Lab 10: Op Amps 2

Introduction:
This experiment will continuously study the operational amplifier from the previous experiment by changing the the circuit with a separate voltage source and the impact of changing the input and feedback resistors. The picture shown below illustrates how the circuit is changed.

Procedure:

1. We fix R₁ = 10 KΩ, and we want to achieve a gain of -10. Then, R_f = 100 KΩ

Resistor	Nominal Value	Measured Value
R1	10 kΩ	9.79 +/- 0.01 kΩ
Rf	100 kΩ	95.2 +/- 0.1 kΩ

2.  If V_sen = 1V, the prediction of the value of current leaving the op-amp is  I_op = 1V/10kΩ = 0.1 mA

3.Build the circuit.

4. Measurement:

Calculation for I_OP = V_Rf/ R_f = (2.50 V)/(95.2 kΩ) = 0.0263 mA

Vin Desired	Vin actual	Vout Measured	VRf Measured	IOP Calculated
0.25 V	0.251 +/- 0.001 V	-2.49   +/- 0.01 V	2.50   +/- 0.01 V	0.0263 mA
0.5 V	0.432 +/- 0.001 V	-4.25   +/- 0.01 V	4.26   +/- 0.01 V	0.0447 mA
1.0 V	1.195 +/- 0.001 V	-10.76 +/- 0.01 V	10.79 +/- 0.01 V	0.113 mA

5. Measure the current I_CC and I_EE :
 I_CC  = 1.00mA and I_EE = -0.91 mA.


6. Confirm the KCL for the op-amp at this operating point:

I_rail = I_EE + I_CC = 1.01 + (-0.91) = 0.100 mA

I­_op = 0.113 mA .

 From above, we can see that the Values of I_op and I_EE+CC are about the same. We can conclude the KCL hold true at this operating point

7. The power of each power supply:

P_Icc = (12V)*(1.00mA) = 12 mW

P_Iee = (12V)*(-0.91mA) = -10.92 mW

8.Add a 1 kΩ resistor across the op-amp output:

9. Set V_sen  = 1V, and take measurement:

Vin desired	Vout Mesured	VRf Measured	Iop Calculated	ICC Measured	I­EE Meaured
0.93 V	-9.39 +/- 0.01V	8.99 +/- 0.01V	0.094 mA	0.93+/-0.01mA	-1.04+/-0.01mA

Sum of the Iee and Icc = 0.93 - 1.04= 0.11mA
We can see the values Iee+cc and Iop are very close. Therefore, the KCL still holds for the op-amp.

P_Icc = (12V)*(0.93mA) = 11.16 mW

P_Iee = (12V)*(-1.04mA) = -12.48 mW

Bonus:
Replace Rf with a new resistor so we can have the gain of -5.
Rf = 50K ohms.

Vin Desired	Vout Measured	VRf Measured	IOP Calculated	ICC Measured	IEE Measured
1.08 V	-5.44 +/-0.01V	5.47 +/-0.01V	0.109 mA	0.90 +/-0.01mA	-1.01+/-0.01mA

Sum of Iee and Icc = -1.01 + 0.90 = 0.11 mA
Iop = 0.109 mA
Therefore, the KCL still holds true.

Conclusion:
The gain of an inverting Op Amp is always equal to -Rf/Rin. If the gain is -5, the ratio of Rf/Rin has to be 5. As the same time, the current at the operating point is equal to the sum of the two rails coming into the op amp, and the KCL always holds correct.