Switched varactor circuit for a voltage controlled oscillator

A switched varactor circuit for use at at least one operating frequency, the switched varactor circuit comprising:

• a first resistive element having a first terminal and a second terminal, wherein the first terminal is coupled to receive a switching voltage;
• a hetero-junction bipolar transistor (HBT) having a base terminal, an emitter terminal, and a collector terminal, wherein the base terminal of the HBT is coupled to a second terminal of the first resistive element, and wherein the emitter terminal is coupled to a first circuit node;
• and
• a first varactor having an anode coupled to the collector terminal of the HBT and a cathode coupled to a second circuit node, and wherein a capacitance value at the first circuit node is a function of the switching voltage, and wherein the first circuit node is coupled to an anode of a second varactor and the second circuit node is coupled to a cathode of the second varactor;
• and
• a second resistive element having a first terminal coupled to the anode of the first varactor and a second terminal coupled to a first power supply terminal,
• wherein:
• when the HBT is substantially conducting, the capacitance value at the first circuit node is substantially equal to a sum of a capacitance of the first varactor and a capacitance of the second varactor, and
• when the HBT is substantially non-conducting, the capacitance value at the first circuit node is substantially equal to a capacitance of the second varactor.

A switched varactor circuit for use at at least one operating frequency, the switched varactor circuit comprising:

• a first resistive element having a first terminal and a second terminal, wherein the first terminal is coupled to receive a switching voltage;
• a hetero-junction bipolar transistor (HBT) having a base terminal, an emitter terminal, and a collector terminal, wherein the base terminal of the HBT is coupled to a second terminal of the first resistive element, and wherein the emitter terminal is coupled to a first circuit node;
• and
• a first varactor having an anode coupled to the collector terminal of the HBT and a cathode coupled to a second circuit node, and wherein a capacitance value at the first circuit node is a function of the switching voltage, and wherein the first circuit node is coupled to an anode of a second varactor and the second circuit node is coupled to a cathode of the second varactor,
• wherein:
• when the HBT is substantially conducting, the capacitance value at the first circuit node is substantially equal to a sum of a capacitance of the first varactor and a capacitance of the second varactor, and
• when the HBT is substantially non-conducting, the capacitance value at the first circuit node is substantially equal to a capacitance of the second varactor;
• and
• the first circuit node is coupled to a first terminal of a second resistive element and a first terminal of a capacitor, wherein a second terminal of the second resistive element is coupled to the first power supply terminal.

A differential voltage controlled oscillator (VCO) for providing at least one operating frequency, the differential VCO comprising:

• a first switched varactor circuit comprising:
• a first resistive element having a first terminal and a second terminal, wherein the first terminal is coupled to receive a switching voltage;
• a first hetero-junction bipolar transistor (HBT) having a base terminal, a first conducting terminal, and a second conducting terminal, wherein the base terminal of the HBT is coupled to a second terminal of the first resistive element, and wherein the first conducting terminal is coupled to a first circuit node;
• a first varactor having an anode coupled to the second conducting terminal of the HBT and a cathode coupled to a second circuit node, wherein a capacitance value at the first circuit node is a function of the switching voltage, and
• a second varactor, wherein the first circuit node is coupled to an anode of the second varactor and the second circuit node is coupled to a cathode of the second varactor;
• and
• a second switched varactor circuit comprising:
• a second resistive element having a first terminal and a second terminal, wherein the first terminal is coupled to receive the switching voltage;
• a second hetero-junction bipolar transistor (HBT) having a base terminal, a first conducting terminal, and a second conducting terminal, wherein the base terminal of the second HBT is coupled to a second terminal of the second resistive element, and wherein the first conducting terminal of the second HBT is coupled to a third circuit node;
• a third varactor having an anode coupled to the second conducting terminal of the second HBT and a cathode coupled to the second circuit node, wherein a capacitance value at the third circuit node is a function of the switching voltage, and
• a fourth varactor, wherein the third circuit node is coupled to an anode of the fourth varactor and the second circuit node is coupled to a cathode of the fourth varactor,
• wherein:
• the first switched varactor circuit further comprises:
• a third resistive element having a first terminal coupled to the anode of the first varactor and a second terminal coupled to a first power supply terminal, and
• the second switched varactor circuit further comprises:
• a fourth resistive element having a first terminal coupled to the anode of the third varactor and a second terminal coupled to the first power supply terminal.