JPS623612B2 - - Google Patents

Description

DETAILED DESCRIPTION OF THE INVENTION The present invention relates to an analog switch circuit suitable for switching high frequency signals, and an object of the present invention is to provide an analog switch circuit with good high frequency isolation characteristics.

Generally, monolithic analog switches are formed using a single transistor switch as shown in Figure 1, so the coupling capacitance between the input and output of the package and the junction type or MOS type transistor switch are When using a field effect transistor, the source-drain capacitance C _ds or the gate-source and gate-source
Coupling capacitances C _gs and C _gd between the drains act, and even when the switch is in an open state, a high frequency signal leaks to the output side, resulting in deterioration of off-isolation characteristics.

Incidentally, the analog switch circuit consisting of MOS transistors shown in FIG. 1 will be shown and its equivalent circuit will be shown and explained.

In FIG. 1, 1 is an input terminal, 2 is an output terminal, and 7 is a driver that controls the MOS transistor 6. FIG. 2 is an equivalent circuit when the switch circuit is in the OFF state. As is clear from the equivalent circuit in Figure 2, the input/output capacitance C _ds is about 0.5 pF, the gate-source and gate-drain capacitances C _gs and C _gd are about 5.5 pF, and the driver If the impedance Z of 7 is large, the high frequency signal will be transmitted to the output terminal 2 even when the switch is off.
It is understandable that this can be leaked.

For the analog switch circuit shown in Figure 1,
There is a limit to the isolation characteristics for high frequency signals. Therefore, three switches 8, 9, and 10 as shown in Fig.
It has been considered to improve off-isolation characteristics by forming a switch circuit to sufficiently reduce the on-resistance of the switch 10 when the switch is turned off. Analog switch circuits for such purposes are generally formed by combining switch circuits such as the one shown in Figure 1 on a printed circuit board, and an analog switch circuit with relatively good off-isolation characteristics is realized. . However, such an analog switch circuit is a combination of transistor switches as shown in FIG. 1, and is not entirely formed into a semiconductor integrated circuit, but is formed within a package or by connecting external leads. Therefore, it is difficult to reduce the strain capacitance or the coupling capacitance between input and output, and there is a limit to the improvement of high frequency off-isolation characteristics. However, since it is a combination of transistors and switches, it has the disadvantage of being expensive. Furthermore, the shape of the analog switch circuit becomes large, which is not preferable for a circuit using a large number of analog switch circuits, such as an AD converter.

The present invention solves the above-mentioned problems, and aims to provide a simple monolithic analog switch circuit with good high frequency and atsolation characteristics.
The switch circuit includes first and second transistor switches connected in series on a semiconductor substrate, and a third transistor switch that controls the bias voltage of the transistor switch.
and a fourth transistor switch, and
An analog switch circuit including a fifth transistor switch that maintains the semiconductor body at low impedance when the switch is off.

The analog switch circuit of the present invention will be explained below with reference to the drawings.

FIG. 4 is a circuit diagram showing one embodiment of the present invention.

1 is an input terminal, 2 is an output terminal, and 4 and 5 are power supply terminals. Q ₁ to Q ₅ are field-effect MOS transistors (hereinafter abbreviated as MOS transistors).
, and R ₁ and R ₂ are bias resistances. The sources of MOS transistors Q ₁ and Q ₂ are commonly connected,
The drain of the MOS transistor Q ₁ is the input terminal 1 for a high frequency signal, and the drain of the MOS transistor Q ₂ is the output terminal 2. The sources of the MOS transistors Q ₃ , Q ₄ , and Q ₅ are commonly connected and connected to the power supply terminal 5 , and their gates are commonly connected and connected to the control terminal 3 . The drain of the MOS transistor Q ₃ is connected to the gate of the MOS transistor Q ₁ and also to one end of the bias resistor R ₁ , and the other end is connected to the power supply terminal 4 . Also, the drain of MOS transistor _Q4 is
It is connected to the gate of the MOS transistor Q ₂ and to one end of the resistor R ₂ , and the other end is connected to the power supply terminal 4. is supplied, and a negative power is supplied to the power supply terminal 5.

FIG. 5 is a diagram for explaining the operation of the analog switch circuit of the present invention.
5 corresponds to MOS transistors Q ₁ to Q ₅ . The analog switch circuit shown in FIG. 5 is an example where the signal input from terminal 1 is cut off, and the signal from terminal 3 causes switch
3 to 15 are turned on, and switches 11 and 1 are turned on.
2 is in the off state. Also, when the analog switch circuit is on, switches 13 to 1
5 is set to the off state, and switches 11 and 12
is set to the on state.

In Fig. 4, a positive power supply voltage is supplied to power supply terminal 4, a negative voltage source is supplied to power supply terminal 5, and when a negative voltage signal is applied to control terminal 3,
MOS transistors Q ₃ to Q ₅ are in the off state, and the analog switch circuit is connected to the bias resistor.
MOS transistor biased through R ₁ and R ₂
Q ₁ and Q ₂ are active, and the analog switch circuit is turned on. Ideally, an analog switch circuit should have low on-resistance, high-speed switching, and large current capacity.From these points of view, transistor switches should be
MOS transistors etc. are effective. Since the effective area of the gate of the V-MOS transistor is small, the input/output capacitance can be extremely small, and an on-resistance of about several ohms can be easily obtained. Such characteristics are important for improving the performance of analog switch circuits.
Furthermore, when the OV signal is applied to the control terminal 3,
MOS transistors Q ₃ , Q ₄ , and Q ₅ are turned on, MOS transistors Q ₁ and Q ₂ are turned off, and the analog switch circuit is turned off.
Regarding the high frequency characteristics of analog switch circuits when the switch is turned off, it is important to have good off isolation characteristics, and the gate of the transistor switch must be kept at a sufficiently low impedance. . For this reason, MOS transistors Q ₃ , Q ₄ , and Q ₅ are kept active when the switch is turned off to prevent deterioration of off-isolation characteristics.

