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Is it possible to amplify weak lows and weak highs?

Started by Kevin Simonson January 2, 2022
I am in possession of a book that says if the gate of an N-channel MOSFET is low (say 0 volts), then the output is high impedance; and that if that gate is high (say 5 volts), then the voltage at the drain depends on the voltage 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-channel MOSFET, if the gate is high, then the output is high impedance; and if that 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 voltage at the drain is high (5 volts); but if the voltage at the source is low (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 possible to design a circuit with such an N-channel transistor that has its source 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 its source at 5 volts and its gate at 1.5 volts that results in its drain being 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} and the output {rs} is defined as the value of {lw} if {pv} is low, and alternately {hg} if {pv} is high. I have N-channel transistors {pivotHigh}, {amplifyHigh}, and {negateLow}; and P-channel transistors {pivotLow}, {amplifyLow}, and {negateHigh}; and I have wires {weakResult}, {lowNegated}, and {highNegated}.

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 each of the gates of {amplifyHigh} and {amplifyLow}, the source of {amplifyHigh} to ground, the source of {amplifyLow} to high voltage, the drain of {amplifyHigh} to {highNegated}, the drain of {amplifyLow} to {lowNegated}, the other end of {highNegated} to the gate of {negateLow}, the other end of {lowNegated} to the gate of {negateHigh}, the source of {negateLow} to ground, the source of {negateHigh} to high voltage, and the drains of each of {negateLow} and {negateHigh} to {rs}.

Will this work? I guess it depends on where the threshold is that determines whether the transistors {amplifyHigh} and {amplifyLow} turn on and off. If that threshold is somewhere between 1.5 volts and 3.5 volts, then it seems like this circuit should work as a multiplexer.

If it does work, are people aware of such a design? When I've heard of implementations of multiplexers, they usually have involved at least fourteen transistors, not six, three NAND gates with four transistors each and one NOT gate with two transistors.

Is there a problem with noise? Is it possible that noise can cause a 1.5 volt 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 transistor? If so, is there some way to bundle up the four transistors {pivotLow}, {pivotHigh}, {amplifyLow}, and {amplifyHigh} to reduce the risk of noise keeping the multiplexer from functioning correctly?
On Sat, 1 Jan 2022 21:15:01 -0800 (PST)
Kevin Simonson <kvnsmnsn@hotmail.com> wrote:

> I am in possession of a book that says if the gate of an N-channel MOSFET is low [...]
The operation of a FET depends on the type and external components. For more clues goggle "fet transconductance graphs" Jan Coombs --