JPH0722933A - Analog switch - Google Patents

Abstract

(57) [Abstract] [Purpose] The purpose is to increase the switching speed of an analog switch using an FET with a simple configuration. [Structure] A switch element composed of an FET and connected to an analog signal line, a resistor connected between the gate and source of the switch element, a gate of the switch element
A FET composed of a source and a drain connected to the source, a resistor connected between the gate and the source of the FET, and a transistor connected between the gate of the FET and a negative power source.

Description

Detailed Description of the Invention

[0001]

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to an analog switch formed by using a field effect transistor as a switch element.

[0002]

2. Description of the Related Art Field effect transistor (hereinafter referred to as FET
A variety of circuits are known as analog switches using the above) as a switch element. A typical circuit thereof is shown in FIG. In FIG. 10, T _r1 Analog switch element constituted by FET, V _IN is an analog input applied to the source of the switch element T _r1, at V _OUT analog output which is switched by T _r1, taken from the drain of T _r1 Be done. T _r2 is a transistor for driving the switching element T _r1, its collector to the gate of T _r1, emitter negative supply V _dd ^- is connected to.

In the analog switch having such a structure, when the transistor T _r2 is turned on, the switch element T
The gate of _r1 is negative supply V _dd ^- drawn, the voltage V _GS between the gate and source of the T _r1 is V _GS = V _dd ^- a relationship -V _IN <V _p (pinch-off voltage of T _r1) Switch element T _r1
Turns off.

When the driving transistor T _r2 is turned off,
The gate voltage v _G of T _r1 becomes V _IN through the resistor R, the gate-source voltage V _GS becomes zero, and T _r1 is turned on. At this time, the gate voltage v _G is equal to the input capacitance C _gs and R of T _r1.
Since it goes to V _IN with a time constant of, it takes time to turn on compared to when turning off. Since this switching time determines the switching speed, there is a problem that the circuit of FIG. 10 cannot perform high-speed switching operation. FIG. 11 shows switching operation waveforms of the circuit shown in FIG.

Various methods have been tried so far in order to increase the switching speed of such FET switches. However, each method has some drawbacks, such as the need for a bias power supply and the large number of parts required for discrete assembly, and the current situation is that satisfactory ones are not always obtained.

[0006]

SUMMARY OF THE INVENTION The present invention has been made in view of the above points, and an analog circuit using an FET
The purpose is to increase the switching speed of the switch with a simple configuration.

[0007]

DISCLOSURE OF THE INVENTION The present invention provides a switch element composed of a FET and connected to an analog signal line,
A resistor connected between the gate and the source of the switch element, and a source connected to the gate and the source of the switch element.
It is composed of a FET connected to the drain, a resistor connected between the gate and the source of the FET, and a transistor connected between the gate of the FET and the negative power source.

[0008]

In the present invention as described above, high-speed switching operation is performed by the action of the driving FET in which the source and drain are connected to the gate and source of the FET switch element.

[0009]

The present invention will be described below with reference to the drawings. FIG. 1 is a circuit configuration diagram showing an embodiment of an analog switch according to the present invention. 1, the same elements as those in FIG. 10 are designated by the same reference numerals as those in FIG. 10, and re-explanation of those elements is omitted. In FIG. 1, T _r3 is a driving FET, the source and drain of which are connected to the gate and source of the switch element T _r1 , and the gate of T _r3 is connected to the collector of the transistor T _r2 . R ₁ and R ₂ are resistors, R ₁ is connected between the gate and source of T _r1 , and R ₂ is T
It is connected between the gate and source of _r3 . The operation of the switch according to the present invention having such a structure will be described below.

