(1) Field of the Invention
The present invention relates to a boosting circuit, more particularly to a boosting circuit used as a multichannel input circuit.
(2) Description of the Prior Art
As is widely known, most semiconductor integrated circuits (IC's) are driven by rated driving voltage sources. For example, an IC fabricated by transistor-transistor logic (TTL) is usually driven by a rated voltage of 5 V. This does not mean, however, that each signal on the IC has a level of 5 V.
One reason for this is that the metal oxide semiconductor (MOS) transistors comprising the IC each have their own threshold voltage levels, i.e., V.sub.th. Therefore, even if an external signal of 5 V is applied to an input of the IC, the input signal is changed to one having a reduced voltage of, for example, (5-V.sub.th) V, when introduced inside the IC.
Such a reduction of the voltage level of signals is not serious when the signals are treated as digital signals in the IC however, when the reduction in the signals are treated as analog signals, the voltage level becomes a serious concern. For example, when input analog signals are converted into digital signals, the result would be erroneously converted digital signals lower in value than the correct value by the threshold voltage level V.sub.th.
In any case, it is necessary to prevent a reduction in the voltage level in the IC. In the prior art, this has conventionally been done by applying a voltage higher than 5 V, i.e., (5+.alpha.) V to a control gate of each transfer gate forming the input circuit. This has usually been effected by a boosting circuit based on the so-called bootstrap effect.
The above-mentioned prior art boosting circuit, however, suffers from the problem that the boosting circuit requires formation of many channels. The multichannel input circuit of the IC makes realization of a highly dense IC package impossible. Also, a complicated circuit construction is needed for forming the boosting circuit.