Explain current sinking and sourcing when two standard TTL gates are connected
1 Answer

Current Sinking:

  • The current sinking will take place for Y=0. The current flows from the input stage of load gate into the output stage of driving gate. Transistor Q4 of driving gate acts as a closed switch since it is saturated. This will forward bias the base emitter junction of Q1 of the load gate.

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Current Sourcing:

  • The current sourcing will take place when output of driving gate becomes high. Q3 will be ON and Q4 will be OFF. So the base emitter junction of Q1 of load gate will be reverse biased. Therefore the reverse leakage current of Q1 will be supplied (sourced) by the driving gate as shown.

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Transistor Q3 is called current sourcing transistor and Q4 is called current sinking transistor.

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