Column: Circuit drill


The performance of a function generator under different loads


By Sulaiman Algharbi Alsayed, Managing Director, Smart PCB Solutions F


unction generator circuits output different waveforms – sinusoidal, triangular and rectangular. These circuits can generate just one or multiple waveforms.


Here we will examine a popular


function generator’s (Figure 1) behaviour and performance with one of its outputs connected to a low-resistance load. The results are important for circuit designers when choosing applications for this circuit.


The setup The selected circuit consists of three op- amps, each able to generate rectangular, triangular and sinusoidal signals. Figure 2 shows the signals when the


circuit’s outputs are not under load; as it can be seen, all signals look stable. The circuit’s power is fixed at +5Vdc


and -5Vdc. A variable load resistor is connected to each output, to examine the stability of the circuit’s outputs for any given scenario. The resistor is first connected to the


rectangular output, with the signal shapes monitored as the resistor’s value decreases. The same steps are repeated with the load resistor connected to the triangular and sinusoidal signal outputs.


Scenario 1: Loading the rectangular output By plotting the output signals’ peak-to- peak values at various load resistances on the rectangular output, it can be seen that the circuit delivers stable outputs for the rectangular, triangular and sinusoidal signals when the load


10 July/August 2022 www.electronicsworld.co.uk


resistance is over 254 Ohms. Below this value, all signals start to rapidly deteriorate; see Figure 3.


We will examine a popular function


generator’s behaviour and performance when one of its outputs is connected to a low-resistance load.


The selected circuit consists of three op-amps, each able to generate rectangular, triangular and sinusoidal signals


Scenario 2: Loading the triangular output When the load resistor is connected to the triangular signal output, the circuit shows a very stable rectangular signal. However, the triangular and sinusoidal output signals deteriorate rapidly for load resistances below 130 Ohms; see Figure 4. Still, circuit performance is better with this setup than with Scenario 1.


Scenario 3: Loading the sinusoidal output With the load resistor connected to the sinusoidal signal output, the circuit shows very stable behaviour at both the rectangular and triangular signal outputs. However, only the sinusoidal output signal deteriorates rapidly for load resistances below 132 Ohms. The circuit performs better in Scenario 3 than Scenarios 1 or 2; see Figure 5. Hence, results show that the


function generator circuit does not provide ideal performance when load resistance drops below a certain threshold level, and it’s not even the same with each output signal. This clearly shows that loading one output of the circuit will impact the other outputs, and that some applications call for more robust and complex function-generator circuits, to operate outside the operating windows defined in this experiment.


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