Gm-enhanced differential colpitts VCO

Abstract

V AbstractThis thesis proposes a gm-enhanced differential Colpitts voltage-control oscillator (VCO) that initiates oscillation with a lower biasing current and exhibits a better phase noise performance. The proposed gm-enhanced topology can improve performance of VCO, whereby the oscillator constructed based on the suggested topology demonstrated operation from 1.53 GHz to 2.1 GHz. The tank inductors engaged in the oscillator are each designed with a quality factor of 9. To evaluate the design, a conventional cross-coupled differential LC VCO, a conventional differential Colpitts VCO and the proposed VCO are simulated using 1.5 V supply voltage, under the 0.13 μm standard CMOS process. Biased with a current of 3 mA, the simulation results show that the proposed VCO's phase noise performance is of -128.9 dBc/Hz at 1 MHz offset, which is 3.6 dBc/Hz better than that of the cross-coupled differential LC VCO and 2 dBc/Hz better than that of the conventional differential Colpitts VCO. In the post-layout simulations of the proposed VCO and conventional cross-coupled differential LC VCO, the former has a 2.1 dBc/Hz phase noise better than that of the conventional one.Based on the developed designed gm-enhanced topology, a gm-enhanced differential Colpiits Quadrature VCO (QVCO) is designed. Operating at 1.8 GHz, the proposed QVCO shows a phase noise of -132.4 dBc/Hz at 1-MHz offset, when biased with 4-mA current and 1.5 V supply voltage. Compared with the conventional Colpitts QVCO, the proposed gm-enhanced differential Colpiits QVCO demonstrates better phase noise of 4 dB under the same operating condition.

Analysis and review of main characteristics of Colpitts oscillators

“…As reported in Hajimiri and Lee, 13 if the value of C 2 is chosen larger than that of C 1 , the oscillation amplitude will be increased, which leads to a decrease in‐phase noise. The negative conductance of this circuit is derived in Xiao et al 14 as where g m 1 and g m 3 represent the transconductances of M 1 and M 3 , respectively.…”

“…Differential g m ‐boosted DS Colpitts VCOs introduced in (A) Li et al 16 (B) Hong and Lee, 18 and (C) Xiao et al 14 …”

“…In another Colpitts configuration, 14 two pairs of cross‐connected transistors are stacked as a cascoding scheme to enhance the circuit overall loop gain as shown in Figure 5C. Although such direct cross‐connections provide the self‐biasing possibilities for each pair of MOSFETs without using extra bias circuit, the voltage headroom requirements have been limited.…”

“…Compared to the conventional DS Colpitts oscillator and also the Colpitts structure of Figure 3B, this configuration can oscillate at lower start‐up current because of stronger positive feedbacks. As pointed out in Xiao et al 14 the small signal conductance of this circuit can be derived as (6). By assuming the same transconductance for MOS transistors ( g m 1,2 = g m 3,4 = g m ), the negative conductance seen by the resonator is – g m , which compared to that of the DS Colpitts oscillator of Figure 3B, is increased by a factor of 1 + 2 C 2 / C 1 .…”