When the switch element T _r1 is turned off. The base of the transistor T _r2 "H" level voltage V _s added, turn on the transistor T _r2. When T _r2 is turned on, the gate voltage v _G3 of FET · T _r3 for driving the negative power supply V _dd ^- pulled. Therefore, the gate-source voltage v _Gs3 of the transistor T _r3 obtained via the resistor R ₂ increases in the negative direction, and the voltage V _Gs3 is the pinch-off voltage V of T _{r3.
When P3} or less, T _r3 is turned off. When T _r3 is turned off, the gate-source voltage v _Gs1 of the switch element T _r1 obtained via the resistor R ₁ increases in the negative direction, and the value of v _Gs1 is equal to or less than the pinch-off voltage V _{P1 of} T _r1. Then, T _r1 is turned off. In this case, v _Gs1 = {R ₁ / (R ₁ + R ₂ )} (V _dd ⁻ −V _IN ) <V _P1
And the values of the resistors R ₁ and R ₂ must be selected so that v _Gs3 = {R ₂ / (R ₁ + R ₂ )} (V _dd ⁻ −V _IN ) <V _P3 . FIG. 2 shows the circuit state of the analog switch according to the present invention when the FET switch T _r1 is turned off.

Switch element T_r1When is turned on. Transistor T_r2"L" level voltage V at the base of_sTo
In addition, this transistor T_r2Turn off. T_r2Is off
Then, as shown in FIG. 3, T_r3Gate voltage v _G3,as well as
Switch element T_r1Gate voltage v_G1Are both analog inputs
Voltage V_INIt comes to the potential of. In this case, T_r3of
Gate-source capacitance C_gs2The charge stored in
Resistance R₂Through R₂・ C_gs2Discharge with a time constant of_G3≧
V_P3Then T_r3Turns on. T_r3When is turned on,
Until then, switch element T_r1Gate-source capacitance C
_gS1Stored in the resistor R₁Through R₁・ C_gS1With the time constant of
The discharged charge is T_r3Through the drain and source of
r_ON3・ C_gS1(R_ON3Is T_{r 3}ON resistance of)
To charge. In this case, T_r3ON resistance of_ON3The value of
Since it is small, the time constant r_ON3・ C_gS1Is small, resulting in C
_gS1The electric charge of is discharged rapidly. By the discharge, T_r1
Gate voltage v_G1Is v_G1≧ V_P1Then T_r1Turned on
It In this way, the switch element T_r1When is turned on
A driving circuit according to the present invention is shown in FIG. In addition, T_r1And T
_r3Is off, V_GS3Gate-source voltage v_Gs3
Is T_r1Gate-source voltage v_Gs1Shallower via
Resistance R₁, R₂By selecting the value of
Faster than T_r1Can be turned on.

[0012] indicates the change with time of the gate-source voltage v _Gs3 of the switch element T between _r1 gate-source voltage v _Gs1 and driving FET-T _r3, and the on / off state of the T _r1 in FIG. In FIG. 5, (a) _shows the change in v _Gs3 ,
(B) shows changes in v _Gs1 . (I),
In (b) view, t _oFF the gate voltage V _G3 of T _r3 is a negative power source V _dd ^- drawn, time and the v _Gs3 is required to reach the pinch-off voltage V _P3, T _r3 is turned off gate voltage v _G1 of the switch element T _r1 from the V _dd ^- drawn into and v _Gs1
Is the sum of the time required to reach the pinch-off voltage V _P1 , but this time is extremely short. (B) t _ON1 in the figure corresponds to the operation in FIG. 3, and shows the discharge period at the time constant of R ₁ · C _gS1 . Similarly, t _ON2 in (b) corresponds to the operation in FIG. 4, and shows the discharge period at the time constant of r _ON3 · C _gS1 . As described in FIG. 4, the value of the on resistance r _{O N3} of T _r3 is very small, which due to the discharge time constant is small,
t _ON2 is extremely short. As a result, the switch according to the present invention performs the switching operation at an extremely high speed. By the way, for comparison with Fig.
FIG. 5 shows changes in the gate-source voltage v _{Gs 1} of the switch element T _r1 when there is no T _r3 (FIG. 10 given as a conventional example).
(C) of. It can be understood from this figure that in the present invention in which T _r3 is provided and t _ON2 is shortened, the switching operation is much faster than that of the conventional example of FIG.

