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I have made these circuits (DTL or TTL). And I want to enter these transistors (T1, T2, T3) in the cutoff region. But I am not able to do so. Which parameter I have to change so these transistors will enter the cutoff region? enter image description here

Circuit simulation

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3 Answers 3

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With the steering diodes there will be 1 diode drop (~0.6V) so the transistor base is never going to be below that and it will not fully cut off.

You could add a resistor from the base to ground to pull it a bit lower, but that brings additional current draw.

Back when they used this sort of logic one thing they would do is use Germanium diodes which have a voltage drop of around 0.3V, so the base would be pulled closer to ground. Another way of doing it was to use a negative voltage, which is why you see older logic with both positive and negative supplies. Here is a circuit using a resistor from the base to a negative voltage: By Inductiveload - Own work, Public Domain, https://commons.wikimedia.org/w/index.php?curid=5741538

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Typical old-timey DTL 3-input NAND gate made with discrete components :

schematic

simulate this circuit – Schematic created using CircuitLab

enter image description here

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This is DTL logic. You have to ground any input to turn off the input transistor.

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