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| Does This Transistor AND Gate Work? |
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| eev_carl:
Hi, I've built a two-transistor AND gate. It's the first schematic in the link. When I set Voltage A to 0V and Voltage V to 5V, I get a Vout of 1.4V. This seems too high for what's supposed to be a zero value. These are the readings of my circuit. My Vcc is 6V. VA VB Vout 0 0 0 5 0 0 0 5 1.4 5 5 4.2 Is it missing a pulldown resistor? Thanks, Carl http://hyperphysics.phy-astr.gsu.edu/hbase/Electronic/trangate.html |
| capt bullshot:
The output voltage is as one would expect. IMO still valid logic levels for a circuit running off a 6V supply, Because current flows through the 10k base resistor, the B-E junction into the 4k7 pull-down resistor. For better results, make the 10k resistors larger, e.g. 100k. |
| JS:
Usualy better to do a NAND and later a NOT, you have NOR gates in your link that whould do, just one transistor and don't even put the second resistor, unless you needed an OR gate following the AND. Propagation delay could be higher this way but you are probably not going to get high speeds with this circuit anyway. JS |
| Zero999:
That's because the base-emitter junction is conducting, passing current through. Yes, it's an emitter follower, so the output voltage is also always lower. than the input, which won't matter for a single stage, but it will be no good if more than two stages are cascaded. Some voltage gain is required. As mentioned above, a better AND gate can be built by connecting a NOT gate after a NAND gate. When the transistors are in common emitter configuration, there's plenty of voltage gain. |
| eev_carl:
Thanks, the 100k resistors bring the Vout down to 200mV when only VB is on. With both on, Vout is 3.7V, |
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