GB2363923A - Variable capacitance circuit - Google Patents
Variable capacitance circuit Download PDFInfo
- Publication number
- GB2363923A GB2363923A GB0015105A GB0015105A GB2363923A GB 2363923 A GB2363923 A GB 2363923A GB 0015105 A GB0015105 A GB 0015105A GB 0015105 A GB0015105 A GB 0015105A GB 2363923 A GB2363923 A GB 2363923A
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- Prior art keywords
- variable capacitance
- voltage
- capacitance
- circuit according
- devices
- Prior art date
- Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
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- 239000003990 capacitor Substances 0.000 claims description 9
- 230000008878 coupling Effects 0.000 claims description 6
- 238000010168 coupling process Methods 0.000 claims description 6
- 238000005859 coupling reaction Methods 0.000 claims description 6
- 238000000034 method Methods 0.000 claims description 3
- 230000008569 process Effects 0.000 claims description 3
- 239000000758 substrate Substances 0.000 claims description 2
- 238000010079 rubber tapping Methods 0.000 claims 2
- 230000007704 transition Effects 0.000 description 7
- 230000035945 sensitivity Effects 0.000 description 6
- 238000010586 diagram Methods 0.000 description 5
- 230000004044 response Effects 0.000 description 5
- 230000000694 effects Effects 0.000 description 2
- 230000009471 action Effects 0.000 description 1
- 230000008901 benefit Effects 0.000 description 1
- 230000008859 change Effects 0.000 description 1
- 239000004020 conductor Substances 0.000 description 1
- 238000010276 construction Methods 0.000 description 1
- 230000001186 cumulative effect Effects 0.000 description 1
- 230000003247 decreasing effect Effects 0.000 description 1
- 210000003127 knee Anatomy 0.000 description 1
- 230000007246 mechanism Effects 0.000 description 1
- 239000002184 metal Substances 0.000 description 1
- 230000004048 modification Effects 0.000 description 1
- 238000012986 modification Methods 0.000 description 1
- 229910021420 polycrystalline silicon Inorganic materials 0.000 description 1
- 229920005591 polysilicon Polymers 0.000 description 1
- 230000000717 retained effect Effects 0.000 description 1
- 230000007480 spreading Effects 0.000 description 1
Classifications
-
- H—ELECTRICITY
- H01—ELECTRIC ELEMENTS
- H01L—SEMICONDUCTOR DEVICES NOT COVERED BY CLASS H10
- H01L29/00—Semiconductor devices specially adapted for rectifying, amplifying, oscillating or switching and having potential barriers; Capacitors or resistors having potential barriers, e.g. a PN-junction depletion layer or carrier concentration layer; Details of semiconductor bodies or of electrodes thereof ; Multistep manufacturing processes therefor
- H01L29/66—Types of semiconductor device ; Multistep manufacturing processes therefor
- H01L29/86—Types of semiconductor device ; Multistep manufacturing processes therefor controllable only by variation of the electric current supplied, or only the electric potential applied, to one or more of the electrodes carrying the current to be rectified, amplified, oscillated or switched
- H01L29/92—Capacitors having potential barriers
- H01L29/94—Metal-insulator-semiconductors, e.g. MOS
-
- H—ELECTRICITY
- H03—ELECTRONIC CIRCUITRY
- H03J—TUNING RESONANT CIRCUITS; SELECTING RESONANT CIRCUITS
- H03J3/00—Continuous tuning
- H03J3/02—Details
- H03J3/16—Tuning without displacement of reactive element, e.g. by varying permeability
- H03J3/18—Tuning without displacement of reactive element, e.g. by varying permeability by discharge tube or semiconductor device simulating variable reactance
- H03J3/185—Tuning without displacement of reactive element, e.g. by varying permeability by discharge tube or semiconductor device simulating variable reactance with varactors, i.e. voltage variable reactive diodes
Landscapes
- Engineering & Computer Science (AREA)
- Power Engineering (AREA)
- Microelectronics & Electronic Packaging (AREA)
- Physics & Mathematics (AREA)
- Ceramic Engineering (AREA)
- Condensed Matter Physics & Semiconductors (AREA)
- General Physics & Mathematics (AREA)
- Computer Hardware Design (AREA)
- Inductance-Capacitance Distribution Constants And Capacitance-Resistance Oscillators (AREA)
Abstract
In order to form a variable capacitance circuit 36 with a desired linear characteristic from variable capacitance devices with highly non-linear characteristics, the circuit 36 comprises a plurality of variable capacitance devices 38n coupled to a common control voltage source 30, each variable capacitance device being connected to a respective voltage offset means such as a drive amplifiers 40n, diodes (figure 8) or a resistance network (figure 7). In this way, for the range of control voltages, the variable capacitance devices are operative in different regions of their voltage capacitance characteristics each to contribute an incremental capacitance as the control voltage rises, whereby to provide an overall desired voltage capacitance characteristic. The invention makes the use of such variable capacitance devices practicable in circuits such as integrated circuit voltage controlled oscillators for application in wireless systems.
