Dear Mohammed Gholami, In the link text is not readable.

Are you able to read this link u have provided?

Http://www.ops.dti.ne.jp/~yanaka/digital/digital_ic/1_2_purpose.html

Please use this link instead of the previous:

http://www.allaboutcircuits.com/vol_4/chpt_3/5.html

The new link shows open collector structure of NAND gate which is very similar to TTL logic.

Mohammad, "in the NAND gate in TTL logic, when only one input is logical zero" the input bipolar transistor does not work in the active reverse condition; it works in this mode when all the inputs are logical "1". Follow the link below:

https://en.wikipedia.org/w/index.php?title=Transistor%E2%80%93transistor_logic&oldid=394474156#Implementation

I would be happy if you join the labs with my students dedicated to this topic:

https://www.researchgate.net/post/I_intend_incorporating_my_students_during_the_labs_on_Digital_circuits_in_our_discussions_Do_you_have_something_against_Will_you_support_me_or_not

Here is the first question about the relation between DL, DTL and TTL:

https://www.researchgate.net/post/What_is_the_basic_idea_of_the_input_logical_part_of_transistor-transistor_TTL_diode-transistor_DTL_and_diode_DL_logic_gates_Are_they_related

Regards, Cyril

Yes. You are right. I mean why it works in the saturation region. It was a typo. I correct the question.

Here are my answers visualized by LEDs:)

http://www.circuit-fantasia.com/ResearchGate/LED%20current%20steering.AVI

http://www.circuit-fantasia.com/ResearchGate/Lubomir/LED_current_steering_Lubomir.AVI

http://www.circuit-fantasia.com/ResearchGate/LEDTL.AVI

http://www.circuit-fantasia.com/ResearchGate/LEDTL_2.AVI

Enjoy! Cyril