RF circuits employ constant capacitors for various purposes e.g.

1. to adjust the resonance frequency of LC tanks

2. to provide ac coupling between stages

3. to bypass the supply rail to ground

The critical parameters of capacitors used in RF ICs include the capacitance density, the parasitic capacitances and the Q.

MOSFETs configured as capacitors offer the highest density in integrated circuits because Cox is larger than other capacitances in CMOS processes. However, the use of MOS capacitors entails two issues. First, to provide maximum capacitance. Second, the channel resistance limits the Q of MOS capacitors at high frequencies.

Both the above issues make MOS capacitors a poor choice for inter-stage coupling. As shown in figure (a), wherein M3 sustains a bias gate source voltage approximately equal to Vdd-Vgs. With typical values of Vdd = 1V and Vgs2 = 0.5V, M3 suffers from a small overdrive voltage and hence a high channel resistance. Moreover, the non-linearity of the capacitance of M3 may manifest itself if the circuit senses large interferers. For these reasons, MOS capacitors rarely serve as coupling devices.

One application of MOS capacitors is in supply bypass. As shown in figure (b), the supply line may include significant bond wire inductance, allowing feedback from the second stage to the first at high frequencies. The bypass capacitor, M3 creates a low impedance between the supply and the ground, suppressing the feedback. In this case, the Q of M3 is still important if the equivalent series resistance of the device becomes comparable with the reactance of its capacitance, then the bypass impedance may not be low enough to suppress the feedback.