Electronics Lab - Day 6

Today we learned all about Operational Amplifiers which just amplify the input signal.

Lab 8-1 : Open Loop Gain

We created the circuit below with a potentiometer to supply the variable resistance which allows us to vary the voltage between -12v and +12v.

We found that with a positive input voltage, the output is the maximum 12 v for the opAmp and with a negative input voltage, the output is the minimum 12 v for the opAmp.

These results imply that the gain is so high that the opAmp is maxing out its voltages no matter how small of an input voltage we put in.

Lab 8-2 : Inverting Amplifier

In order to lower the gain of the opAmp circuit so that we aren't always hitting the maximum voltage, we added negative feedback to the circuit.

When we have negative feedback in place, we have to keep in mind two golden rules:

1. The opAmp will keep the voltages at the two inputs the same.

2. The inputs draw virtually no current.

With these two rules in mind, we calculated in class that the gain between the output and input voltages would be.

In the circuit that we created, R1=1 kohm and R2=10 kohm. So we calculated that the gain should be -10. We found this to be true experimentally since Vin=1.5v and Vout=-15v.

We noticed at high frequencies the amplification is bad and that there was a phase shift between the output and input voltages.

We were also asked to measure the input and output impedances of this circuit. We measured the input impedance to be Zin = 1 kohm. For the output impedance, we added a 56 ohm load resistor and found that Vin~= Vout which implies that the output impedance of the circuit is much smaller than the 56 ohm load resistor.

Lab 8-3 : Non-Inverting Amplifier

As the name implies, this opAmp circuit does not invert the input voltage at the output as the inverting amplifier does.

Again, we used the golden rules for an opAmp circuit with negative feedback to calculate that the gain for this type of circuit is:

Again we made R1 = 1 kohm and R2 = 10 kohm and calculated our gain to be 11. Experimentally, our gain almost matches the theoretical gain. Our maximum input voltage was 0.5 v and our maximum output voltage was around 5 v which gives us a gain of 10.

We also measured the input impedance of this circuit and found it to be very high. Vin=500mV and Vout=450mV so we calculated that Zin was just very large. This is because the input is connected directly to the input of the opAmp and the impedance of the opAmp is very large. The output impedance was still very small but it seemed to be higher than that of the inverting amplifier.

Lab 8-4 : Follower

The follower circuit is basically a non inverting amplifier with R1=infinity and R2=0. This means that according to the formula for a non inverting amplifier, the gain for this circuit should be 1 and the input and output voltages should be the same.

We found that the input and output voltages were the same for this circuit.