The analog switch circuit of the present invention includes MOS transistors Q ₁ , Q ₃ and MOS transistors Q ₂ , Q ₄
voltage sources are individually supplied via bias resistors R ₁ and R ₂ , and the individual voltage sources supplied from bias resistors R ₁ and R ₂ are connected to the MOS transistors.
It acts as a bias voltage for Q ₁ and Q ₂ ,
It is simply a voltage source for MOS transistors Q ₃ and Q ₄ . In this way, by separately supplying voltages through the resistors R ₁ and R ₂ , it is possible to eliminate the interaction between the MOS transistors Q ₁ and Q ₂ . On the other hand, if the voltage source is supplied through the same bias resistor, the gates of MOS transistors Q ₁ and Q ₂ are equivalently shorted, resulting in off-isolation. The characteristics deteriorate, and there is no point in forming a T-type analog switch circuit.

Incidentally, FIG. 6 is an equivalent circuit diagram of the analog switch circuit according to the present invention when it is off. A portion corresponding to the transistor switch in FIG. 4 is surrounded by a dotted line, and the symbol of the transistor switch is attached. Further, FIG. 7 shows the off-isolation characteristics of the analog switch circuit of the present invention. The horizontal axis represents the frequency of the control signal, and the vertical axis represents the attenuation amount corresponding to each frequency. The solid curve indicates the OFF/OFF state of the analog switch circuit of the present invention.
This is the isolation characteristic, and the dotted line is the isolation characteristic of the analog switch circuit explained earlier.

As explained above, OFF/OFF for high frequency signals
In order to improve isolation characteristics, it is necessary to have a small strain capacitance and to maintain a sufficiently low impedance at the gate of the transistor switch when the switch is turned off. The analog switch circuit of the present invention includes first and second transistor switches connected in series between input and output terminals, and third and fourth transistor switches that share a voltage source with the first and second transistor switches, respectively. It has a transistor switch,
first and third transistor switches and second
and a fourth transistor switch are supplied with a power supply voltage through respective load resistors, and the first and second
The third and fourth transistor switches are driven individually, and when the switch is off, the third and fourth transistor switches are driven individually.
driving the switch to cut off the first and second transistor switches, thereby keeping the gates of the first and second transistor switches at low impedance, and the connection point between the first and second transistors; By driving the fifth transistor switch, the stray capacitance generated between the semiconductor substrate and the semiconductor substrate is erased and the off-isolation characteristics are improved.

When the first to fourth transistor switches are supplied with the power supply voltage through a common bias resistor, the capacitance between the input and output becomes as if it were a single transistor switch, and the fifth transistor switch The effect of the switch will be drastically reduced. Therefore, the off-isolation characteristics of such an analog switch circuit with respect to high frequency signals deteriorate as shown by the dotted line in FIG.

As explained above, the analog switch circuit of the present invention has good off-isolation characteristics for high-frequency signals, and as shown in FIG. It exhibits isolation characteristics and can provide analog switch circuits that are extremely suitable for high-frequency signals.

In addition, in the embodiment, the transistor. Although a MOS transistor is shown as a switch, a junction field effect transistor may also be used, or a V-MOS transistor (V-MOS transistor) with good high frequency characteristics may be used.
It is clear that the use of superior transistor elements such as grooved MOS (grooved MOS) or DI-MOS transistors (DI-electric isolated MOS) would be more effective.

It goes without saying that the individual elements forming the analog switch circuit of the present invention are formed on the same semiconductor substrate.

[Brief explanation of the drawing]

FIG. 1 shows an analog switch circuit consisting of MOS transistors, and FIG. 2 shows its equivalent circuit. FIG. 3 shows an analog switch circuit formed in a T-shape. FIG. 4 shows an embodiment of the analog switch circuit according to the present invention, FIG. 5 is an explanatory diagram thereof, and FIG. 6 is an equivalent circuit diagram thereof. Figure 7 shows the off-frequency response to the frequency of the input signal.
FIG. 3 is a diagram showing isolation characteristics.

Claims (1)

[Claims] 1 comprising a signal input/output terminal and a control terminal to which a signal for controlling opening/closing of an analog switch circuit is supplied, first and second field effect transistors connected in series between the input/output terminals; the first and the second
An analog switch circuit comprising a third field effect transistor whose potential at a connection point with a field effect transistor is controlled by a control signal from the control terminal, wherein the first field effect transistor and the second field effect transistor gate electrodes of the field effect transistors are individually connected to a bias voltage source via resistive components, and a bias voltage source is provided for controlling the bias voltage supplied to the respective gate electrodes of the first and second field effect transistors. 4
and a fifth field effect transistor, and by turning off the third to fifth field effect transistors, conduction is established between the input and output terminals, and the third to fifth electric field By turning on the effect type transistor, the potential at the connection point of the first and second field effect transistors is made low, and the potential of their gate electrodes is made low so that the first field effect transistor is turned on. 1. An analog switch circuit characterized in that the gates of the field effect transistor 1 and the second field effect transistor are electrically separated to cut off the input and output terminals.