IEEE Circuits and Systems Magazine - Q4 2020 - 15

The common drain oscillator shown in this section is
a parallel oscillator, meaning that it runs the resonator
or the FBAR close to its anti-resonance frequency. Butler [24] proposed a circuit where the crystal is placed
in the feedback as shown in Fig. 16, between the output and the splitted capacitors to drive the crystal at its
resonance frequency.
A. Differential Push-Push Common
Drain Colpitts Oscillator
The single-ended common drain configuration, presented earlier, can be connected " back to back " to another
single-ended common drain through an inductor, as
shown in Fig. 17 to create a differential Colpitts oscillator
[76]. This configuration is also referred to as push-push
or balanced oscillator in literature. This configuration
can be used to drive a resonator as in [77], where the
inductor is replaced by a crystal or a FBAR. Differential
oscillator output is desired in modern circuit design
because a differential signal is easier to process, for instance in frequency down or up conversion circuits.

The principle of common gate cascoding of the output can also be applied to the differential Colpitts [80].
In [80] the capacitances in the tank have been omitted
and internal parasitic capacitances have been used as
the necessary capacitances in the tank.
The capacitors C2 and C4 in Fig. 17 can be combined
to one capacitor C2 as shown in Fig. 18. The Capacitor C2 can be replaced by a variable capacitor to make
the circuit tunable if an inductor is used instead of the
FBAR as presented in [76]. To increase the transconductance of the differential oscillator, a cross-coupled pair
as shown in Fig. 19 may be added [35] to the oscillator.
To our best knowledge [35] is the first circuit that combines a differential common drain Colpitts and crosscoupled transistors to increase the transconductance of
the oscillator.
Authors in [35] realized this circuit in bipolar technology using only npn transistors, which can also be easily
implemented in MOSFET technology. Care must be taken in order to avoid latching as the cross-coupled pair
is a latch-circuit often used in digital circuits to hold a
binary value. This can be avoided by properly choosing
the transistor sizes in cross-coupled compared to the
rest of the circuitry [81] and use DC blocking capacitors
in the signal path.

MN1
C1
FBAR

Vout
C2

FBAR

LNF

C
MN2

Vbias

Vout1

Vbias

Vout2
Vbias

Figure 15. The inductor LNF is used for noise filtering in the
oscillator [53].
Figure 17. Balanced differential common drain Colpitts topology [78], [79].

MN1
C1

FBAR
Vout

Vout1
C2

FBAR
MN2

Vbias

Figure 16. The Butler configuration drives the FBAR at its
series resonance [24].

FOURTH QUARTER 2020

Vbias

C3
C2

Vout2
Vbias

Figure 18. Differential common drain Colpitts topology with
single capacitor in parallel with the FBAR.

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IEEE Circuits and Systems Magazine - Q4 2020

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