Question 8.PE.1: To investigate the Schmitt trigger using (a) the 741 operati...
To investigate the Schmitt trigger using (a) the 741 operational amplifier ic, (b) the 7414 Schmitt trigger ic
For this exercise you will need the following components and equipment:
1 – 741 ic (operational amplifier)
1 – 74LS14 ic (hex inverting Schmitt trigger) (six Schmitt triggers in one chip)
1 – ±15 V dc supply
1 – variable ±5 V dc supply
1 – resistor 2.2 kΩ
1 – resistor 10 kΩ
1 – capacitor 0.1 μF
1 – signal generator (1 kHz) with approximately 10 V peak to peak sinusoidal output voltage
1 – double-beam cathode ray oscilloscope
1 – dc voltmeter
Procedure (a): using the 741 operational amplifier
1 Connect up the circuit of Figure 8.3. The pin connection diagram for the 741 ic is shown in Figure 8.4.
Notice that the 741 ic is being used as an inverting amplifier (the input is connected to the inverting ‘-‘ input of the ic but the feedback is from the output to the non-inverting ‘+’ input). The amplifier gain is (theoretically) infinite!
2 Set the input voltage to 0 V. The output voltage should be just below the positive supply voltage (about +13 V).
3 Increase the input voltage from 0 V towards +5 V and observe the output voltmeter. Notice that there is no change at the output until a certain input voltage is reached, at which point the output will switch to a negative voltage (about – 1 3 V). Note the value of this upper threshold input voltage.
4 Decrease the input voltage from this upper threshold value through zero towards – 5 V, and note the value of the lower threshold voltage (when the output switches back to +13 V).
5 Note that it is the inverting action of the 741 amplifier which causes the output voltage to fall at the upper threshold and rise at the lower threshold. The important point to grasp here is that a definite rapid switching action occurs at the two threshold values.
6 Replace the ±5 V dc supply with the signal generator, and the voltmeter with the oscilloscope. Arrange to display both the input and output voltages.
7 Increase the input signal voltage from the generator and notice that at a certain input level a square waveform appears at the output. Draw on graph paper the input and output waveforms, one underneath the other. The waveforms should resemble those shown in Figure 8.5.
Make sure that the appropriate voltage levels are marked on the drawing (upper and lower threshold points for the input and the output levels).
8 It may be helpful, in completing the statements 3 and 4 which follow, to display together the waveforms at pins 2 and 3 of the 741 ic.
Conclusions
The general principles of operation of this Schmitt Trigger are now dealt with. Work through each of the statements and compare your calculated answers with those obtained from the practical measurements.
Important points
- For a supply voltage of ±V_{S}, it is reasonable to assume that the amplifier output will be limited to a (saturated) value (V_{Osat}) \text{ of} ±(V_{S} – 2) V. Hence if V_{S} = 15 \ V , V_{Osat} = 13 \ V.
- The resistors R_{1} \text{ and} R_{2} divide the output and determine the feedback voltage applied to the non-inverting input.
Copy out and complete the following statements:
1 Upper threshold point (UTP)
=\frac{R_1}{R_1+R_2} \times(+) V_{\text {Osat }} V = + ____ V
From the practical exercise, UTP = + ____ V
2 Lower threshold point (LTP)
=\frac{R_1}{R_1+R_2} \times(-) \quad V_{\text {Osat }} \ V= – ____ V
From the practical exercise, LTP = – _____ V
See Figure 8.5.
Copy out the following items 3, 4 and 5, select the correct word from within the square brackets and fill in the spaces with values or names as appropriate.
3 When the inverting input becomes more [positive/negative] than the noninverting input, the output switches rapidly to – _____ V.
4 When the inverting input becomes more [positive/negative] than the noninverting input, the output switches rapidly to + _____ V.
5 The difference between the two switching threshold points (UTP-LTP) is called ____.
Its value for this exercise = ___ V
Procedure (b): using the 7414 hex inverting Schmitt trigger
1 Connect up the circuit of Figure 8.6. The pin connection diagram of the 7414 ic is shown in Figure 8.7.
Note (a) the addition of the ‘bubble’ which describes the inverting Schmitt trigger and (b) the shape inside the triangle, which resembles the well-known hysteresis loop for a ferromagnetic material.
2 Set the input signal level such that there is an output square waveform being displayed. Draw on graph paper the input and output waveforms, one underneath the other.
This time, the waveforms should resemble those shown in Figure 8.8.
Again, make sure that the appropriate voltage levels are marked on the drawing.
Conclusions
Important points
• For the ic version of the Schmitt trigger, the threshold voltages are both positive values, but are described as a positive-going threshold voltage and a negative-going threshold voltage (see Figure 8.8).
From the waveforms obtained in procedure (b) 2, estimate the following values:
(a) Positive-going and negative-going threshold voltages, comparing these values with those given in the manufacturers’ data.
(b) The voltage hysteresis.
(c) The high and low switching voltages (these were referred to in the opening paragraph of this chapter). The difference between these values will give the actual voltage height of the pulse.
(d) The duration of the output pulse.
Finally, to illustrate the hysteresis effect in diagram form, see Figure 8.9.
