Bipolar bandwidth switch for use in a phase-locked loop

摘要

An integrated circuit phase-locked loop includes a bipolar bandwidth switch for selectively coupling circuit elements of an external bandwidth filter network so as to control loop bandwidth. The bandwidth switch includes first and second switched terminal means which are coupled to the bandwidth filter network. A differential amplifier has a first control terminal coupled to the first switched terminal and a second control terminal coupled to the second switched terminal. The differential amplifier controls first and second currents as a function of a difference between potentials applied to the first and second switched terminals. A first current mirror couples the differential amplifier to the second switched terminal and controls current flow between a first supply terminal and the second switched terminal as a function of the first current. A second current mirror couples the differential amplifier to the second switched terminal and controls current flow between the second switched terminal and a second supply terminal as a function of the second current. Control means responsive to digital control signals control current flow through the differential amplifier, and thereby the impedance between the first and second switched terminals.