TTL logic

TTL-Nand Gate

Transistor -Transistor Logic is a Digital circuit built from transistors and resistors.It is called TTL because the transistors perform the required logic function

Case 1)
When B input is low and A is high . All current flows through Q1 emitter. I.e Base current of Q1 and colector current of Q1 .So Q2 is off,  q3 is off. So all current flows through Q4.Q4 will be on(low resistance) So output will be high
Similarly, When A is low and B is high | when A and B are low. Output will be logic 1.
Case 2)
When Both A and B are high. The multi emitter transistor will be in reverse active mode
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Read Modes of operation here

Bipolar transistors have five distinct regions of operation, defined by BJT junction biases.

The modes of operation can be described in terms of the applied voltages (this description applies to NPN transistors; polarities are reversed for PNP transistors):

• Forward-active: base higher than emitter, collector higher than base (in this mode the collector current is proportional to base current by ).
• Saturation: base higher than emitter, but collector is not higher than base.
• Cut-Off: base lower than emitter, but collector is higher than base. It means the transistor is not letting conventional current go through from collector to emitter.
• Reverse-active: base lower than emitter, collector lower than base: reverse conventional current goes through transistor.

In terms of junction biasing: ('reverse biased base–collector junction' means Vbc < 0 for NPN, opposite for PNP)

• Forward-active (or simply, active): The base–emitter junction is forward biased and the base–collector junction is reverse biased. Most bipolar transistors are designed to afford the greatest common-emitter current gain, β_F, in forward-active mode. If this is the case, the collector–emitter current is approximately proportional to the base current, but many times larger, for small base current variations.
• Reverse-active (or inverse-active or inverted): By reversing the biasing conditions of the forward-active region, a bipolar transistor goes into reverse-active mode. In this mode, the emitter and collector regions switch roles. Because most BJTs are designed to maximize current gain in forward-active mode, the β_F in inverted mode is several (2–3 for the ordinary germanium transistor) times smaller. This transistor mode is seldom used, usually being considered only for failsafe conditions and some types of bipolar logic. The reverse bias breakdown voltage to the base may be an order of magnitude lower in this region.
• Saturation: With both junctions forward-biased, a BJT is in saturation mode and facilitates high current conduction from the emitter to the collector (or the other direction in the case of NPN, with negatively charged carriers flowing from emitter to collector). This mode corresponds to a logical "on", or a closed switch.
• Cutoff: In cutoff, biasing conditions opposite of saturation (both junctions reverse biased) are present. There is very little current, which corresponds to a logical "off", or an open switch.
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In reverse active Beta will be very low. So In reverse active mode ( Emitter becomes collector and collector becomes emitter).Now Q1 collector is emitter. So Ie=Ib+Ic. here Ie refers to Collector current of Q1. Since Beta is very very low.Ic can be ignored.

So the collector current of Q1 will be =Ib(base current of Q1) when both A and B are high.Base current flows through q2. So Q2 and Q3 will be on. So output will be pulled to zero.Logic 0

Need of  Diode.Tottempole connection

If D1 is not added to the circuit. When A and B inputs are high.Q2 and Q3 would be on.So 0.95volt would be at the base of Q4.So Q4 would have been on for all cases.So its vital.

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