Logic gate using current source and diodes

Question

This circuit is supposed to work as a logic gate. I need to find out the logic operation it performs. Suppose the diodes are ideal.

https://www.circuitlab.com/circuit/egg3wh/logicgate/

^{simulate this circuit – Schematic created using CircuitLab}

Now when the circuit is implemented with voltage sources, I know the general approach to solve this. You check the voltage at the anode and cathode of the diodes and determine whether or not it is conducting....

With a current source I am kind of confused on what to do. I can't determine the state of the diodes for a given input combination (00, 01, 10, 11) because I don't know the voltage at the anode of the diodes.

I think the problem is that I am not used to current source driven diodes.

Can anyone provide some guidance?

Thank you.

Answer 1

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.

Answer 2

I can understand your confusion. It all comes down to how you define the logic levels.

If either of the diodes is connected to a low input it will pull current through it.

Normally this circuit pulls a VOLTAGE low and it is the voltage that is used to define the output.

By using a current source, current will only flow if either or both diodes are grounded.

If we define the presence of current as '1' and its absence '0'

then the grounding of a diode = current = '1'

                      A        B          Out

                      0        0           0
                      0        1           1
                      1        0           1
                      1        1           1

it forms an OR GATE

If we define '1' as a voltage HIGH, and '0' as a voltage LOW we get, the output will be HIGH with both inputs not connected. (Both inputs HIGH)

                      A        B           Out
                      0        0            0
                      0        1            0
                      1        0            0
                      1        1            1

it forms an AND GATE

If we mix definitions - Inputs as high/low voltages Output as a current we get

                      A        B            Out 
                      0        0             1
                      0        1             1
                      1        0             1
                      1        1             0

it forms a NAND gate

So just like you I am confused because what constitutes a logic state (current, voltage or a mixture) is not defined.

As using LTSpice seems to like the "both high in gives a high out" it must define the logic in terms of the voltages (the middle option in this answer) so it will be an AND gate.