Description
2363923 VARIABLE CAPACITANCE CIRCUIT The present invention relates to
variable capacitance circuits, employing voltage controlled variable capacitance devices.
Voltage controlled variable capacitance devices, commonly known as varicap devices, are widely used in circuits such as voltage controlled oscillators, voltage controlled phase shifters and voltage controlled filters It is desirable to use varicap devices whose capacitance varies smoothly and linearly with control voltage from a minimum capacitance value to a maximum capacitance value across some given voltage range (e g a voltage between two supply voltages) available within a system.
However, there may occur situations where it is necessary to use a varicap device with a far from desirable voltage/capacitance characteristic, but which may for example be produced at low cost.
For example, referring to Figure la a nearly abrupt voltage/capacitance transfer function of a varicap device is shown Figure lb shows such varicap device 2 in a circuit with a control voltage source 4 and a series resistance 6 In this circuit the nearly abrupt transition of device 2 gives a high sensitivity to noise at the control voltage input The nearly abrupt transition also gives the circuit very high sensitivity to control voltage over a small range of voltage at the transition, as shown in the region below IV It has a very low sensitivity to control voltage over the remaining voltage range, as shown above 1 V, in which the capacitance has a generally flat characteristic and remains close to a maximum value.
A typical voltage controlled system employing the circuit of Figure lb is shown in Figure 2, wherein control voltage source 4 is connected via series resistance 6 to the cathode of varicap device 6 The cathode of device 6 is also connected via a coupling capacitor 8 to the remainder of the system 10, which may, for example, be a voltage controlled oscillator In use as a voltage controlled oscillator (VCO), the VCO would have a very high voltage to frequency gain over the small voltage range at the transition, and near zero gain over the rest of the voltage range This 2 would disrupt the operation, for example, of a phase locked loop system incorporating such a VCO The VCO would, in addition, be highly sensitive to control voltage drifts and offsets.
Summary of the Invention
It is an object of the invention to employ variable capacitance devices with undesirable transfer characteristics in such a way that the effects of these undesirable characteristics are reduced.
The present invention provides a variable capacitance circuit comprising a plurality of variable capacitance devices coupled to means for providing a control voltage, and the variable capacitance devices being responsive to respective voltage offset means which have values for ensuring that for at least part of the range of the control voltage values, the variable capacitance devices are operative in different regions of their respective voltage/capacitance characteristics, whereby to provide an overall desired voltage capacitance characteristic of the variable capacitance circuit.
As preferred, the desired characteristic is linear, but may be some other function, for example quadratic square law The precise characteristic will depend on the intended application A VCO may require a linear characteristic, whereas for example a voltage controlled delay device may require a non-linear characteristic.
As preferred, a multiplicity of variable capacitance devices are provided, each having a relatively small value of capacitance, the devices contributing their capacitance value in sequence as the control voltage is raised, so that the resultant characteristic approximates to the desired characteristic more closely In one preferred embodiment, the devices are formed in a CMOS integrated circuit as an array of a large number of devices, each device comprising an insulated gate positioned over a conductive well The gate provides one electrode of the capacitance device, and the other electrode is provided by the well connected to terminal means.
3 The variable capacitance devices may be connected to the control voltage source in any suitable configuration In one preferred form, the variable capacitance devices are connected in a configuration, with each device being coupled to the control voltage source via a driver amplifier and a series impedance The driver amplifier is preferably a limiting amplifier having a limiting voltage output corresponding to the flat part of the capacitance characteristic, for determining the maximum capacitance value The series impedance is connected to one electrode of the variable capacitance device; this one electrode is also connected via a coupling capacitor to a common output The driver amplifiers are coupled to offset voltages which have incremental values so as to move the variable capacitance devices to their transition regions in sequence as the control 1 o voltage is raised.
