ELECTRONIC CIRCUIT FOR IMITATING A CAPACITOR WITH VARIABLE CAPACITANCE

Abstract: An electronic circuit for imitating a capacitor with a controllable capacitance comprises a chain (CHN) of at least two differential pairs (DF1-DF3), the tail of each differential pair (DF1-DF3) being connected to a reference terminal (GND) via a respective separate current source (I1-I3). Each differential pair comprises a first and a second input. All the first inputs of the differential pairs (DF1-DF3) are connected to a signal terminal (CPVR) of the electronic unit. All the second inputs of the differential pairs (DF1-DF3) are interconnected via means (SPMNS) for supplying separate DC voltages between the second inputs of the differential pairs (DF1-DF3). Between the signal terminal (CPVR) and the reference terminal (GND) a capacitance is formed which is the sum of the input capacitances of the differential pairs (DF1-DF3). The capacitance is determined by the amount of charge per unit of voltage which is delivered to the first inputs of the differential pairs (DF1-DF3). If the separate DC voltages between the second inputs of the differential pairs (DF1-DF3) increase, then said amount of charge will decrease and as a consequence the capacitance will decrease. Thus the capacitance can be varied in that said separate DC voltages are varied.

Description: Electronic circuit for imitating a capacitor with variable capacitance. The invention relates to an electronic circuit for imitating a capacitor with variable capacitance. Such an electronic circuit is known from the prior art, for example, where a so- called varicap diode is used as a variable capacitor. The capacitance ofthe varicap diode can be varied in that a reverse bias voltage across the varicap diode is modified. A disadvantage ofthe prior art is that the application of a varicap diode is not always desirable. This is the case, for example, in integrated circuits in which the possibility of integrating a varicap diode into the integrated circuit is not given. It is an object ofthe invention to provide an electronic circuit for the imitation of a capacitor with variable capacitance which does not require the use of a varicap diode. According to the invention, the electronic circuit mentioned in the opening paragraph is for this purpose characterized in that the electronic circuit comprises a chain of at least two differential pairs, the tail of each differential pair being connected to a reference terminal ofthe electronic circuit via a separate current source, while each differential pair has a first input and a second input, all said first inputs being coupled to a signal terminal ofthe electronic circuit and all said second inputs being coupled to one another via separate means for providing separate DC voltages between the second inputs. The invention is based on the recognition that the input capacitance of a differential pair can be varied in that the potential of at least one ofthe inputs (first or second) ofthe differential pair is varied. An embodiment of an electronic circuit according to the invention is characterized in that said separate means for providing the separate DC voltages between the second inputs comprise: a chain of resistors connected in series and a controllable current source coupled to the chain of resistors connected in series for controlling said separate DC voltages. A simple implementation ofthe separate means mentioned above is realized thereby. The controllable current source delivers a controllable current which flows through the chain of resistors connected in series, so that said separate DC voltages are realized across the resistors. An embodiment of an electronic circuit according to the invention is characterized in that the separate means for providing the separate DC voltages between the second inputs comprise: a chain of resistors connected in series and a controllable voltage source coupled to the chain of resistors connected in series for controlling the separate DC voltages. A simple implementation ofthe separate means mentioned above is realized also by these means. It is an alternative solution to the embodiment mentioned in the preceding paragraph, the controllable current source being replaced by a controllable voltage source, while the chain of resistors coupled in series comprises an additional resistor which is connected in series with the controllable voltage source. An embodiment of an electronic circuit according to the invention is characterized in that the differential pairs comprise field effect transistors. The field effect transistors may be constructed, for example, with MOS transistors. An electronic circuit according to the invention offers the possibility inter alia of realizing frequency-tunable LC oscillators in MOS technology. A coil may be realized, for example, in passive technology or be implemented with a bonding wire of a chip. The tunable LC oscillator may form part, for example, of a phase-locked loop circuit. The electronic circuit according to the invention can be used in electronic circuits of various kinds which require a variable capacitance. The invention will be explained in more detail with reference to the accompanying drawing, in which: Fig. 1 is a circuit diagram of a first embodiment of an electronic circuit for imitating a capacitor with variable capacitance, Fig. 2 is a circuit diagram of a second embodiment of an electronic circuit for imitating a capacitor with variable capacitance; and Fig. 3 is a circuit diagram of a third embodiment of an electronic circuit for imitating a capacitor with variable capacitance. Corresponding components or elements have been given the same reference symbols in these Figures. Fig. 1 is a circuit diagram of a first embodiment of an electronic circuit for imitating a capacitor with variable capacitance. The electronic circuit comprises a chain of three differential pairs DFpDF 3 with three separate current sources I 1 -I 3 for the DC adjustment ofthe three differential pairs DFι-DF 3 , and separate means SPMNS constructed with voltage sources Vi and V 2 . The three separate current sources Iι-I 3 are coupled respectively between the tail ofthe first differential pair DFi and a reference terminal GND, between the tail ofthe second differential pair DF 2 and the reference terminal GND, and between the tail ofthe third differential pair DF 3 and the reference terminal GND. The three differential pairs DFι-DF are fitted with a first field effect transistor Ti and a second field effect transistor T , a third field effect transistor T 3 and a fourth field effect transistor T 4 , and a fifth field effect transistor T 5 and a sixth field effect transistor T , respectively. The gates ofthe first, third, and fifth field effect transistors Tr, T 3 , T 5 are connected to a signal terminal CPVR ofthe electronic circuit. The gates ofthe second and fourth field effect transistors T 2 , T 4 are interconnected via a voltage source Vi. The gates ofthe fourth and sixth field effect transistors T 4 , T are interconnected via a voltage source V 2 . The tails ofthe first, second, and third differential pairs DFι-DF 3 are formed by the sources ofthe first and second field effect transistors Tι-T 2 , the sources ofthe third and fourth field effect transistors T 3 -T 4 , and the sources ofthe fifth and sixth field effect transistors T 5 -T 6 , respectively. The electronic circuit operates as follows. A capacitance is formed between the signal terminal CPVR and the reference terminal GND which is the sum ofthe input capacitances ofthe differential pairs DFpDF 3 . The capacitance is determined by the quantity of charge which is transported per voltage unit to the gates ofthe first, third, and fifth field effect transistors T l5 T 3 , T 5 . The transported charge mentioned above will decrease when at least one ofthe voltages supplied by the voltage sources Vi and V 2 increases, with the result that the capacitance between the signal terminal CPVR and the reference terminal GND decreases. The capacitance can thus be varied in that the voltages supplied by the voltage sources Vi and V 2 are varied. An additional advantage ofthe electronic circuit is that the voltage (or voltages), to be varied for varying the capacitance between the signal terminal CPVR and the reference terminal GND do(es) not influence the voltage between the signal terminal CPVR and the reference terminal GND. This is in contrast to the conventional application of a varicap diode as a capacitor with variable capacitance. The adjustment range ofthe electronic circuit is limited by the parasitic capacitances ofthe field effect transistors Tι-T 6 to the substrate. The adjustment range ofthe electronic circuit is wider if it is implemented in the SOI technology. This is because said parasitic capacitances are lower then. Fig. 2 is a circuit diagram of a second embodiment of an electronic circuit for imitating a capacitor with variable capacitance. The circuit diagram of Fig. 2 is largely identical to that of Fig. 1. In this embodiment, the separate means SPMNS are implemented in the form of a chain CHN RSΓ of three resistors Rι-R 3 connected in series and a controllable current source ISRC VR - The separate DC voltages between the gates ofthe second, fourth, and sixth transistors T 2 , T 4 , T 6 are varied in that the current supplied by the controllable current source ISRC VR and flowing through the three resistors RrR 3 connected in series is varied. Fig. 3 is a circuit diagram of a third embodiment ofthe electronic circuit for imitating a capacitor with variable capacitance. The circuit diagram of Fig. 3 is largely identical to that of Fig. 2. In this embodiment, however, the controllable current source ISRC VR is replaced with a controllable voltage source VSRC VR . The chain CHN RSΓ in addition comprises a fourth resistor j which is connected in series between the third resistor R 3 and the controllable voltage source VSRC VR .

