Analog Switch Circuit Having Output Offset Compensation Circuit

This invention relates to an analog switch circuit of a CMOS (complementary metal oxide semiconductor) FET construction.

As shown in FIG. 1, an analog switch circuit of the CMOS type is fundamentally comprised of a transmission gate of the CMOS type. Specifically, the analog circuit is formed of a p channel MOS transistor P.sub.1 and an n channel MOS transistor N.sub.1, which transistors are connected in parallel. An input signal voltage Vin (FIG. 3C) is supplied to an input side node J.sub.1 of the p and n channel MOS transistors P.sub.1, N.sub.1. An output signal voltage Vout (FIG. 3D) is taken out from an output side node J.sub.2. The gate of the n channel MOS transistor N.sub.1 is supplied with a control signal .phi. (FIG. 3A). The gate of the p channel MOS transistor P.sub.1 is supplied with a control signal .phi. (FIG. 3B) having a phase which is opposite that of signal .phi.. A mirror capacitance C.sub.mN is present between the output side node J.sub.2 of the n channel MOS transistor N.sub.1 and the p channel MOS transistor P.sub.1, and the gate of said n channel MOS transistor N.sub.1. A mirror capacitance C.sub.mP is present between the output side node J.sub.2 and the gate of the p channel MOS transistor P.sub.1. A load capacitance C.sub.L is present between the output side node J.sub.2 and ground. FIG. 2 is a curve diagram showing the characteristics of a resistance against the input signal voltage Vin, which resistance arises between the input and output terminals of the subject switch circuit; and the characteristics of the resistance prevailing in the p channel MOS transistor P.sub.1 and n channel MOS transistor N.sub.1. With reference to FIG. 2, the abscissa denotes the input signl voltage Vin, and the ordinate represents a resistance value R. Roman numerals I and II respectively indicate the curves characterizing the resistances prevailing in the p channel MOS transistor P.sub.1 and the n channel MOS transistor N.sub.1. Roman numeral III shows a curve characterizing a resistance prevailing throughout the subject analog switch circuit; or, specifically, the resistance between the input and output terminals thereof. In this case, it is preferred that a resistance value R prevailing between the input and output terminals of the subject analog switch circuit be rendered constant throughout the entire range of the input signal voltage Vin. To stabilize said resistance value R, the conventional practice is to let the p channel MOS transistor P.sub.1 and n channel MOS transistor N.sub.1 have an equal resistance value. To attain this object, the p channel MOS transistor P.sub.1 is chosen in such a way as to have a channel width twice as broad as that of the n channel MOS transistor N.sub.1. However, the conventional procedure has a drawback, in that capacitance C.sub.mP assumes a value twice that of capacitance C.sub.mN ; and, as a result, the control pulse .phi. has its voltage level changed from that of the V.sub.DD voltage to that of the ground voltage GND, and the control pulse .phi. has its voltage level varied from that of the ground voltage GND to that of the V.sub.DD voltage. Therefore, when the transmission gate is rendered nonconductive, the output signal voltage Vout is offset from a correct value to the extent of .DELTA.V, in accordance with the difference between capacitance C.sub.mP and capacitance C.sub.mN.

The offset voltage V.sub.1 occurring when the control pulse .phi. has its logic level varied from "0" to "1", and the offset voltage V.sub.2 arising when the control pulse .phi. has its logic level changed from "1" to "0" are respectively expressed as follows: ##EQU1## Therefore, the output voltage Vout undesirably appears in a form offset to the extent of the sum .DELTA.V of voltages V.sub.1 and V.sub.2, which sum is expressed as: ##EQU2##

Since this invention has been contrived in view of the above, it is intended to provide an analog switch circuit which comprises a CMOS type transmission gate and is characterized in that, when the transmission gate is rendered nonconductive, it is possible to prevent the offsetting of an output voltage by a difference between the mirror capacitances of the p channel and n channel MOS transistors.

According to the invention, an analog switch circuit is provided, which circuit comprises a transmission gate consisting of a first n channel MOS transistor and a first p channel MOS transistor which are connected in parallel and whose gates are respectively supplied with control signals having opposite logic levels, and a second n channel MOS transistor and second p channel MOS transistor, whose gates are connected to the output terminal of said transmission gate.

FIG. 1 is a circuit diagram of a transmission gate which basically constitutes an analog switch;

FIG. 2 is a curve diagram showing the level of a resistance presented by the transmission gate of FIG. 1 to an input signal voltage;

FIGS. 3A to 3D are time charts of signals appearing in various sections of the transmission gate of FIG. 1; and

FIG. 4 is a circuit diagram of an analog switch embodying this invention.