In a further form of the invention, first electrodes of the variable capacitance devices are connected to a common nodal junction, the nodal junction being coupled to receive the control voltage via a common series impedance The nodal junction is also coupled to a common coupling capacitor for connection to an output Second electrodes of the devices are connected to respective offset voltages for causing operation in different regions of their characteristics for a given control voltage value.
Brief Description of the Drawings
Preferred embodiments of the invention will now be described with reference to the accompanying drawings wherein:
Figure la is a voltage/capacitance characteristic of a variable capacitance device for use in the present invention, and Figure lb shows the device in a typical control circuit; Figure 2 is a schematic diagram of a voltage controlled system employing the control circuit of Figure ib; Figure 3 is a circuit diagram of a first embodiment of a variable capacitance circuit according to the invention; Figure 4 is a diagram showing the various voltage characteristics for the driver amplifier circuits of Figure 3; Figure 5 shows the overall voltage capacitance characteristic for the circuit of Figure 3; 4 Figure 6 is a schematic circuit diagram of a second preferred embodiment of the invention; Figures 7 and 8 are diagrams of two circuits for providing offset voltages to the embodiments of Figures 3 and 6; Figure 9 is a sectional view of one example of a variable capacitance device which may be employed in the invention; and Figure 10 is a plan view of an array of devices according to Figure 9.
Description of the Preferred Embodiments
The invention proposes splitting a given nearly abrupt varicap device of active value A, as shown in Figure 2, into a number N of smaller devices of value A/n, as shown in Figure 3 Each such small device is controlled by the output voltage of one of a set of N drivers Each driver has an output voltage which depends differently on the overall system control voltage.
Each driver is one of a family of limiting linear amplifiers, having a peak output magnitude, which is stable The responses of the drivers, when referred to the input, are substantially identical, except that each has a different input referred DC offset The offset of the amplifier number m is (Vos +(m-l)Vos),) where V 0,,, is the offset of amplifier number 1, and V, is a small offset voltage The responses of such a family of N drivers is shown in Figure 4 (where N = 10).
The offset may be provided by the construction of the amplifier, e g the thresholds of internal transistors may be adjusted; alternatively an external source may provide the voltage offset.
Referring to Figure 3, a control voltage source 30 is coupled via positive and negative lines 32, 34 to a variable capacitance arrangement 36 comprising a large number of individual variable capacitance devices 381 38 n 3810 in this example 10 devices are provided Line 32 is coupled to each device 38 N via a driver amplifier 40 N and a series resistance 42 n The cathode of each device 38 N is coupled via a coupling capacitor 44 N to the rest of the system 10 The anodes of devices 38 N are coupled to negative line 34.
As shown conceptually, each driver 40 N has a different offset voltage 46 n, so that the offset voltages are incremented by equal amounts from amplifier 401 to amplifier 4010 The family of driver responses is shown in Figure 4 Each capacitance 38 N has a value of about 0 1 p F.
In operation, as the control voltage 30 rises gradually from a zero value, firstly a voltage is provided by amplifier 401 to act on device 381 when the offset voltage 461 is overcome, so that device 381 goes through its transition region to reach the flat part of its characteristic, as shown in Figure 4, to provide an increment of capacitance In this flat region, the limiting amplifier 401 reaches its peak value to determine the maximum capacitance value While device 381 is going through its transition region, and when the control voltage has risen by a further increment V,, relative to offset voltage 461, driver 402 overcomes its offset voltage 462 and acts on device 382 in a similar manner as driver 401 acts on device 381 This process is repeated as the control voltage rises until all 10 devices provide their maximum capacitance, to give a stepwise actuation of the devices with a resultant characteristic as shown in Figure 5 It may be seen the characteristic is linear, at least in the range from 0 5 volts to 2 5 volts, with only a small ripple voltage caused by the stepwise actuation of the devices This ripple voltage may be decreased by employing a larger number of devices, each contributing a smaller capacitance value.
Thus, as the overall input control voltage is raised e g from OV, successive driver outputs are raised from OV to some voltage, e g supply voltage, according to the individual driver response.
Consequently, the control voltage of each individual smaller varicap is in turn raised The overall effect, as shown in Figure 5, is that the cumulative capacitance change is spread across a wider input control voltage Thus, by means of this invention, a group of small near-abruptly changing varicap diodes is controlled to achieve an overall larger varicap response which demonstrates the desired property of a smooth capacitance versus voltage characteristic, spread over a wide input control voltage.
