Operational Amplifiers I

We are presented with a sensor that outputs a signal up to +1 V. Because the signal is small, we want to insert a signal conditioning circuit in between to amplify the signal so the microcontroller can be powered.

In this model, the voltage divider on the left will represent the sensor, the 741 op-amp as the signal conditioning circuit, and Vout as the microcontroller's output. We wanted a gain of -10 through the op-amp and +12 V and -12 V on the top and bottom rails, respectively. We determined the following values for the resistors and voltage sources:

Component	Nominal Value
Ri	1 kΩ
Rf	10 kΩ
RX	1125 Ω
RY	Potentiometer (max 104.17 Ω)
V1	12 V
V2	-12 V

The Thevenin resistance, looking back form the inverting amplifier, was 96 Ω, which was at least 20 times smaller than the resistance for R_i. Therefore, there was no loading effect on our circuit.

We assembled the circuit and measured Vout, VRi, and VRf.

Our results were:

Vin	Vout	GAIN	VRi	IRi	VRf
0.0 V	0.0 V	N/A	0 V	0 mA	0 V
0.25 V	-2.762 V	-11.05	0.2785 V	0.279 mA	2.906 V
0.50 V	-5.06 V	-10.12	0.513 V	0.513 mA	5.17 V
0.75 V	-7.59 V	-10.12	0.753 V	0.753 mA	7.55 V
1.00 V	-10.22 V	-10.22	1.060 V	1.06 mA	10.20 V

Sure enough, we saw a gain of at least -10 in our output and the current drew no more than 1 mA as the sensor approached 1 V.