Transistor Not Entering in Cutoff region

Question

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?

Circuit simulation

Answer 1

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:

Answer 2

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

^{simulate this circuit – Schematic created using CircuitLab}

Answer 3

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