CN102916686A - Improved switch capacitor structure with low parasitic effect and high quality factor - Google Patents

Abstract

The invention discloses an improved switch capacitor structure with low parasitic effect and high quality factor. Compared with a traditional switch capacitor structure implemented by an NMOS (N-channel metal oxide semiconductor) switching tube and a capacitor, the switch capacitor structure is improved in the following way: the gate voltage of the NMOS switching tube is connected to the drain of the NMOS switching tube through a phase inverter and a resistor, the equivalent parasitic resistance of a switching tube in the starting state is reduced, the equivalent parasitic capacitance of the switching tube in the off state is reduced, and the switching characteristic of the NMOS switching tube is optimized. The improved switch capacitor structure has the characteristics of low parasitic effect, high quality factor and large effective tolerance.

Description

The modified model switch capacitance structure that a kind of ghost effect low quality factor is high

Technical field

The present invention relates generally to discrete capacitance structure, especially refers to the modified model switch capacitance structure that a kind of ghost effect is low, quality factor is high.

Background technology

In CMOS technique, Switch capacitor structure is widely used in the design field of LC oscillator.As the capacitive part of LC oscillator resonator, switching capacity has extremely important influence to the Key Performance Indicators such as phase noise, tuning range and power consumption of LC oscillator.Therefore, the electric capacity adjustable extent is large, and the Switch capacitor structure that ghost effect is little and quality factor is high is one of difficult point in the LC oscillator design.In integrated circuit, the variable capacitor structure that adopts CMOS technique to realize is mainly contained three kinds: PN junction electric capacity, metal-oxide-semiconductor electric capacity and equivalent switch electric capacity.PN junction electric capacity is when its shortcoming is that resonance potential is large during as variable capacitance, and PN junction may enter the positively biased state, has increased leakage current, causes quality factor to descend, so that the phase noise variation.Metal-oxide-semiconductor electric capacity is variable capacitance commonly used at present, but it realizes that the electric capacity adjustable extent is little, is subjected to the restriction of technology library.Because equivalent switch electric capacity adopts metal capacitance and switch MOS pipe to realize, is not subjected to the restriction of technology library, can realize the arbitrary size specific capacitance, and the electric capacity adjustable extent is large, realize that the LC oscillator frequency gain curve linearity is good, K _VCOValue is little, reaches preferably phase noise performance.Therefore, equivalent switch electric capacity has preferably application prospect in System on Chip/SoC.

Fig. 1 has provided a kind of conventional switch capacitance structure of being realized by nmos switch pipe and metal capacitance, wherein NMOS pipe N1 is the radio-frequency (RF) switch pipe, grid is the input of switch controlling signal D, an end of drain electrode connection metal capacitor C, and the other end of electric capacity is output OUT.When switch controlling signal D was low level, switching tube N1 did not open, and switching capacity is in closed condition, this moment Switch capacitor structure equivalent electric circuit as shown in Figure 3, its parasitic capacitance can be expressed as:

$C_{\mathrm{OFF}}=C//(C_{N1\mathrm{par}}+C_{\mathrm{par}})=\frac{C\·(C_{N1\mathrm{par}}+C_{\mathrm{par}})}{C+(C_{N1\mathrm{par}}+C_{\mathrm{par}})}---\left(1\right)$

C wherein _ParBe the bottom crown of metal capacitance C and the parasitic capacitance of ground formation, C _N1parFor the drain electrode of nmos switch pipe produces edge capacitance, its value equals W _SwC _Dd, W wherein _SwThe width of switching tube, C _DdBe drain terminal edge unit width capacitance, unit is fF/ μ m.Because when switch disconnected, metal capacitance was not isolated in outside the resonator fully, access in the resonator afterwards but connect with parasitic capacitance, this ghost effect greatly reduces the frequency of waveform, affects the oscillator tuning scope.

When switch controlling signal D was high level, switching tube N1 opened, and switching capacity is in running order, this moment the switching capacity equivalent electric circuit as shown in Figure 4, R wherein _ONEquivalent resistance during for the switching tube conducting.This moment, switching capacity appearance value size can be expressed as:

C _ON≈C (2)

Its quality factor q can be expressed as:

$Q=\frac{1}{{\mathrm{\ω}}_0{\mathrm{CR}}_{\mathrm{ON}}}\∝\frac{W_{\mathrm{SW}}}{{\mathrm{\ω}}_{0}C}---\left(3\right)$

ω in the formula ₀Be operating frequency, R _ON=[(μ _nC _Ox) (W/L) (V _GS-V _TH)] ^-1The conducting resistance of switching tube N1.From formula (3), can draw the conducting resistance R of switching tube during owing to conducting _ONExistence, the effective Q value of switching capacity reduces, thereby reduces the phase noise performance of LC oscillator.

For the defective that the single-ended Switch capacitor structure of tradition exists, the designer has proposed the high modified model switch capacitance structure (shown in Figure 2) of a kind of ghost effect low quality factor.Switch controlling signal D receives the grid of nmos switch pipe and the input of inverter INV simultaneously, and the drain electrode of nmos switch pipe connects an end of capacitor C, and the other end of capacitor C is output OUT, the output of inverter INV and resistance R ₀An end join resistance R ₀The other end connect the drain terminal of nmos switch pipe N1.

