I am in possession of a book that says if the gate of an N-channel MOSFET i s low (say 0 volts), then the output is high impedance; and that if that ga te is high (say 5 volts), then the voltage at the drain depends on the volt age at the source; if the voltage at the source is low, then the voltage at the drain is low; but if the voltage at the source is high (say 5 volts), then the voltage at the drain is a weak high (say 3.5 volts). For a P-chann el MOSFET, if the gate is high, then the output is high impedance; and if t hat gate is low, then the voltage at the drain again depends on the voltage at the source; if the voltage at the source is high (5 volts), then the vo ltage at the drain is high (5 volts); but if the voltage at the source is l ow (0 volts), then the voltage at the drain is a weak low (say 1.5 volts). Does this sound about right?
But I've also heard that a transistor can act as an amplifier. Is it possib le to design a circuit with such an N-channel transistor that has its sourc e at 0 volts and its gate at 3.5 volts that results in its drain being at 0 volts? And to design a circuit with such a P-channel transistor that has i ts source at 5 volts and its gate at 1.5 volts that results in its drain be ing at 5 volts?
If so, then couldn't I design a multiplexer with just six transistors?
Assume the inputs to my multiplexer are logical bits {pv}, {lw}, and {hg} a nd the output {rs} is defined as the value of {lw} if {pv} is low, and alte rnately {hg} if {pv} is high. I have N-channel transistors {pivotHigh}, {am plifyHigh}, and {negateLow}; and P-channel transistors {pivotLow}, {amplify Low}, and {negateHigh}; and I have wires {weakResult}, {lowNegated}, and {h ighNegated}.
Then I attach {pv} to the gates of each of {pivotHigh} and {pivotLow}, {lw} to the source of {pivotLow}, {hg} to the source of {pivotHigh}, the drains of each of {pivotHigh} and {pivotLow} to {weakResult}, {weakResult} to eac h of the gates of {amplifyHigh} and {amplifyLow}, the source of {amplifyHig h} to ground, the source of {amplifyLow} to high voltage, the drain of {amp lifyHigh} to {highNegated}, the drain of {amplifyLow} to {lowNegated}, the other end of {highNegated} to the gate of {negateLow}, the other end of {lo wNegated} to the gate of {negateHigh}, the source of {negateLow} to ground, the source of {negateHigh} to high voltage, and the drains of each of {neg ateLow} and {negateHigh} to {rs}.
Will this work? I guess it depends on where the threshold is that determine s whether the transistors {amplifyHigh} and {amplifyLow} turn on and off. I f that threshold is somewhere between 1.5 volts and 3.5 volts, then it seem s like this circuit should work as a multiplexer.
If it does work, are people aware of such a design? When I've heard of impl ementations of multiplexers, they usually have involved at least fourteen t ransistors, not six, three NAND gates with four transistors each and one NO T gate with two transistors.
Is there a problem with noise? Is it possible that noise can cause a 1.5 vo lt value to turn on an N-channel transistor? Or that noise can cause a 2.5 volt value to turn off such a transistor? And vice versa for a P-channel tr ansistor? If so, is there some way to bundle up the four transistors {pivot Low}, {pivotHigh}, {amplifyLow}, and {amplifyHigh} to reduce the risk of no ise keeping the multiplexer from functioning correctly?