Although the use place of the analog switch according to the present invention which performs such high speed switching operation is not particularly limited, FIG. 6 shows a case where the present invention is applied to an auto-zero circuit as an example of implementation. . In FIG. 6, AS1 and AS2 are analog switches according to the present invention, and AS1 is connected in series to an analog signal line L and is used to turn on / off this line. AS
2 is connected between the line L and the common COM, and when the line L is turned off by the AS1, the line L is dropped to the common COM via this AS2, and when the AS1 is turned on, the line L and the common COM are turned off. To act. OA is a non-inverting amplifier, which is connected to the line L, and the analog output V _OUT turned on / off is taken out from this amplifier. When the switches AS1 and AS2 according to the present invention are used in an auto-zero circuit that performs such an on / off operation, the shorter the period in which both switches are on or both are, the longer the waiting time for the calculation There are great merits such as shortening. FIG. 7 shows changes in the gate-source voltage v _Gs1 of both switch elements T _r1 forming the switches AS1 and AS2.
Shown in. The conventional switch shown in FIG. 8 has a waveform like a dotted line, but in the present invention, it has a waveform like a solid line. In addition,
In FIG. 6, each transistor _Tr2 uses an output transistor inside the open-collector comparators COMP1 and COMP2. In this case, as the base voltage V _s of each transistor T _r2 , V _COMP and V _ref used in the open collector comparator are V ₋ and V _+.
Has been used. _Without the resistor R _d connected in series to the drains of the driving FETs T _r3 of both the switches AS1 and AS2, both t _ON1 shown in FIG. 5 are too short, and a constant pulse whisker appears on the output V _OUT. Therefore, it is used for damping.

FIG. 8 shows another embodiment in which the switch according to the present invention is applied to an auto-zero circuit. 8 and 6
6 is that a non-inverting amplifier is used as the amplifier OA in FIG. 6, but an inverting amplifier is used as the amplifier OA in FIG. Therefore, a field effect transistor T _r1b is provided in the analog switch AS2, and this T _r1b is used as an input resistor R
_An additional connection is made between in and T _r1 of the analog switch AS1. T _r1b is turned on together with T _r1a to be short-circuited to common so that the terminal on the T _r1 side of the input resistance R _in becomes a voltage of V _in when the switch AS1 is off and T _r1 is not destroyed. It is for the purpose. This circuit
When the switch AS1 is on, the circuit equivalently becomes as shown in FIG. Here, C _gs1 is the switching element T _{r 1} of AS1.
It shows the input capacitance of. The frequency characteristics of the amplifier are affected by this C _gs1 , but the resistance element Rd is required to suppress it. This resistance element Rd is also necessary to take a "whisker" as in the example of FIG.

[0015]

According to the present invention, an analog switch which performs a switching operation at an extremely high speed can be obtained by adding a simple structure of providing a driving FET T _r3 to the conventional analog switch.

[Brief description of drawings]

FIG. 1 is a circuit configuration diagram of a switch according to an embodiment of the present invention.

FIG. 2 is a circuit diagram for explaining the operation of FIG.

FIG. 3 is a circuit diagram for explaining the operation of FIG.

FIG. 4 is a circuit diagram for explaining the operation of FIG.

FIG. 5 is a waveform diagram for explaining the operation of FIG.

FIG. 6 is an application example of the switch of FIG.

7 is a waveform chart for explaining the operation of FIG.

FIG. 8 is another application example of the switch of FIG.

9 is an equivalent circuit diagram of FIG.

FIG. 10 is a circuit configuration diagram of an example of a conventional switch.

11 is a waveform chart for explaining the operation of FIG.

[Explanation of symbols]

T _r1 switch element T _r2 , transistor T _r3 FET R ₁ , R ₂ resistance

Claims (1)

[Claims] 1. A switch element comprising an FET and connected to an analog signal line, and a gate of the switch element.
A resistor connected between sources, a FET whose source and drain are connected to the gate and source of the switch element,
An analog switch comprising a resistor connected between the gate and the source of the FET, and a transistor connected between the gate of the FET and the negative power source.