With reference to FIG. 4 which shows the circuit arrangement of an analog switch embodying this invention, an n channel MOS transistor N.sub.11 and p channel MOS transistor P.sub.11 are connected in parallel to constitute a transmission gate TG. An input signal voltage Vin is impressed on the junction J.sub.1 of said transistors N.sub.11, P.sub.11, which junction faces the input side. An output signal voltage Vout is taken out of the other junction J.sub.2 of said transistors N.sub.11 and P.sub.11. A control signal .phi. (FIG. 3A) is supplied to the gate of the n channel MOS transistor N.sub.11. A control signal .phi. (FIG. 3B) having an opposite phase to the control signal .phi. is delivered to the gate of the p channel MOS transistor P.sub.11. A mirror capacitance C.sub.mN11 is present between that junction J.sub.2 of the parallel connected n channel MOS transistor N.sub.11 and p channel MOS transistor P.sub.11 which faces the output side of the transmission gate TG and the gate of n channel MOS transistor N.sub.11. A similar mirror capacitance C.sub.mP11 prevails between the junction J.sub.2 facing said output side and the gate of the p channel MOS transistor P.sub.11. The gates of an n channel MOS transistor N.sub.12 and a p channel MOS transistor P.sub.12 are connected to the junction J.sub.2 of the aforesaid transistors N.sub.11, P.sub.11, which faces the output side of the transmission gate TG. Therefore, an output signal Vout from the transmission gate TG is supplied as a gate signal to the MOS transistors N.sub.12, P.sub.12. The source and drain of MOS transistor N.sub.12 are connected to the gate of MOS transistor P.sub.11. The drain and source of MOS transistor P.sub.12 are connected to the gate of MOS transistor N.sub.11. Therefore, the drain and source of MOS transistor N.sub.12 are supplied with a signal .phi. which is delivered to the gate of the p channel MOS transistor P.sub.11 of the transmission gate TG. The drain and source of MOS transistor P.sub.12 are supplied with a signal .phi. which is delivered to the gate of the n channel MOS transistor N.sub.11. A mirror capacitance C.sub.mN12 is present between the gate of the n channel MOS transistor N.sub.12 and, e.g., the drain thereof. Further, a mirror capacitance C.sub.mN13 exists between the gate of said n channel MOS transistor N.sub.12 and, e.g., the source thereof. A mirror capacitance C.sub.mP12 is present between the gate of the p channel MOS transistor P.sub.12 and, e.g., the drain thereof. A mirror capacitance C.sub.mP13 exists between the gate of said p channel MOS transistor P.sub.12 and, e.g., the source thereof. A load capacitance C.sub.L is present between the aforesaid junction J.sub.2 facing the output side of the transmission gate TG. An output capacitance Cout, as viewed from the output side of the whole circuit of the subject analog switch, is expressed as:

Assuming that the channel widths of the respective transistors P.sub.11, P.sub.12, N.sub.11, N.sub.12 are indicated by the symbols W.sub.P11, W.sub.P12, W.sub.N11 and W.sub.N12 ; if, in this case, the respective channel widths are as chosen as to have relationships such as the following:

The following equations result:

Therefore, the forming equations are established as follows:

When the channel widths of the transistors P.sub.11, P.sub.12, N.sub.11, N.sub.12 are chosen as described above. The following facts are recognized.

(i) When the control pulse .phi. has its logic level changed frm "0" to "1", the resultant output voltage Vout indicates an offset voltage V.sub.1 expressed as: ##EQU3##

(ii) When the control pulse .phi. has its logic level varied from "1" to "0", the resultant output voltage Vout shows an offset voltage V.sub.2 expressed as: ##EQU4##

A composite offset voltage .DELTA.V related to the output voltage Vout is represented by the sum of offset voltage V.sub.1 and V.sub.2, i.e., by a value arrived at by adding equations (3) and (4), and is expressed as follows: ##EQU5## When the above equations (1), (2) are substituted for equation (5), the following equation results:

This means that when, in the analog switch arranged as shown in FIG. 4, the channel widths of the MOS transistors P.sub.11, P.sub.12, N.sub.11, N.sub.12 are so chosen as to have such relationships as W.sub.P12 =W.sub.P11 /2 and W.sub.N12 =W.sub.N11 /2, no offset appears in the output voltage Vout when the transmission gate TG is rendered nonconductive.

The foregoing embodiment of this invention refers to the case wherein the channel width W.sub.P12 was so chosen as to be half of channel width W.sub.P11, and channel width W.sub.N12 was so chosen as to be half of channel width W.sub.N11. However, channel widths W.sub.P12 and W.sub.N12 need not be limited to half the corresponding channel widths (W.sub.P11 and W.sub.N11). If offset voltage can be overlooked to some extent, it is possible to let channel widths W.sub.P12 and W.sub.N12 bear any other ratio to the corresponding channel widths W.sub.P11 and W.sub.N11. Further, the aforementioned embodiment refers to the case wherein mirror capacitances C.sub.mN12 and C.sub.mP12 were respectively chosen to exit between the gates of the MOS transistors N.sub.12, P.sub.12, and the drains thereof. However, it is also possible to let said mirror capacitances C.sub.mN12 and C.sub.mP12 be present between the gates of said MOS transistors N.sub.12, P.sub.12 and the sources thereof. In the latter case, mirror capacitances C.sub.mN13 and C.sub.mP13 are obviously meant to be located between the gates of the MOS transistors N.sub.12, P.sub.12 and the drains thereof.

It should be noted that this invention is not limited to the above-mentioned embodiment, but can be practiced with various changes and modifications known to those skilled in the art, without departing from the scope and object of the invention.

As mentioned above, this invention provides an analog switch wherein the output side of a transmission gate consisting of a p channel MOS transistor and n channel MOS transistor is provided with a p channel MOS transistor and n channel MOS transistor which are supplied with an output voltage from said output side; and a capacitance is provided on the output side of the transmission gate, to thereby suppress the occurrence of any offset voltage resulting from a difference between the mirror capacitances of the p and n type MOS transistors constituting the transmission gate, when the transmisson gate is rendered nonconductive.