AND Gate

[cyclops_returns]

I'm looking for an and gate with a positive output(I know that sounds stupid but i found one where the out put is negative:Transistor Gates). I also found one which uses diodes but it doesn't seem to work. Can you suggest something. Alternatively can you suggest a pnp transistor that's commonly available. I'm from India, if that helps.

Thanks in advance

 

[Nigel Goodwin]

If the output is the wrong polarity, then follow it with an inverter.

 

[Noggin]

The first one in the link is a positive output AND gate, further down the page is a negative output AND gate called a NAND.

 

[Willbe]

I'm looking for an and gate with a positive output(I know that sounds stupid but i found one where the out put is negative:Transistor Gates). I also found one which uses diodes but it doesn't seem to work. Can you suggest something. Alternatively can you suggest a pnp transistor that's commonly available. I'm from India, if that helps.

Thanks in advance

Post the schematic that this gate fits into. Sometimes just two diodes will work as an AND gate.

 

[cyclops_returns]

Thanks for the help. It turns out I was just being stupid, the transistor gate works fine. But can you please explain me how the diode gate will work, I feel that the output will always be high regardless of the input

Thanks for the idea of attaching the inverter, quite practical but i just wanted to be minimal.

 

[Dean Huster]

If both the A and B input is held HIGH, neither diode will have a voltage drop, there will be no current flow; no current flow means no IR drop across the resistor, hence V will appear at the output (a HIGH).

If either or both the A and/or B input is grounded (a logical LOW), either or both diodes will be forward-biased and current will flow between V and ground through one or both diodes. This current will cause an IR drop across the resistor equal to about (V-0.7v) leaving the remainder (0.7v) at the output for a logical LOW. It's not the best circuit in the world, for that's a sloppy LOW, assuming silicon diodes. Using germanium diodes, the output would be closer to 0.3v for a logical LOW, maybe 0.1 to 0.2v using hot carrier (Schottky) diodes. So, it does work as an AND gate: if both A AND B is HIGH, the output is HIGH; in all other cases, the output is LOW. This also translates to a DeMorgan equivalent AND: If either A or B goes LOW, the output goes LOW; in all other cases the output is HIGH.

Dean