The high input case (logic 1) is trivial - the left 'diode' is off and the current is diverted (steered) entirely through the right 'diode' string. As a result, the base current of the input transistor is diverted ('steered') through the lower output part of the previous stage to ground and the circled transistor is off. The right diode is the string of three 'diodes' in series: the base-collector junction of the input transistor, the base-emitter junction of the circled transistor and the base-emitter junction of Q2. The left diode is the base-emitter junction of the input transistor. Thus, at low input voltage (logical 0), there are two 'diodes' in parallel. The best examples of TTL are logic gates namely the 7402 NOR Gate & the 7400 NAND gate. As the name suggests, the transistor performs two functions like logic as well as amplifying.

The base-emitter junctions of the circled transistor and the output transistor Q2 can be also considered as 'diodes'. What is Transistor-Transistor Logic (TTL) The Transistor-Transistor Logic (TTL) is a logic family made up of BJTs (bipolar junction transistors). The input transistor can be presented as two 'diodes' ( base-emitter and base-collector junctions) with common anodes (the base). The input part of the TTL logic gate above can be easily explained at low input voltage (logic 0) by the phenomenon of current steering, the essence of which is: If we connect two diodes with different forward voltages in parallel, the current will flow through the diode with lower forward voltage.

What is totem pole output