CN101764584A - Linear source follower capable of eliminating bulk effect and channel length modulation effect - Google Patents

Abstract

The invention discloses a linear source follower capable of eliminating bulk effect and channel length modulation effect, belonging to the voltage follower technical field. The linear source follower is characterized in that transistors M2 and M3 are cascaded; and grids are all connected to Vin, wherein, M2 is a low threshold transistor and M3 is a high threshold transistor. The transistors M2 and M3 always work in a saturation region, thereby reducing the fluctuation of the M3 drain-source voltage and eliminating the channel length modulation effect of the transistor M3. The sources S of M2 and M3 are respectively connected with a substrate B, and the bulk effect of the transistor M3 is eliminated. The invention has the advantage of high linearity.

Description

Can eliminate the linear source follower of bulk effect and channel length modulation effect

Technical field

The invention belongs to the VLSI (very large scale integrated circuit) designs in Microelectronics and Solid State Electronics field, relate to a kind of novel source follower circuit, can be widely used in fields such as Analog Circuit Design and integrated circuit testing.

Background technology

Voltage follower is one of most important module in the analog circuit, is widely used in various Analog Circuit Design.Usually require voltage follower to have very high linearity, make output as far as possible near input.

Traditional source follower as shown in Figure 1.Though realize very simply, have some shortcomings.As input signal V _InDuring variation, the drain D of transistor M1 and the voltage difference of source S, the voltage difference of source S and substrate B all changes with the variation of input signal, the bulk effect of Yin Ruing therefrom, channel length modulation effect etc. has limited the linearity of conventional source follower.

At above situation, the present invention proposes a kind of high linear source follower that can eliminate bulk effect and channel length modulation effect.

Summary of the invention

The objective of the invention is to eliminate in the conventional source follower bulk effect and channel length modulation effect and avoid the output signal of source follower to produce distortion, have the source follower of high linearity for the influence of the linearity.The invention is characterized in,

The one N type field effect transistor M2, the 2nd N type field effect transistor M3 and current source I2, wherein:

The one N type field effect transistor M2, grid meets input voltage V _In, drain electrode meets power vd D, and substrate links to each other with source electrode, and a described N type field effect transistor M2 always works in the saturation region, and the 2nd N type field effect transistor M3 grid meets described input voltage V _InThe drain electrode of described the 2nd N type field effect transistor M3 links to each other with the source electrode of a described N type field effect transistor M2, substrate and the source electrode of self link to each other, the threshold value of described the 2nd N type field effect transistor M3 and overdrive voltage all are higher than threshold value and the overdrive voltage of a described N type field effect transistor M2, make the described second field effect transistor M3 always work in the saturation region;

Current source I2 input links to each other with the source electrode of described the 2nd N type field effect transistor M3, constitutes the output of described linear source follower, and output voltage is V _Out, described current source I2 output head grounding.

Transistor M2 and M3 always work in the saturation region among the present invention, and the drain voltage of M3 is followed input V _InChange, thereby reduced M3 drain-source voltage V _DS3Fluctuation, eliminated the channel length modulation effect of transistor M3.The source S of M2 and M3 all is connected with substrate B, has eliminated the bulk effect of transistor M3.The present invention has avoided the distortion introduced in output by bulk effect and channel length modulation effect, has advantage of high linearity.

The invention has the beneficial effects as follows: carry out SPICE emulation at the CADENCE platform, simulation result shows, at the 30pF capacitive load, and the 2.5V supply voltage, under the 20mA total current, the 1Vp-p amplitude of oscillation, harmonic distortion is-107dB.

Description of drawings

Fig. 1. conventional source follower circuit schematic diagram.

Fig. 2. source follower circuit schematic diagram of the present invention.

Fig. 3. the source follower circuit schematic diagram of the present invention that difference realizes.

The source follower circuit schematic diagram of the present invention that Fig. 4 .P transistor npn npn is realized.

The source follower circuit schematic diagram of the present invention that Fig. 5 .P transistor npn npn difference realizes.

Embodiment

The invention is characterized in and contain:

Contain low threshold transistor M2, high-threshold transistors M3 and current source I2, transistor M2 and M3 form cascade system, and grid all meets input V _InThe drain electrode of M2 meets power vd D, and the source electrode of M2 is connected with the drain electrode of M3, and the source electrode of M3 is drawn output V _Out, the substrate of M2, M3 is equal to link to each other with source electrode.The source electrode of the termination M3 of current source I2, other end ground connection GND.

Fig. 1 is conventional source follower configuration figure.V wherein _InBe input signal, V _OutBe output signal.I1 is a current source, and its current value size is I, and M1 is a field-effect transistor, its substrate B ground connection GND.As input signal V _InDuring variation, output signal V _OutFollow input signal V _InChange.Among the transistor M1, the voltage difference V of drain D and source S _DS1, the voltage difference V of source S and substrate B _SB1All the variation with input signal changes.

