To measure output voltages for different input voltages, to measure source current, sink current and output voltage for various loads of NOT gates.
Name– study of noise margin of inverter logic circuit.
Aim – to measure output voltages for different input voltages, to measure source current, sink current and output voltage for various loads of NOT gates.
Apparatus – IC 7404 – 2 Nos; circuit board, power supply +5V DC, voltmeter (0–5V), ammeter (0–100mA), microammeter (0–500A), connecting wires, soldering iron, cutter etc.
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Procedure –
1) Measure the output voltages for different input voltages starting from 0.0V to 1.5V. Take at least 15 different readings.
2) Tabulate the readings.
3) Plot a graph of input voltage versus output voltage.
4) Measure source current; sink current and output voltage for various loads of NOT gates.
5) Connect different number of NOT gates as loads (at least five NOT gates).
6) Tabulate the readings.
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Brief theory – the stray electric and magnetic fields may induce unwanted voltages, known as NOISE on the connecting wires between logic circuits. This may cause the voltage at the input to a logic circuit to fall below high-level input voltage (VIH) or rise above low-level input voltage (VIL). This may produce undesired operation of the circuit.
The ability of the circuit to tolerate noise signal is called noise immunity of the circuit. And the qualitative measure of noise immunity is called as noise margin. It is indicated as
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Fan-in & Fan-out –
1) Fan-in is defined as the number of inputs to a logic gate.
2) Fan-out is defined as the number of similar gates which can be driven by a previous gate.
3) For a standard TTL circuit the fan-out is generally equal to 10.

Sourcing & Sinking –
1) When a standard TTL output is HIGH, a reverse emitter current exists in the IC from the output transistor to the input transistor. For HIGH output the driver can source 400A for 10 TTL loads of 40A each. For this maximum loading, the output voltage is 2.4V or more.
2) When a standard TTL output is LOW, the output transistor in the IC acts like a current sink and changes the flow through it to the ground. Any 74xx series device can sink up to 16mA of current i.e. IOL = 16mA. The output voltage is ideally up to 0.4V.

Conclusion –
1) The noise margin of a logic circuit decides its characteristic working, in terms of logic–0 and logic–1 output & input conditions.
2) The fan–out of a given NOT gate is found to be ________.
3) The fan–in of a given NOT gate is found to be ________.