# Logic gate using current source and diodes

Posted on Feb 7, 2014

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 have simulated the circuit using

LTSpice and it seems like the output is high when both A and B is high. If both A and B are low or if one of A and B is low, the output is low. Why How do you solve this kind of circuit How is a current source usually dealt with in this case Douglas Edward Jun 15 `13 at 14:04 You have concluded it is a NAND gate and I agree. Do you understand the forward conduction characteristics of a diode This will help you, and if you are uncertain by the current source just imagine it is a high value resistor connected to (say) +5V Andy aka Jun 15 `13 at 14:26 I have managed to figure out 00, 01, 10. If any one of the input is low and the output is high, diode will be forward biased and act as a short, and there will be a contradiction of voltage levels. Therefore the output has to be low if any of the input is low. Now for the 11 case I don`t see how to reason since both cases (output 1 and 0) seem possible. Douglas Edward Jun 15 `13 at 15:40 Maybe a thought experiment helps If you consider a single diode, in series with a very low resistor, say 0\$Omega\$ what is the output voltage in that case Then slowly increase that resistor 1\$Omega\$, 10\$Omega\$, . what happens to the output in that case What happens if you increase the resistor even further (to \$infty\$) jippie Jun 15 `13 at 16:03

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