Consider bulk effect and channel length modulation effect, the current-voltage correlation of transistor M1 meets as follows

$I=\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}{(V_{\mathrm{in}}-V_{\mathrm{th}1}-V_{\mathrm{out}})}^2(1+\mathrm{\λ}{V}_{\mathrm{DS}1})$ Formula (1)

Wherein $V_{\mathrm{th}1}=V_{\mathrm{th}01}+\mathrm{\υ}(\sqrt{2{\mathrm{\Φ}}_F+V_{\mathrm{SB}1}}-\sqrt{2{\mathrm{\Φ}}_F})$

V _DS1＝V _DD-V _out??V _SB1＝V _S1-V _B1＝V _out

V wherein _Th01Threshold voltage when not considering bulk effect, υ is called body-effect coefficient, Ф _FBe Fermi's electromotive force.

Suppose that current source I1 output impedance is enough big, at input signal V _InCurrent value remains unchanged during variation, by formula (1) as seen, owing to have bulk effect and channel length modulation effect, V _SB1And V _DS1Variation cause conventional source follower output V _OutWith input signal V _InBetween be not simple linear relationship, output V _OutIn introduced a lot of input signal V _InNonlinear terms, therefore introduced a lot of harmonic waves, the linearity of conventional source follower is lower.

Fig. 2 is circuit diagram of the present invention, wherein V _InBe input signal, V _OutBe output signal.I2 is a current source, and its current value size is I.Wherein M2 is the lower transistor of threshold voltage, and M3 is that threshold voltage is than higher transistor.The substrate B of M2, M3 all is connected with source S.Suppose that current source I1 output impedance is enough big, at input signal V _InCurrent value remains unchanged during variation.

The drain-source voltage of transistor M3

V _DS3＝V _in-V _GS2-V _out＝(V _in-V _GS2)-(V _in-V _GS3)

＝V _GS3-V _GS2＝(V _th3+V _ov3)-(V _th2+V _ov2)

＝(V _th3-V _th2)+(V _ov3-V _ov2)

If will make transistor M3 be operated in the saturation region, must V _DS3＞V _GS3-V _Th3=V _Ov3

Because what M3 adopted is the higher thresholds transistor, what M2 adopted is low threshold transistor; By adjusting circuit parameter, get the overdrive voltage V of M3 in the design _Ov3The overdrive voltage V that is higher than M2 _Ov2, guarantee V _DS3＞V _Ov3, make transistor M3 be operated in the saturation region.Transistor M2, M3 always work in the saturation region among the present invention.

The current-voltage correlation of transistor M3 meets as follows

$I=\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}{(V_{\mathrm{in}}-V_{\mathrm{th}3}-V_{\mathrm{out}})}^2(1+\mathrm{\λ}{V}_{\mathrm{DS}3})$ Formula (2)

V _DS3＝V _in-V _GS2-V _out＝V _in-V _GS2-(V _in-V _GS3)

＝V _GS3-V _GS2＝(V _th3+V _ov3)-(V _th2+V _ov2)

Wherein $V_{\mathrm{th}3}=V_{\mathrm{th}03}+\mathrm{\υ}(\sqrt{2{\mathrm{\Φ}}_F+V_{\mathrm{SB}3}}-\sqrt{2{\mathrm{\Φ}}_F}),$ V _Th03The threshold voltage of M3 when not considering bulk effect.

Because the substrate B of M2, M3 all is connected with source S, VSB=0 has eliminated the inclined to one side effect of lining, so

V _th3＝V _th03

Substitution formula (2),

$I=\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}{(V_{\mathrm{in}}-V_{\mathrm{th}03}-V_{\mathrm{out}})}^2(1+\mathrm{\λ}{V}_{\mathrm{DS}3})$ Formula (3)

So

$\frac{I}{\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}(1+\mathrm{\λ}{V}_{\mathrm{DS}3})}={(V_{\mathrm{in}}-V_{\mathrm{th}03}-V_{\mathrm{out}})}^2$

$V_{\mathrm{out}}=V_{\mathrm{in}}-V_{\mathrm{th}03}-\sqrt{\frac{I}{\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}(1+\mathrm{\λ}{V}_{\mathrm{DS}3})}}$

$V_{\mathrm{out}}=V_{\mathrm{in}}-(V_{\mathrm{th}03}+\sqrt{\frac{I}{\frac{1}{2}{\mathrm{\μ}}_n{C}_{\mathrm{ox}}(1+\mathrm{\λ}{V}_{\mathrm{DS}3})}})$ Formula (4)

μ in the formula (4) _nC _Oxλ V _Th03All are constants by the technology decision, V _DS3=(V _Th3-V _Th2)+(V _Ov3-V _Ov2) be a steady state value that does not change with input signal, so output signal has just been carried out level shift, V on the basis of input signal _OutAnd V _InRelation become simple linear relation, do not introduce and input signal V _InTherefore relevant harmonic distortion have high linearity.