Referring now to Figure 6, a second preferred embodiment is shown, similar parts to those shown in Figure 3 being denoted by the same reference numeral In this embodiment, the cathodes of capacitance devices 38 N are connected to a common nodal junction, which is connected via a common series resistance 60 to positive line 32 from control voltage source 30, 6 and via a common coupling capacitor 62 to the rest of the system 10 The anodes of devices 38 n are connected to respective offset voltage sources 64 N which are in turn connected to negative line 34 Voltage sources 64 are incremented in value across the series of capacitance devices.
The operation of Figure 6 is essentially the same as Figure 3 wherein each device 38 N is actuated in turn as the control voltage rises to give the overall characteristic of Figure 5, and will not be described in further detail.
In Figure 7 an arrangement is shown for providing the offset voltages for the circuits of Figures 3 and 6 It comprises a resistive divider chain comprising resistors Ri to Rn and RS connected in series across a voltage supply VDD-VSS The nodes between the resistors provide respective offset voltages V 1 Vn Such nodes are also connected via respective capacitors Cl Cn to voltage rail VSS in order to prevent disturbing AC signal components disturbing a stable DC voltage The resistors Ri to RS may all have the same value Alternatively, terminating resistor RS may have a different value It also possible for one or both of RI and RS to be zero ohms.
Referring now to Figure 8, this shows a further arrangement for providing offset voltages wherein the resistors RI to RN of Figure 7 are replaced by diodes DI to Dn Terminating resistor RS is retained The knee voltage of the diode characteristic provides the voltage division between the offset voltages V 1 to Vn This arrangement has the advantage over Figure 7 that for a given supply current, it is likely that the diodes will give less noise than the resistors of Figure 7 In both Figures 7 and 8, capacitors Cl to Cn are required to pass AC signal components to prevent disturbance of DC voltages V 1 to Vn In a modification, the arrangements of Figures 7 and 8 are combined and D 1 and R 2 to Rn give fine steps over an offset; one or more diodes D 1 to Dn may be employed in parallel with respective resistors RI to Rn.
Referring now to Figure 9, there is shown an example of a variable capacitance device for use in the present invention formed as part of a CMOS process A p substrate 90 has an n well 92 containing n+ source and drain areas 94, 96 A thin oxide insulating layer 98 carries a polysilicon gate 100 Metal terminals 102, 104, 106 are coupled to respective source 94, gate 100 and drain 106 Insulating oxide regions 108, and field oxide regions 110 are also provided.
The structure has similarities to a PMOS transistor, except that the source and drain terminals 94, 7 96 are n+, thus preventing transistor action Terminals 102, 106 are tied together in voltage so that the variable capacitance is formed between terminal 104 connected to gate 100, and well 92 coupled to terminals 102, 106.
A number of such devices may be formed as an array on an integrated circuit, as shown schematically in Figure 10 wherein, each device 38 N has terminals 102 n, 104 n, 106 n, and wherein terminals of adjacent devices are merged, e g terminals 1061, 1022 of devices 381, 382.
The terminals are connected by conductor lines (not shown) in order to implement the circuits of Figures 3 or 6.
Thus, the present invention provides a single controlled sensitivity circuit from a plurality of small devices, each driven by a separate control voltage generator In accordance with the invention, a high sensitivity varicap device with a nearly abrupt capacitance/voltage characteristic is transformed into a more practical device with a smoother capacitance/voltage control characteristic The basic varicap structure is high quality factor Q, and the invention introduces no series resistance into the varicap signal path, avoiding the introduction of noise.
Thus the invention provides a mechanism for spreading the control voltage over a wider range, and lowering the sensitivity The invention makes the use of such varicaps practicable in circuits such as integrated circuit voltage controlled oscillators for application in wireless systems, voltage controlled phase shifters, and voltage controlled filters.
Claims (14)
1 A variable capacitance circuit comprising a plurality of variable capacitance devices coupled to means for providing a control voltage, and the variable capacitance devices being responsive to respective voltage offset means which have values for ensuring that for at least part of the range of control voltage values, the variable capacitance devices are operative in different regions of their respective voltage/capacitance characteristics, whereby to provide an overall desired voltage capacitance characteristic of the variable capacitance circuit.
2 A circuit according to claim 1, wherein each variable capacitance device has a relatively small capacitance value, the arrangement such that the capacitance values are contributed is in sequence to an overall capacitance as the control voltage progressively changes.
