this LTSpice simulation

e.x.c.e.l.l.e.n.t

ak

 

Now that you understand how the circuit works, the next task is to understand why that diode in the output is there. Notice that it's presence lowers the HI output voltage by another roughly 0.7 V on top of the Vbe drop of the transistor above it. So the highest that a 5 V TTL output can drive is about 3.6 V and that's before any drop in that resistor is taken into account. This is why TTL output levels of around 3.2 V to 3.5 V are so commonly observed and give people fits when they encounter them for the first time because they have been led to believe that 5 V logic means 5 V.

It would seem like removing that diode would let the output get up into 4.3 V range which would seem to be a good thing.

So why is it there?

Try answering this question by analyzing the circuit assuming it weren't there.

 

Now that you understand how the circuit works, the next task is to understand why that diode in the output is there. Notice that it's presence lowers the HI output voltage by another roughly 0.7 V on top of the Vbe drop of the transistor above it. So the highest that a 5 V TTL output can drive is about 3.6 V and that's before any drop in that resistor is taken into account. This is why TTL output levels of around 3.2 V to 3.5 V are so commonly observed and give people fits when they encounter them for the first time because they have been led to believe that 5 V logic means 5 V.

It would seem like removing that diode would let the output get up into 4.3 V range which would seem to be a good thing.

So why is it there?

Try answering this question by analyzing the circuit assuming it weren't there.

Oh thanks sir. I will analyze