The function generator circuits generate different waveforms – sine, triangular and rectangular. Such a circuit can generate only one type of signal or multiple waveforms.
Here we will consider the behavior and performance of the popular function generator (Figure 1) when one of its outputs is connected to a low resistance load. This information is important for circuit designers when selecting the applications for this circuit.
The configuration
The selected circuit consists of three operational amplifiers, each of which can generate rectangular, triangular and sinusoidal signals.
Figure 2 shows the signals when the circuit outputs are not connected to a load; as you can see all the signals look stable.
Figure 1: A popular function generator scheme
Figure 2: The function generator signals when none of the outputs are connected to a load
The circuit power is kept fixed at +5Vdc and -5Vdc. A variable load resistor is connected to each output to test the stability of the circuit outputs under any given scenario.
The resistor is first connected to the square output, and the waveforms are observed as the resistor value decreases. The same steps are repeated with the load resistor connected to the delta and sine wave outputs.
Scenario 1: Loading the rectangular output
By plotting the peak-to-peak values of the circuit’s signals at various load resistances at the square-wave output, the circuit provided stable outputs for square-wave, triangle-wave and sine-wave signals when the load resistance was above 254 ohms. Below this value, all signals begin to degrade rapidly; see Figure 3.
Figure 3: Signal behavior when a load resistor is connected to the square output of the circuit
Scenario 2: Loading the triangular output
When the load resistor is connected to the delta signal output, the circuit exhibits a very stable square wave signal. However, triangle and sinusoidal output signals degrade rapidly for load resistances below 130 Ohms; see Figure 4. However, this scheme shows better performance in this setting compared to Scenario 1.
Figure 4: Signal behavior when a load resistor is connected to the delta output of the circuit
Scenario 3: Loading the sinusoidal output
When the load resistor is connected to the sinusoidal signal output, the circuit shows very stable behavior for both square and delta signal outputs. However, only sinusoidal output signals degrade rapidly for load resistances below 132 Ohms. The chain shows better performance in scenario 3 compared to scenarios 1 and 2; see Figure 5.
Figure 5: Signal behavior when a load resistor is connected to the sinusoidal output of the circuit
Therefore, the results show that the function generator circuit does not provide ideal performance when the load resistance falls below a certain level (threshold resistance). This threshold resistance is not even the same for every output signal. This clearly shows that charging one output of the circuit will affect the other outputs and that some applications require more robust and complex function generator circuits to operate outside the operating windows defined in this experiment.