Fig. 5 has provided the equivalent circuit structure when modified model switch capacitance structure is closed.Because transistor N1 disconnects, transistorized drain electrode is by large resistance R ₀Be biased to power supply V _DD, this moment, transistor drain one substrate PN junction presented greatly instead partially, so that the depletion region of PN junction increases, had greatly reduced the parasitic capacitance of drain electrode to the impact of switching capacity, Simultaneous Switching electric capacity bottom crown parasitic capacitance C over the ground _ParAlso very little, can ignore.Therefore, the filtering characteristic that shows low pass of modified model switch capacitance structure under the closed condition, its transfer function can represent:

$H\left(s\right)=R_0+\frac{1}{C\·s}---\left(4\right)$

Fig. 6 has provided the equivalent circuit structure when modified model switch electric capacity is opened.The dead resistance R that introduced by switching tube N1 this moment _ONAnd resistance R ₀Parallel connection has reduced equivalent dead resistance, has increased the effective quality factor of switching capacity, that is:

$Q=\frac{1}{{\mathrm{\ω}}_{0}C(R_{\mathrm{ON}}//R_0)}---\left(5\right)$

In sum, follow-on Switch capacitor structure has reduced the parasitic capacitance when closing, and the dead resistance when having reduced to open has improved the tuning range of switching capacity, has increased the quality factor of switching capacity.

Summary of the invention

The problem to be solved in the present invention is: the technical problem for prior art exists the invention provides the high Switch capacitor structure of a kind of ghost effect low quality factor.

For realizing above-mentioned technical problem, the solution that the present invention proposes is: the modified model switch capacitance structure that a kind of ghost effect low quality factor is high is characterized in that: it comprises a nmos switch pipe (N1), an inverter (INV), a resistance (R ₀), an electric capacity (C), described nmos switch pipe (N1) grid is the input of switch controlling signal D, drain electrode is connected with an end of electric capacity (C), source electrode and substrate be ground connection (GND) all, the input that is input as switch controlling signal D of described inverter (INV), output connecting resistance (R ₀) an end, resistance (R ₀) the drain electrode of another termination nmos switch pipe, the other end of electric capacity is the output OUT of switching capacity.

Compared with prior art, the invention has the advantages that:

1, reduced the ghost effect of switching capacity.Compare with the conventional switch capacitance structure, the dead resistance of introducing when Switch capacitor structure of the present invention is opened reduces, and the parasitic capacitance that produces when closing reduces, and has good switching characteristic.

2, increased the quality factor of switching capacity.Compare with the conventional switch capacitance structure, dead resistance was little when Switch capacitor structure of the present invention was opened, and had realized the high characteristic of electric capacity quality factor.

3, improved the tuning capability of switching capacity.Compare with the conventional switch capacitance structure, Switch capacitor structure ghost effect of the present invention is little, and effectively holding value difference during the switching tube opening and closing increases, thereby has improved the tuning capability of switching capacity.

Description of drawings

Fig. 1 is conventional switch capacitance structure schematic diagram;

Fig. 2 is Switch capacitor structure schematic diagram of the present invention;

Fig. 3 is conventional switch electric capacity schematic equivalent circuit when closing;

Schematic equivalent circuit when Fig. 4 is the unlatching of conventional switch electric capacity;

Fig. 5 is switching capacity of the present invention schematic equivalent circuit when closing;

Schematic equivalent circuit when Fig. 6 is switching capacity unlatching of the present invention;

Embodiment

Below with reference to the drawings and specific embodiments the present invention is described in further details.

As shown in Figure 2, the present invention is the high modified model switch capacitance structure of a kind of ghost effect low quality factor, and it comprises a nmos switch pipe (N1), an inverter (INV), a resistance (R ₀), an electric capacity (C).Wherein, nmos switch pipe N1 grid is the input of switch controlling signal D, and drain electrode is connected with an end of capacitor C, and source electrode and substrate be ground connection GND all, the input that is input as switch controlling signal D of described inverter INV, output connecting resistance R ₀An end, resistance R ₀The drain electrode of another termination nmos switch pipe N1 so that switching tube N1 when closing its drain electrode and substrate form PN junction and present large partially instead, reduce drain parasitic capacitance, increase effective tolerance of switching capacity; Reduce dead resistance during Simultaneous Switching pipe N1 conducting, increase the quality factor of switching capacity.The other end of capacitor C is the output OUT of switching capacity.

Operation principle: when switch controlling signal D was high level, switching tube N1 opened, and its conducting resistance is R _ONAnd resistance R ₀Parallel connection has reduced equivalent dead resistance, has increased the quality factor of switching capacity.When switch controlling signal D was low level, switching tube N1 closed, and this moment, node E voltage was near V _DD, so that the drain electrode of switching tube N1 and substrate PN junction present greatly instead partially drain parasitic capacitance C _N1parVery little, thus parasitic capacitance when Switch capacitor structure is closed reduced, increased its tuning range.

Claims (2)

1. modified model switch capacitance structure that ghost effect is low, quality factor is high, it is characterized in that: it comprises a nmos switch pipe (N1), an inverter (INV), a resistance (R ₀), an electric capacity (C), described nmos switch pipe (N1) grid is the input of switch controlling signal D, drain electrode is connected with an end of electric capacity (C), source electrode and substrate all connect power supply ground (GND), the input that is input as switch controlling signal D of described inverter (INV), output connecting resistance (R ₀) an end, resistance (R ₀) the drain electrode of another termination nmos switch pipe, the other end of electric capacity is the output OUT of switching capacity.

2. the high Switch capacitor structure of ghost effect low quality factor according to claim 1 is characterized in that resistance R ₀Value will be in M Ω level.

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