Differential LC voltage-controlled oscillator

• US 6,469,587 B2
• Filed: 12/04/2000
• Issued: 10/22/2002
• Est. Priority Date: 12/04/2000
• Status: Expired due to Term

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Agere Systems Guardian Corp. (Broadcom, Inc.)
• Inventors: Scoggins, John E.
• Primary Examiner(s): KINKEAD, ARNOLD M
• Application Number:
  US09/729,002
• Publication Number:
  US 20020113659A1
• Time in Patent Office: 687 Days
• Field of Search: 331/36.C, 331/177.V, 331/117.R, 331/117.FE, 331/182, 331/183
• US Class Current: 331/117.00R
• CPC Class Codes: H03B 2200/0038 including a current mirror
• CPC Class Codes: H03B 5/1215 the current source or degeneration circuit being in common to both transistors of the pair, e.g. a cross-coupled long-tailed pair
• CPC Class Codes: H03B 5/1228 the amplifier comprising one or more field effect transistors
• CPC Class Codes: H03B 5/124 the means comprising a voltage dependent capacitance

Abstract

A differential voltage-controlled oscillator (VCO) employs a pair of accumulation-mode varactors driven with a differential control voltage to generate a differential oscillating waveform. The differential control voltage is formed from a pair of level-shifted input differential control voltage components. Level shifting of the input control voltages and driving the varactors with a differential control voltage allows for biasing of the varactors over a substantial range of capacitance variation. Such differential VCO may be employed within a phase-locked loop (PLL) circuit, with the pair of input control voltages being provided by the loop filter of the PLL circuit. The differential VCO comprises a differential control voltage to voltage converter (CV2VC) coupled to an LC-tank VCO. To improve common-mode noise rejection of the LC-VCO, the inductors of the LC-tank may be AC-coupled to the supply voltage, and the output differential oscillating waveform may be AC-coupled to the LC-tank through capacitors.

Claims

1. A voltage-controlled oscillator comprising:

a voltage-to-voltage converter configured to adjust a gain of an input differential voltage and to shift a DC level of the differential input voltage to generate a differential control voltage, the voltage to voltage converter comprising;

a voltage-follower amplifier is configured to provide a relatively high input impedance to a circuit providing the differential input voltage;

a first differential amplifier stage is configured to shift the DC level of the differential input voltage; and

a second amplifier stage is configured so as to provide a predetermined overall gain of the voltage-to-voltage converter, the overall gain including a gain of the voltage-follower amplifier and the first differential amplifier stage,

wherein the voltage to voltage converter applies the overall gain to the differential input voltage; and

a differential VCO comprising;

an inductance/capacitance (LC) tank circuit having a pair of varactors configured back-to-back, wherein capacitance of the back-to-back varactors contributes to capacitance of the LC-tank circuit; and

a differential amplifier having its differential output terminals coupled through the LC-tank circuit to its differential input terminals in a feedback path configuration,

wherein the VCO receives the differential control voltage applied across each varactor to set capacitance of each varactor; and

the differential amplifier generates an oscillating waveform in the feedback path at a frequency that is a function of the capacitance and inductance of the LC-tank circuit.

2. A voltage-controlled oscillator (VCO), comprising:

an inductance/capacitance (LC) tank circuit having a pair of varactors configured back-to-back, wherein capacitance of the back-to-back varactors contributes to capacitance of the LC-tank circuit;

a differential amplifier having its differential output terminals coupled through the LC-tank circuit to its differential input terminals in a feedback path configuration; and

a voltage-to-voltage converter configured to adjust a gain of an input differential voltage and to shift a DC level of the differential input voltage,

wherein the VCO receives a differential control voltage applied across each varactor to set capacitance of each varactor; and

the differential amplifier generates an oscillating waveform in the feedback path at a frequency that is a function of the capacitance and inductance of the LC-tank circuit.

3. The VCO as recited in claim 2, wherein each varactor includes 1) a common terminal where the pair of back-to-back varactors are coupled back-to-back, the common terminal coupled to one component of the differential control voltage, and 2) a separate terminal coupled to a complement component of the differential control voltage.

4. The VCO as recited in claim 3, wherein the inductance of the LC-tank circuit includes a pair of inductors, each inductor coupled between a source voltage driving the differential amplifier and the separate terminal of a corresponding varactor.

5. The VCO as recited in claim 4, wherein each inductor is coupled between the source voltage and the separate terminal of a corresponding varactor with a capacitor.

6. The VCO as recited in claim 3, wherein the oscillating waveform includes a pair of differential oscillation components, each differential oscillation component being AC-coupled to a corresponding output terminal through a capacitor.

7. The VCO as recited in claim 2, wherein the differential amplifier comprises a pair of cross-coupled transistors.

8. The VCO as recited in claim 2, wherein the gain and the DC level is set to adjust a DC bias point across each varactor so as to increase an available range of capacitance for the varactor.

9. The VCO as recited in claim 2, wherein the voltage-to-voltage converter comprises a voltage-follower amplifier, a first differential amplifier stage, and a second differential amplifier stage, wherein:

the voltage-follower amplifier is configured to provide a relatively high input impedance for the voltage-to-voltage converter,

the first differential amplifier stage is configured to shift the DC level, and

the second amplifier stage is configured so as to provide a predetermined overall gain of the voltage-to-voltage converter, the overall gain including a gain of the voltage-follower amplifier and the first differential amplifier stage.

10. The VCO as recited in claim 9, wherein each of the first and second differential amplifier stages is coupled to a corresponding voltage driver pair of transistors to isolate the differential amplifier stage and drive its differential output signal.

11. The VCO as recited in claim 2, wherein each varactor is either i) an accumulation mode NMOS varactor, ii) a depletion mode NMOS varactor, iii) a bipolar Base-Emitter Junction varactor, iv) a bipolar Base-Collector Junction varactor, v) a P+ to Ntub Junction varactor, or vi) a N+ to Ptub Junction varactor.

12. The VCO as recited in claim 2, wherein the circuit is embodied in an integrated circuit.

13. A method of generating an oscillation waveform with a voltage controlled oscillator (VCO) comprising the steps of:

(a) coupling an inductance/capacitance (LC) tank circuit in a feedback path between input and output terminals of a differential amplifier, the LC tank having a pair of varactors configured back-to-back and capacitance of the back-to-back varactors included in capacitance of the LC-tank circuit; and

(b) applying a differential control voltage across each varactor to set capacitance of each varactor;

(c) generating, by the differential amplifier, an oscillating waveform in the feedback path at a frequency that is a function of the capacitance and inductance of the LC-tank circuit; and

(d) adjusting a gain of an input differential voltage and shifting a DC level of the differential input voltage to generate the differential control voltage that increases an available range of capacitance for the varactor.

14. The method as recited in claim 13, wherein step (a) comprises the step of:

(a1) AC-coupling the inductance of the LC tank between the pair of output terminals and a source voltage; and

wherein step (b) comprises the step of;

(c2) AC-coupling the oscillating waveform in the feedback path across the pair of output terminals.