# NAND gate TTL - How it works

#### pj33000

Joined Feb 1, 2021
11
Hello,
I am studying digital electronics and I am a bit confused about the saturation conditions of the bjt.
I knew that the bjt saturates when the Vce is close to 0 and Vbe >= 0.7. However, when I was reading about TTL I got confused.
Can someone give me some intuition and explain me why in the case where both inputs are high, why Q2 and Q4 saturate and Q3 is OFF please.

I believe I understand something wrong because I cannot see how the saturation conditions are met.
Thank you

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#### LesJones

Joined Jan 8, 2017
4,205
Think about where the current through R1 goes when both inputs are high. (Tip. Think about the way you test a transistor using the diode test mode on a multi meter.)

Les.

#### dl324

Joined Mar 30, 2015
16,988
For homework, you need to show your work before we can offer any hints. You'll learn more that way...

#### pj33000

Joined Feb 1, 2021
11
Think about where the current through R1 goes when both inputs are high. (Tip. Think about the way you test a transistor using the diode test mode on a multi meter)
For Q3 I know it must be OFF but not sure why. But you give me a more intuitive explanation about the saturation mode please? Because I know they must be in saturation but not sure why.

Les.
I know that Q1 is in reverse mode, current flow from R1 to Q2 (don't know whether current is large or not)

#### pj33000

Joined Feb 1, 2021
11
For homework, you need to show your work before we can offer any hints. You'll learn more that way...
I don't know how to start calculating. Current flows from R1 to Q2 but I don't know how to calculate how much. Also I am not sure whywhy Q3 is OFF

#### ericgibbs

Joined Jan 29, 2010
18,976
hi pj,
As this homework, we cannot give you the answer.
Perhaps this simulation [with one input] will give you a clue.
E

#### crutschow

Joined Mar 14, 2008
34,669
I know that Q1 is in reverse mode, current flow from R1 to Q2 (don't know whether current is large or not)
I don't know how to start calculating. Current flows from R1 to Q2 but I don't know how to calculate how much. Also I am not sure whywhy Q3 is OFF
You have the value of the resistor, you should know the value of the nominal diode (transistor junction) forward voltage drop, and the supply voltage.
From that you can calculate the current using Ohm's law.

After that calculation, you should be able to determine why Q3 is off.

You do know Ohm's law I would hope.

#### crutschow

Joined Mar 14, 2008
34,669
It may help to draw just the path from V+ through the resistor, and the transistor junctions to ground.

#### pj33000

Joined Feb 1, 2021
11
It may help to draw just the path from V+ through the resistor, and the transistor junctions to ground.
I have been thinking of the simulation given above. In that case Q1 is reversed thus current flow from R1 to Q2. Q2 is saturated as Vbe >= 0.7V and Vce is close to 0V so current flows to Q4. Given that the forward bias voltage of the diode is almost 0.7V then Vb3 across Q3 is 0.1V thus Q3 is OFF and Q4 is ON for similar reason.

So current flows from R2 to Q2 to Q4 and ground.
How can I calculate the voltage va as shown in the diagram? I mean the transistor is reversed which formula helps to calculate that?

#### crutschow

Joined Mar 14, 2008
34,669
How can I calculate the voltage va as shown in the diagram? I mean the transistor is reversed which formula helps to calculate that?
Yes, Q1 is off as a transistor.
But current can flow through Q1's base-collector junction, which is now forward biased (draw in the NPN base-collector junction direction).

#### ericgibbs

Joined Jan 29, 2010
18,976
hi pj,
I have set Vin=5V, so the voltage levels are more detailed/visible.
Should help.
E

#### LesJones

Joined Jan 8, 2017
4,205
How many forward biased PN junctions are there between va and the 0 volt rail ?

Les.

#### pj33000

Joined Feb 1, 2021
11
Yes, Q1 is off as a transistor.
But current can flow through Q1's base-collector junction, which is now forward biased (draw in the NPN base-collector junction direction).
You mean it is forward bias thinking of the Ebers-Moll model?
If yes, how can I calculate the current and the voltage drop? Sorry that the questions are elementary but I am confused as the notes that I have are not that good.

#### pj33000

Joined Feb 1, 2021
11
hi pj,
I have set Vin=5V, so the voltage levels are more detailed/visible.
Should help.
E
View attachment 263286
Thank you
I think the path of the current though is not different to what I described above, right?

#### crutschow

Joined Mar 14, 2008
34,669
You mean it is forward bias thinking of the Ebers-Moll model?
You don't need the model.
The base-collector junction just looks like a forward-biased silicon junction diode, the same as the base-emitter junction.

#### pj33000

Joined Feb 1, 2021
11
You don't need the model.
The base-collector junction just looks like a forward-biased silicon junction diode, the same as the base-emitter junction.
Ok, but how can I solve the circuit. I mean I have a 5V source, R2 a diode but no ground.
Explain this please to clear everything out as well as how can I know by inspection that a transistor is saturated or OFF as done here:
https://www.vedantu.com/question-an...lass-12-physics-cbse-603774e71bab3a5dc7202907
Which just by inspection it says that T3 is saturated (annoyingly all the sources of that I found do this without explaining why it saturated as no calculations are done)

#### dl324

Joined Mar 30, 2015
16,988
Ok, but how can I solve the circuit. I mean I have a 5V source, R2 a diode but no ground
Since it now seems to be okay to provide more than hints. What are R4 and Q4 connected to?

#### ericgibbs

Joined Jan 29, 2010
18,976
hi pj,
This image gives all the information, it is up to you to put it together, post your calculations.
E

#### crutschow

Joined Mar 14, 2008
34,669
Ok, but how can I solve the circuit. I mean I have a 5V source, R2 a diode but no ground.
Of course you have a ground.
You follow the current from the resistor through to the emitter of Q4 at ground.
Why is that difficult to understand?

#### ericgibbs

Joined Jan 29, 2010
18,976
hi @pj33000