Noise Reduction in Transistor Oscillators: Part 3—Noise Shifting Techniques

In modern wireless communication systems, cross-coupled oscillators have been preferred over other topologies for monolithic integrated circuit implementation because they are easily realized using CMOS technology and differential circuitry. However, because the current-source transistor is top or tail located, to improve the phase noise performance, it is necessary to use the special filtering techniques for second harmonic suppression. Therefore, the new oscillator topologies based on classical types of oscillators can overcome this problem when one of the active device ports is grounded [28]. In addition, these topologies provide larger oscillation amplitude for a given bias current because there is no voltage drop of the DC current across the current-source transistor, similar to the differential voltage-biased VCO shown in Figure 23. Figure 27(a) shows a single-ended common base bipolar Colpitts oscillator configuration. The required regeneration factor for the startup oscillation conditions can be chosen using a proper ratio of the feedback capacitances C1 and C2. To maximize the loaded quality factor of the resonant circuit, the choke inductance can be connected between the bias resistor Rbias and emitter. In CMOS implementation, the single-ended bipolar common base oscillator can be replaced by the schematic using nMOS device shown in Figure 27(b) with RF grounded gate and current source instead of bias resistor. Connecting two identical singleended oscillators, as shown in Figure 27(c), can provide the differential output. In a perfectly balanced circuit, identical sides carry 180° (out of phase) signals of equal magnitude. Consequently, there is a midpoint (the center node between C2 capacitances) where the signal magnitude is zero. For absolutely identical circuits in each balanced side, the difference between signal magnitudes becomes equal to zero in each midpoint (dotted line) referring as a virtual ground. Because of a push-pull operation, the DC current flows through each current-source This series concludes with discussions of additional noise reduction methods using circuit topology, matching and feedback.