14-05-2011, 09:28 AM
Name– study of Schmitt trigger inverter circuit.
Aim – to study the working of Schmitt trigger inverter as square wave generator.
Apparatus – IC 7414, circuit board, power supply +5V DC, CRO, resistors, capacitors, connecting wires, soldering iron, cutter etc.
Circuit diagram –
[attachment=13852]
[attachment=13853]
Procedure –
1) You are given IC 7414. Identify the pin numbers of the IC.
2) Write the expression for the period of output waveforms. Calculate the values of R1 & C1 for three different frequencies.
3) Draw the ideal output voltage waveform of the circuit.
4) Connect the circuit with different values of R1 and C1. Observe each output on CRO and trace the waveforms.
5) Measure the periods and frequencies from CRO and compare them with theoretical results.
[attachment=13854]
Brief theory – as shown in the above circuit, simple clock generator can be constructed using Schmitt trigger NOT gates. A LOW value of R1 is used across first inverter to bias it near its linear region and therefore, must remain fixed. Capacitor C1 provides the necessary feedback for oscillations. It is the main component that determines the frequency of the clock and can be changed for adjusting the frequency. The diode in the circuit is used to rectify the AC signal, since negative voltages applied to the inputs of TTL ICs may damage them.
Schmitt trigger – it is a circuit, whose output changes state when the input is at specific threshold voltage levels. It is recognizable by the Hysteresis symbol on its logic symbol. For the Schmitt trigger inverter, when the input voltage reaches the upper threshold voltage (+Vsat) of +1.7V the output goes LOW. When the input voltage falls to the lower threshold voltage (–Vsat) of +0.9V, the output goes HIGH. Following figure shows how the output swings HIGH and LOW with the input threshold voltages and constitutes a Hysteresis loop of curve –
[attachment=13855]
Conclusion – the circuit produces square waves whose frequency can be changed by R1 and C1. Its +Vsat and –Vsat values depend on charging and discharging of timing capacitor.
