Logic gate using current source and diodes

  
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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
Logic gate using current source and diodes - schematic

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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