Figuring out diode logic

Thursday, January 16, 2014

Q. I am studying diode gates in Lesson 3463. In the What Have You Learned? questions we are asked to figure out what kind of gates the circuits are, given just the circuit and the input voltages. I don't know how we are supposed to do this, because we aren't told what kind of gates they are in the first place.

A. The point of these exercises is to analyze each circuit electronically, and then to determine what kind of gate you have. Start by drawing four circuits, one for each line of the truth table. In each circuit, put in the input voltages. It may help to also draw in batteries for the points where voltages are shown, and connect the other end of each battery to ground.

Take each circuit in turn and decide whether each diode will be forward or reverse biased, based on the voltages in the circuit. Remember that Kirchhoff's voltage law must always apply, so the sum of the rises in a path must equal the sum of teh drops. When there is current through a resistor, there will be a voltage drop across it. And remember that ground is zero volts.

You may alos want to simply try one of the logic voltages you think ought to be on the output. If you then analyze the circuit, and find somehting impossible, such as forward-biased diode with a 5V drop across it, then you know you have picked the wrong voltage. Admittedly, this is more guesswork than analysis, but since the output has to be either a logic 1 or a logic 0, there are only two possibilities for each line of the truth table.

Either way, this process is somewhat tedious and time consuming, but in order to build up your skills in figuring out diode logic, you have to go through these exercises.

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