3 A circuit according to claim 1 or 2, wherein each variable capacitance device has a respective input circuit comprising a drive amplifier with an input connected to said means for providing a control voltage, and the drive amplifier being associated with a respective offset voltage means, the output of the drive amplifier being coupled to an electrode of the variable capacitance device.
4 A circuit according to claim 3, wherein each drive amplifier is a limiting amplifier with its peak output value acting to determine the maximum value of the respective variable capacitance device.
A circuit according to claim 3 or 4, wherein each drive amplifier includes a respective voltage offset means.
6 A circuit according to claim 1, wherein each variable capacitance device has first and second electrodes, and wherein first electrodes of the variable capacitance devices are connected in common to said means for providing a control voltage and wherein the second electrodes of the variable capacitance devices are connected to a respective offset voltage means.
7 A circuit according to any preceding claim, wherein said offset voltage means comprises a voltage dividing chain comprising a series of impedances, tapping points between the impedances providing respective offset voltages.
8 A circuit according to claim 7, wherein said impedances comprise resistors and/or diodes.
9 A circuit according to claim 7 or 8, wherein each tapping point is coupled to a capacitance for removing AC signal components.
A circuit according to claim 3, 4 or 5, wherein each variable capacitance device is connected by a respective output coupling capacitor to an output.
11 A circuit according to claim 6, wherein the electrodes of the variable capacitance devices are connected to a common output capacitor.
12 A circuit according to any preceding claim, wherein the voltagecapacitance characteristic of each device is approximately a step function.
13 A circuit according to any preceding claim, wherein each variable capacitance device is formed by a CMOS process and comprises a well of predetermined conductivity type in a substrate, an insulating layer positioned over the well, and a gate electrode positioned on the insulating layer, wherein the gate electrode provides a first electrode and said well provides a second electrode.
14 A circuit according to any preceding claim, wherein the overall desired voltage capacitance characteristic is linear.
Variable capacitance circuits as claimed in claim 1, and substantially as described with reference to the accompanying drawings.
Priority Applications (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0015105A GB2363923A (en) | 2000-06-20 | 2000-06-20 | Variable capacitance circuit |
Applications Claiming Priority (1)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| GB0015105A GB2363923A (en) | 2000-06-20 | 2000-06-20 | Variable capacitance circuit |
Publications (2)
| Publication Number | Publication Date |
|---|---|
| GB0015105D0 GB0015105D0 (en) | 2000-08-09 |
| GB2363923A true GB2363923A (en) | 2002-01-09 |
Family
ID=9894052
Family Applications (1)
| Application Number | Title | Priority Date | Filing Date |
|---|---|---|---|
| GB0015105A Withdrawn GB2363923A (en) | 2000-06-20 | 2000-06-20 | Variable capacitance circuit |
Country Status (1)
| Country | Link |
|---|---|
| GB (1) | GB2363923A (en) |
Cited By (2)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2425007A (en) * | 2004-09-20 | 2006-10-11 | George Hedley Storm Rokos | Variable reactance tuning for a resonant circuit |
| US8022789B2 (en) | 2005-04-05 | 2011-09-20 | George Hedley Storm Rokos | Tuning control arrangement |
Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4456917A (en) * | 1980-12-18 | 1984-06-26 | Clarion Co., Ltd. | Variable capacitor |
-
2000
- 2000-06-20 GB GB0015105A patent/GB2363923A/en not_active Withdrawn
Patent Citations (1)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| US4456917A (en) * | 1980-12-18 | 1984-06-26 | Clarion Co., Ltd. | Variable capacitor |
Cited By (3)
| Publication number | Priority date | Publication date | Assignee | Title |
|---|---|---|---|---|
| GB2425007A (en) * | 2004-09-20 | 2006-10-11 | George Hedley Storm Rokos | Variable reactance tuning for a resonant circuit |
| GB2425007B (en) * | 2004-09-20 | 2010-11-17 | George Hedley Storm Rokos | Tuning control arrangement |
| US8022789B2 (en) | 2005-04-05 | 2011-09-20 | George Hedley Storm Rokos | Tuning control arrangement |
Also Published As
| Publication number | Publication date |
|---|---|
| GB0015105D0 (en) | 2000-08-09 |
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Legal Events
| Date | Code | Title | Description |
|---|---|---|---|
| WAP | Application withdrawn, taken to be withdrawn or refused ** after publication under section 16(1) |