A Differential Complementary Colpitts Oscillator based on Common Drain Topology Mehdi Azadmehr, Igor Paprotny, Yngvar Berg Abstract—In this paper we present a new topology comple- mentary differential Colpitts oscillator. The circuit is realized by stacking one PMOS common drain Colpitts oscillator on top on a NMOS common drain Colpitts oscillators. Measurements of a prototype implemented with discrete components have been used to verify the correct operation of the system. The Oscillator was realized using the integrated circuits ALD1105 connected to a 100μH and a Crystal with a resonance frequency of 4,915MHz. The oscillator connected to the crystal showed a phase noise of -95 dBc/Hz at 100KHz offset. Index Terms—Colpitts, Complementary, Crystal, Oscillator, Common Drain. I. I NTRODUCTION Oscillators are the most important analog circuits today and are being used in different applications such as clock generators in digital systems, in communication systems [1] for modulation and demodulation of signals and as front- end for resonating sensors [2]. Colpitts oscillators [3], [4], invented by Edwin H. Colpitts in 1918 is one of the most popular and oldest oscillator circuit topologies available today. Initially, the purpose of this design was to simplify vacuum tube oscillators, but it has proven to be as effective now as then. Colpitts oscillators are very adaptable and versatile and can be designed using most types of transistors technologies available today. They can be used to produce differential and quadrature signals [5], [6], [7], to operate at frequencies above 100GHz [7], [8] and work with voltages down to 20mV [9] to be powered by a thermoelectric power sources. Figure 1 shows the most common Colpitts conﬁgurations today. What characterizes Colpitts is its feedback which is made of two series connected capacitors in parallel with an inductor. The inductor can be replaced by a crystal or Film Bulk Acoustic Resonator (FBAR) to gain better stability and produce high quality oscillations. In circuits 1 a) and b) the inductor is connected between ground and to the either the input or the output and one capacitance between the input and the output. In these two conﬁgurations the ampliﬁer is non- inverting. In the case of circuit c) where the inductive part is connected between the input and the output, the ampliﬁer is inverting type. If the inductor in this conﬁguration is replaced by a crystal, this circuit is called a Pierce oscillator after its inventor G. E. Pierce. Is it important to note that the grounded nodes in feedback are ac grounds and can also be connected to VDD. These ampliﬁers can be realized by single Manuscript received July 23, 2018; revised Aug. 16, 2018. This work was supported research council of Norway. Mehdi Azadmehr and Yngvar Berg are with department of Microsystems (IMS), University of South-Eastern Norway, Raveien 205, 3184, Horten, Norway, Email: mehdi.azadmehr@usn.no Igor Paprotny is with department of Electrical and Computer Engineering, University of Illinois at Chicago, Il, USA, Email: paprotny@uic.edu. transistor ampliﬁers such as common gate, common drain and common source using MOSFETS or their equivalent circuits in Bipolar Junction Transistors(BJT). One of the main reason for the popularity of Colpitts oscil- lators is their good cyclostationary noise properties compared to other topologies such as ring Oscillators and cross Coupled [10], [6]. Another reason for the popularity of the Colpitts oscillators today is that they can easily be implemented in modern CMOS technologies where high quality capacitors can be made with high precision using different metal layers. This allows the capacitors in the feedback to be made on chip and the inductive part or the high Q resonators such as FBARs placed outside the chip. Fig. 1. Colpitts Topology, a) the inductance is connected to the input, b) the inductor is connected to the output and c) the inductor is connected between the input and the output and the ampliﬁer needs to be inverting. The oscillation frequency of the Colpitts oscillators in Fig. 1, if we ignore the parasitics in the circuit, is equal to the resonance frequency of the LC tank given by: f t = 1 2π  L C1C2 C1+C2 (1) II. COMMON DRAIN COLPITTS OSCILLATOR Figure 2 shows the circuit implementation of Colpitts oscillator shown in Fig. 1 a) using a common drain ampliﬁer. This conﬁguration is described in detail in various literature [11], [12], [13], [14], [15] and widely used in communication systems implemented using either inductor or a crystal. An analysis of the start up characteristics of this circuit can be found in [16] and a detailed design procedure and analysis method can be found in [4]. Single-ended oscillators are more susceptible to external noise and environmental variations caused by changes in temperature and/or power supply. This is the main reason that differential versions are often preferred. Another reason for choosing differential oscillators, especially at high frequen- cies, is that frequency up- and down-conversion of the signals becomes much easier. Figure 3 shows a balanced Colpitts Proceedings of the World Congress on Engineering and Computer Science 2018 Vol I WCECS 2018, October 23-25, 2018, San Francisco, USA ISBN: 978-988-14048-1-7 ISSN: 2078-0958 (Print); ISSN: 2078-0966 (Online) WCECS 2018