Staggered-bias varactor
Techniques are described for staggered-bias varactors. For example, a staggered-bias varactor can include a control voltage node, a number of bias voltage nodes, and a number of sub-varactors coupled in parallel. The control voltage node can be configured to receive a single, variable control voltage; and the bias voltage nodes can each be configured to receive a different, fixed bias voltage. Each sub-varactor is configured, so that its equivalent capacitance is a function of a difference between the control voltage and a respective one of the bias voltages; and the equivalent capacitance of the staggered-bias varactor is a function of the capacitances of the component sub-varactors. The number of varactors and the bias voltages can be configured, so that respective non-linear capacitive responses of the component sub-varactors effectively combine to yield a substantially linear capacitive response for the staggered-bias varactor as a whole.
1. A staggered-bias varactor comprising:
a pair of varactor terminals;
a control voltage node;
N bias voltage input nodes, wherein N is a positive integer greater than one; and
N sub-varactors, each coupled between the pair of varactor terminals, and having a variable capacitance portion coupled with the control voltage node and with a respective one of the N bias voltage nodes, the variable capacitance portion comprising:
a first capacitor coupled between the respective first bias voltage terminal and a respective control voltage terminal; and
a second capacitor coupled between the respective control voltage terminal and a respective second bias voltage terminal,
wherein the first bias voltage terminal is coupled with a first of the pair of varactor terminals and with the respective one of the N bias voltage nodes, and the second bias voltage terminal is coupled with a second of the pair of varactor terminals and with the respective one of the N bias voltage nodes,
wherein, for each sub-varactor, the first capacitor and the second capacitor have a same nominal capacitance,
such that a respective sub-varactor capacitance of each sub-varactor varies as a function of a control voltage applied at the control voltage node and a different respective bias voltage applied at each respective one of the N bias voltage nodes.
2. The staggered-bias varactor of claim 1 , wherein:
for each sub-varactor, the respective sub-varactor capacitance varies non-linearly as a function of the control voltage applied at the control voltage node; and
in response to applying the different respective bias voltage at each respective one of the N bias voltage nodes, a total varactor capacitance across the N sub-varactors in parallel varies substantially linearly to as a function of the control voltage.
3. The staggered-bias varactor of claim 1 , wherein the first bias voltage terminal is coupled with the first of the pair of varactor terminals via a third capacitor.
4. The staggered-bias varactor of claim 1 , wherein the second bias voltage terminal is coupled with the second of the pair of varactor terminals via a fourth capacitor.
5. The staggered-bias varactor of claim 1 , wherein the respective first bias voltage terminal and the respective second bias voltage terminal are each coupled with the respective one of the N bias voltage nodes via a respective resistor.
6. A method for controlling a total capacitance across varactor terminals of a staggered-bias varactor, the method comprising:
applying a respective one of N different fixed bias voltage levels at each of N bias voltage input nodes of the staggered-bias varactor, wherein N is a positive integer greater than one; and
varying a control voltage level applied at a control voltage node,
wherein the staggered-bias varactor comprises N sub-varactors, each coupled between the varactor terminals, and each having a variable capacitance portion coupled with the control voltage node and with a respective one of the N bias voltage nodes,
the variable capacitance portion comprising:
a first capacitor coupled between the respective first bias voltage terminal and a respective control voltage terminal; and
a second capacitor coupled between the respective control voltage terminal and a respective second bias voltage terminal,
wherein the first bias voltage terminal is coupled with a first of the pair of varactor terminals and with the respective one of the N bias voltage nodes, and the second bias voltage terminal is coupled with a second of the pair of varactor terminals and with the respective one of the N bias voltage nodes,
wherein, for each sub-varactor, the first capacitor and the second capacitor have a same nominal capacitance,
such that a respective sub-varactor capacitance of each sub-varactor varies in response to varying the control voltage level.
7. The method of claim 6 , further comprising:
setting the N different fixed bias voltage levels, such that, in response to applying the respective one of the N different fixed bias voltage level at each of the N bias voltage input nodes:
the respective sub-varactor capacitance of each sub-varactor varies non-linearly in response to varying the control voltage level; and
the total capacitance of the staggered-bias varactor varies substantially linearly in response to varying the control voltage level, the total capacitance of the staggered-bias varactor being a capacitance across the N sub-varactors in parallel.
8. An oscillator system comprising:
a capacitor-controlled oscillator to generate an output signal at an oscillator frequency; and
a varactor having varactor terminals coupled with the capacitor-controlled oscillator, the varactor comprising:
a control voltage node;
N bias voltage input nodes, wherein N is a positive integer greater than one; and
N sub-varactors, each coupled between the varactor terminals, and having a variable capacitance portion coupled with the control voltage node and with a respective one of the N bias voltage nodes, the variable capacitance portion comprising:
a first capacitor coupled between the respective first bias voltage terminal and a respective control voltage terminal; and
a second capacitor coupled between the respective control voltage terminal and a respective second bias voltage terminal,
wherein the first bias voltage terminal is coupled with a first of the pair of varactor terminals and with the respective one of the N bias voltage nodes, and the second bias voltage terminal is coupled with a second of the pair of varactor terminals and with the respective one of the N bias voltage nodes,
wherein, for each sub-varactor, the first capacitor and the second capacitor have a same nominal capacitance,
such that a respective sub-varactor capacitance of each sub-varactor varies as a function of a control voltage applied at the control voltage node and a respective bias voltage applied at the respective one of the N bias voltage nodes, and
such that the oscillator frequency is a function of a total capacitance of the N respective sub-varactor capacitances in parallel.
9. The oscillator system of claim 8 , wherein the capacitor-controlled oscillator is an inductive-capacitive (LC) oscillator, wherein a resonant frequency of the LC oscillator is responsive to the total capacitance of the N respective sub-varactor capacitances in parallel.
10. The oscillator system of claim 8 , wherein the capacitor-controlled oscillator is a ring oscillator, wherein a ring delay is responsive to the total capacitance of the N respective sub-varactor capacitances in parallel.
11. The oscillator system of claim 8 , further comprising:
a phase-locked loop (PLL) comprising a voltage-controlled oscillator (VCO), a PLL output frequency of the PLL being controlled by the VCO, the VCO comprising the capacitor-controlled oscillator and the varactor.
12. The oscillator system of claim 8 , wherein:
a different respective bias voltage is applied at each respective one of the N bias voltage nodes.
13. The oscillator system of claim 12 , wherein:
for each sub-varactor, the respective sub-varactor capacitance varies non-linearly as a function of the control voltage applied at the control voltage node; and
in response to applying the different respective bias voltage at each respective one of the N bias voltage nodes, the total capacitance of the N respective sub-varactor capacitances in parallel varies substantially linearly to as a function of the control voltage.
14. The oscillator system of claim 8 , wherein, for each sub-varactor:
the first respective bias voltage terminal is coupled with the first of the pair of varactor terminals via a third capacitor; and
the second respective bias voltage terminal is coupled with the second of the pair of varactor terminals via a fourth capacitor.
15. The oscillator system of claim 8 , wherein, for each sub-varactor:
each of the first respective bias voltage terminal and the second respective bias voltage terminal is coupled with the respective one of the N bias voltage nodes via a respective resistor.