1. Field of the Invention
The present invention relates to analog switches fabricated from a MOS transistor, and more particularly to switches incorporated in circuits operating at a low voltage, for example 3 volts.
2. Discussion of the Related Art
FIG. 1 represents a P-channel MOS transistor usable as an analog switch. The transistor includes a control terminal or gate Vg, a bulk terminal Vb, and two main terminals U1 and U2 (usually called source and drain) which form the switch terminals. The bulk terminal Vb generally corresponds to a substrate terminal for N-channel MOS transistors and to a well terminal for P-channel MOS transistors.
To simplify the drawings and the description, the same reference characters designate a terminal and the terminal voltage.
The two main terminals U1 and U2 of a MOS transistor are symmetrical. The role of each terminal (source or drain) depends upon its voltage. Thus, for a P-channel MOS transistor (FIG. 1), the source is the terminal U1 or U2 having the highest voltage while, for an N-channel MOS transistor, the source is the terminal U1 or U2 having the lowest voltage. A MOS transistor switch is designed to switch a signal present at either of terminals U1 or U2 toward the other terminal. Thus, it cannot be determined beforehand which terminal is the source or the drain.
The impossibility of being able to determine the role of terminals U1 and U2 is a drawback when the MOS transistor is used as a switch in a circuit supplied by a low voltage, for example 3 volts. In fact, for a MOS transistor to suitably operate as a switch, i.e., to behave like a low resistor in the on state, a voltage which sufficiently exceeds the transistor threshold voltage should be applied across its gate and source. It is therefore obvious that it is desirable to reduce the threshold voltage. In practice, because of the so-called bulk effect, the threshold voltage of the MOS transistor increases with the voltage present between the source (U1 or U2) and the bulk terminal Vb. In addition, the threshold voltage varies as a function of temperature.
In order to have a minimum threshold voltage, the ideal solution is to connect the bulk terminal Vb to the transistor source. However, as mentioned above, it cannot be determined which one of the main terminals U1 or U2 of the switch is the source. Thus, the bulk terminal Vb should be connected to a fixed voltage which should be higher than the source voltage for a P-channel MOS transistor, or lower than the source voltage for an N-channel MOS transistor, to prevent forward biasing of a diode present between the bulk terminal Vb and the main terminal U1 or U2.
For a P-channel MOS transistor (FIG. 1), the bulk voltage Vb is generally connected to a high supply voltage Vcc of the circuit and a low supply voltage GND is applied to the gate Vg to turn on the switch. When voltages U1 and U2 approach zero (voltage GND), the voltage between the source (terminal U1 or U2) and the gate (at voltage GND), liable to render the switch conductive, decreases. Moreover, the threshold voltage reaches its maximum value when voltages U1 and U2 are close to zero. As a result, the switch cannot operate within a relatively high range extending from the low voltage GND. This range is close to 3 volts in current techniques, which makes it impossible to use such a switch in a circuit supplied at 3 volts, unless specific measures, described hereinafter, are taken.
The problem is the same for an N-channel MOS transistor switch, the non-operating range being close to the high supply voltage Vcc.
To solve this problem, a conventional approach consists of providing charge-pump circuits which increase the gate voltage Vg of the transistor switch to values beyond the supply voltages. However, the charge pump circuits operate at high frequencies and may introduce spurious pulses in the analog signals which are to be connected. In addition, the relatively complex charge-pump circuits occupy a non-negligible surface.