IEEE Circuits and Systems Magazine - Q4 2020 - 13

f- avor of cross-coupled in the 1/f 2 region of it. On the other hand, Colpitts shows a lower 1/f noise up conversion.
In [61] a differential common base Colpitts is compared
to a classical cross-coupled differential oscillator. In this
work, the Colpitts achieves better phase noise.
III. Common Drain Colpitts Oscillator
Fig. 7 shows the Colpitts oscillator implemented using a
Common Drain Amplifier (CDA). This configuration is one
of the most common in literature [23], [62]-[65] and widely used in communication systems often implemented
with a crystal instead of the inductor. Similar circuit
configuration based on vacuum tube cathode-follower
circuits were described in 1940s [21], [22] and 1950s
[66]. An analysis of the start up characteristics of this
circuit can be found in [67], [68] and a detailed design
procedure and analysis method can be found in [36].
Different modified designs based on this configurations
have been presented, which improves its properties and
will be presented in the following.
The common drain Colpitts is the circuit, which resembles most to the circuit proposed by Clapp [23]. In his

paper he adds a capacitor in series with the inductor to
improve its stability as shown in Fig. 8. This configuration
also makes the circuit more tunable regarding the frequency as the added capacitor can be a variable capacitor. As
mentioned earlier, one of the reasons for popularity of the
Colpitts is the possibility of using it for driving crystals and
FBARs as shown in Fig. 9. Already in 1957 a procedure of
designing Colpitts crystal oscillators was published in [18].
In order to further improve the stability of the common drain Colpitts, a cascode of a common gate amplifier and common drain Colpitts can be implemented as
shown in Fig. 10.
This configuration will in addition to better input and
output isolation, improve the gain and give higher input
and output impedance, better stability and increase the
bandwidth due to the reduction of the Miller effect. This
configuration is used widely in radio communication to

MN1
C1

Vout

FBAR
C2
MN1
C1

MN2

C

Vbias

Vout

L
C2
MN2

Vbias

Figure 9. Common drain Colpitts topology with a FBAR replacing the inductor.

Figure 7. Single-ended common drain Colpitts topology.
Vbias3

MP1

Vbias2

MN1

MN2

MN1
C1

Vout

L
C2
C

MN2

Vout

Vbias

Figure 8. Clapp added a capacitor in series with the inductor
in the common drain Colpitts topology.

FOURTH QUARTER 2020

C1
FBAR
C2
C

MN3

Vbias1

Figure 10. Cascode of a common gate amplifier and common drain Colpitts topology.

IEEE CIRCUITS AND SYSTEMS MAGAZINE

13

IEEE Circuits and Systems Magazine - Q4 2020

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