower phase noise in combination with MEMS resonators such as FBARs,
Bulk and surface acoustic wave (BAW and SAW) resonators.
the authors present a FBAR Pierce oscillator with a power consumption of 22 nW at a frequency of 2 GHz. The
low power is achieved by controlling the bulk voltage
of the transistors in the common source configuration.
In [109], [110] the authors present a low power Pierce
oscillator in 65-nm CMOS process, which exhibits a FoM
of 248 dBc/Hz with a power consumption of 19 W and
PN of 139 dBc/Hz at 1-kHz offset frequency. These results are achieved by stacking several inverters on top
of each other.
The low manufacturing cost of the Pierce oscillator,
and the outstanding frequency stability of the MEMS
resonators, gives it an advantage over other designs
in many consumer electronics applications. Virtually
all digital IC clock oscillators today are of Pierce type,
as the circuit can be implemented using a minimum of
components: a single digital inverter, one resistor, two
capacitors, and the resonator.
VI. Discussion and Comparison
In this paper, we presented many different types of
Colpitts oscillators, each with different characteristics
and properties. There are many ways of grouping
these oscillators, we chose to group them based on
their amplifier topologies since it gives a more streamlined description of the evolution of the Colpitts. We also
showed how various circuit design techniques such as
cross-coupling, push-pull and push-push can be applied
to improve various properties of these oscillators, such
as their phase noise and power consumption.
Colpitts oscillators have played an important role in
development of circuits and systems used in radio communication for over a century. In the past, the Colpitts
was chosen due to its stability and simple design. Today,
it is chosen because of its compatibility with modern
technologies such as ASIC and MEMS. The current and
future trend in oscillator design will be higher oscillator frequencies, higher quality factor and lower phase
noise. Colpitts can achieve all of these in combination
with MEMS resonators such as FBARs, Bulk and surface
acoustic wave (BAW and SAW) resonators. Colpitts oscillators will continue to play an important role in the
further development of oscillators due to their simplicity, compatibility and versatility.
One main question will be, which configuration of
Colpitts oscillator to choose, when designing a system. This is a challenging question to answer as all
FOURTH QUARTER 2020

the presented oscillators can somehow be improved
by various circuit design techniques. However, some
simple circuit design criterias and rules valid for amplifier design can be followed, when looking for the
appropriate circuit. For example, for low voltage applications, cascode oscillators are not the best choice
due to the stacking of transistors as each transistor
requires a minimum voltage to operate properly. For
low voltage and low power applications, the inductive
degenerated techniques may be used, as presented in
Figs. 27 and 29.
The presented oscillators are difficult to compare to
each other and other types of oscillators due to the difference in their amplifier structure or their feedback.
However, some comparison studies have been reported
in literature. To make these comparisons fair, the authors have tried to design the oscillators as equal as
possible. i.e. using the same transistor sizes and voltages before the comparison.
One of the most important oscillator properties,
especially in modern high frequency systems is the
phase noise, which was discussed in chapter 2. The
comparison presented in [59] shows that a differential
Armstrong oscillator has better performance than the
Colpitts at 10 GHz resonance frequency. In [60], the authors show that the differential common gate Colpitts
oscillator shown in Fig. 28 has better phase noise performance than a cross-coupled LC tank oscillator. This paper also compares and discusses other properties such
as the frequency tunability and power consumption for
circuits manufactured in 350 nm SiGe-BiCMOS technology. In another comparison [111], the authors conclude
that a single-ended common gate Colpitts is superior
to Hartley.
In [112], the authors compare the phase noise of a
gm-boosted common gate, similar to the one presented
in Fig. 31, in the two different technologies CMOS and
SiGe Hetro-junction Bipolar Transistor (HBT). In this
work, the feedback tank of the oscillators are made
with an inductor instead of a FBAR. This study shows
that the CMOS is superior to the bipolar since it benefits more from the gm-boosting technique achieved
by cross-coupling.
In [113], the authors show a close-in phase noise comparison of different MEMS oscillators such as FBAR,
SAW and BAW and compare these to a crystal oscillator.
They also show that by adding a non-linear capacitor
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