Diodes exercise logic gates

There are 2 ways to understand this.

1. Replace Current Source with a "Voltage source with a resistor in series (+ve where current goes out)". Now you can imagine that there will be output only if both diodes are blocked, otherwise all current will be grounded.
2. A current is originating from source and it needs a path to ground. It always selects a low resistance path. If N terminal of diode is grounded (logic 0 attached), it is forward bias (ideally there is no +0.7V required). Current takes 0 resistance path irrespective of load and hence 0 output. If any voltage is attached at N terminal (logic 1), it will be reverse bias and so open circuit, current will go to output.

Other than that, practically speaking, HIGH logic state in practice never behaves as ground (0), although at times it may behave as 1.

The method is just like any problem with an ideal diode.

• Guess whether the diode is in forward or reverse bias.

• Solve the circuit under that assumption.

• Check whether the solution actually biases the diode the way you thought.

• If it does not, assume the diode is biased the other way, solve again, and check your answer again.

For your circuits you'll have to essentially do this procedure for each switching event in the forcing function (for the \$t<0\$ initial state, for \$0<t<T\$, and for \$t>T\$). For the second circuit, you also have to consider the possibility that the diode changes state during the transient.

As you gain more experience you'll find it gets easier to guess correctly in the first step.