Fig. 3 is that difference of the present invention realizes.V _Inp, V _InnBe differential input signal, V _Outp, V _OutnBe difference output.Wherein M5, M7 be threshold voltage than higher transistor, M4, M6 are the lower transistor of threshold voltage.The substrate of M4, M5, M6, M7 all is connected with source electrode.I3, I4 are current source, and its current value size is I.

In order to verify performance, Fig. 3 is carried out SPICE emulation at the CADENCE platform.

Simulation result shows, at the 30pF capacitive load, and the 2.5V supply voltage, under the 20mA total current, the 1Vp-p amplitude of oscillation, harmonic distortion is-107dB that the characteristic of high linear source follower is summed up as table 1.

Table 1: high linear source follower characteristic is summed up

Title	High linear source follower
		Supply voltage (V)	??2.5
Power consumption (mW)	??50
		Signal frequency (MHz)	??15.5
The amplitude of oscillation (V)	??1Vp-p
		Capacitive load (pF)	??30

Title	High linear source follower
		Harmonic distortion (dB)	??-107

Fig. 4 is that P transistor npn npn of the present invention is realized.V _InBe input signal, V _OutBe output signal, M12 be threshold voltage than higher transistor, M13 is the lower transistor of threshold voltage.The substrate of M12, M13 all is connected with source electrode.I7 is a current source, and its current value size is I.

Fig. 5 is that P transistor npn npn difference of the present invention realizes.V _Inp, V _InnBe differential input signal, V _Outp, V _OutnBe difference output.Wherein M8, M10 be threshold voltage than higher transistor, M9, M11 are the lower transistor of threshold voltage.The substrate of M8, M9, M10, M11 all is connected with source electrode.I5, I6 are current source, and its current value size is I.

Claims (2)

1. can eliminate the linear source follower of bulk effect and channel length modulation effect, it is characterized in that, a N type field effect transistor (M2), the 2nd N type field effect transistor (M3) and current source (I2), wherein:

The one N type field effect transistor (M2), grid connects input voltage (Vin), and drain electrode connects power supply (VDD), and substrate links to each other with source electrode, and a described N type field effect transistor (M2) always works in the saturation region,

The 2nd N type field effect transistor (M3) grid connects described input voltage (Vin), the drain electrode of described the 2nd N type field effect transistor (M3) links to each other with the source electrode of a described N type field effect transistor (M2), substrate and the source electrode of self link to each other, the threshold value of described the 2nd N type field effect transistor (M3) and overdrive voltage all are higher than the threshold value and the overdrive voltage of a described N type field effect transistor (M2), make described second field effect transistor (M3) always work in the saturation region

The source electrode of current source (I2) input and described the 2nd N type field effect transistor (M3) links to each other, and constitutes the output of described linear source follower, and output voltage is (Vout), described current source (I2) output head grounding.

2. a kind of differential type source follower that can eliminate the linear source follower of bulk effect and channel length modulation effect and realize is characterized in that, contains two described linear source followers parallel with one another, wherein:

First linear source follower, successively by second current source (I5), the 3rd P type field effect transistor (M8), the 4th P type field effect transistor (M9) is in series successively, wherein, the input termination power (VDD) of second current source, the grounded drain of the 4th P type field effect transistor (M9), after linking to each other, the grid of the grid of the 3rd P type field effect transistor (M8) and the 4th P type field effect transistor (M9) connects first input voltage (Vinp), the substrate of the 3rd P type field effect transistor (M8) links to each other with source electrode, the substrate of the 4th P type field effect transistor (M9) links to each other with source electrode, the threshold voltage of the 3rd P type field effect transistor (M8) is higher than the threshold voltage of the 4th P type field effect transistor (M9)

Second linear source follower is successively by the 3rd current source (I6), the 5th P type field effect transistor (M10), the 6th P type field effect transistor (M11) is connected in series successively and forms, the described power supply of input termination (VDD) of the 3rd current source (I6), the grounded drain of the 6th P type field effect transistor (M11), the substrate of the 5th P type field effect transistor (M10) links to each other with source electrode, the substrate of the 6th P type field effect transistor (M11) links to each other with source electrode, the threshold value of the 5th P type field effect transistor (M10) is higher than the threshold value of the 6th P type field effect transistor (M11), after linking to each other, the grid of the grid of the 5th field effect transistor (M10) and the 6th field effect transistor (M11) connects second input voltage (Vinn)

The source electrode of described the 3rd P type field effect transistor (M8) constitutes first output, exports first output voltage (Voutp), and the source electrode of described the 5th P type field effect transistor (M10) constitutes second output, exports second output voltage (Voutn).

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