Claims: CLAIMS: 1. An electronic circuit for imitating a capacitor with variable capacitance, characterized in that the electronic circuit comprises a chain (CHN) of at least two differential pairs (DF 1 -DF 3 ), the tail of each differential pair (DFι-DF 3 ) being connected to a reference terminal (GND) ofthe electronic circuit via a separate current source (I l ), while each differential pair (DFι-DF 3 ) has a first input and a second input, all said first inputs being coupled to a signal terminal (CPVR) ofthe electronic circuit, and all said second inputs being coupled to one another via separate means (SPMNS) for providing separate DC voltages between the second inputs. 2. An electronic circuit as claimed in claim 1 , characterized in that said separate means (SPMNS) for providing the separate DC voltages between the second inputs comprise: a chain (CFTN RSΓ ) of resistors (Rι-R 3 ) connected in series and a controllable current source (ISRC VR ) coupled to the chain (CHN RSΓ ) of resistors (Rj-R ) connected in series for controlling said separate DC voltages. 3. An electronic circuit as claimed in claim 1 , characterized in that the separate means (SPMNS) for providing the separate DC voltages between the second inputs comprise: a chain (CHN RSΓ ) of resistors (Ri-R t ) connected in series and a controllable voltage source (VSRC VR ) coupled to the chain (CHN RSΓ ) of resistors (Ri-Rt) connected in series for controlling the separate DC voltages. 4. An electronic circuit as claimed in claim 1, 2 or 3, characterized in that the differential pairs (DFι-DF 3 ) comprise field effect transistors (Tj-Tό). 5. An electronic circuit as claimed in claim 4, characterized in that the field effect transistors (Tj-T 6 ) are MOS transistors. 6. An oscillator provided with an electronic circuit as claimed in claim 1, 2, 3, 4, 7. A phase-locked loop provided with an oscillator as claimed in claim 6. 8. An integrated circuit provided with an electronic circuit as claimed in claim 1 , 2, 3, 4, or 5. 9. An integrated circuit provided with an oscillator as claimed in claim 6. 10. An integrated circuit provided with a phase-locked loop as claimed in claim 7.

Inventors: VAN DER GOES FRANCISCUS M L, VAN DER GOES, FRANCISCUS, M., L.
Applicants: KONINKL PHILIPS ELECTRONICS NV, KONINKLIJKE PHILIPS ELECTRONICS N.V.
Application year: 2000
Main classification: H03H11
IPC codes: H01L21, H01L27, H03H11