8/18/2023 0 Comments Transistor gate count![]() ![]() The following is known as as a AND/OR gate:Īnd has the following truth table. These functions need not be strictly AND, OR or Xor functions. This is often done for area efficiency, power efficiency or even for speed. the structure is a simple inversion.Įxtending the designs to higher order inputs is easy, as shown by a 3 input NAND.īy placing NMOS and PMOS in series/parallel combinations you can implement various logic functions at a transistor level. There is pleasing symmetry from NOR to NAND. I hope by now you should be able to get the truth table yourself. To turn into a AND gate you just need to invert the output. This truth table shows that if Not(A&B) AKA NAND. ![]() The output is then charged up and Vout = high. In all other conditions at least one of the PMOS will be sourcing current and the NMOS stack will not be able to sink current. ![]() So in that condition Vout will sink current and the Vout will be Low. Notice that for that condition that BOTH the PMOS are off (i.e. The NMOS "stack" will only ever sink current under one condition, and that is when BOTH A & B are high. Now for the first two input gate the NAND gate: Transistors turn gate voltage into controlled currents that then charge and discharge gate capacitances that turns that change in charge back into a change in voltage. Current is the flow of charge per time and capacitance is the storage of charge per voltage. This is deliberate as a MOS transistor IS a capacitor and it is mainly this capacitance that is charged and discharged during operation. If you look at the symbol for both the PMOS and the NMOS you'see that the gate looks like a capacitor on the symbol. as a result, the Vout pin tries to pull charge off of any available capacitance and discharges that capacitance until it reaches the Ground level.Ī "high" on the input gives a "low" on the output, a "low" on the input gives a "high" on the output. Likewise when the Vin is High, the NMOS is turned on and can sink current, but the PMOS is now off and cannot source current. As a result, the Vout pin tries to put charge onto any available capacitance and charges that capacitance up until it reaches the V+ level. When Vin is at ground, the PMOS is turned on and can source current, but the NMOS is off and cannot sink current. You will notice that already we are using logic language to describe function (AND, OR) so now we can start piecing together circuits. You could say that "either A OR B can be low for current to flow" for the PMOS and you could say that "either C OR D can be high for current to flow" for the NMOS circuit. Just like you can "stack" (actually put in series), you can parallel devices. You could say "Both A AND B have to low for current to flow" and "Both C AND D have to be high for current to flow". Also notice that Both C and D have to be higher than Ground (that funny hatched triangle symbol) for current to be sinked (sunk ?). Notice that both Voltage at A (we'll call it A) and B BOTH have to be below +V for current to flow. Stacking two PMOS gives a current source that is controlled by two voltages, stacking two NMOS gives a current sink that is controlled by two voltages. Interestingly you can stack the devices to make various functions. PMOS is usually connected to a positive voltage and NMOS is usually connected to negative voltages typically ground. Please note that I've taken some liberties with naming for the sake of clarity. The NMOS will sink current into ground through the drain into the source (which in this case you should think of as a sink). The PMOS will source current (the dotted line in the diagram shows current flow when on) from a power supply (attached to source) through the drain and into other circuits when the Gate voltage is LOWER than the source. The transistors are electrically controlled current sources/sinks. ![]() There are two kinds of transistors used, PMOS and NMOS, here are their symbols: I will deal with CMOS since 99% of all logic that has ever existed (in number count) exists as CMOS. To start with you have to understand transistors in a simple way. I've turned this into a community wiki so we can collect cool logic gate implementations to which to refer to in the future. ![]()
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