Operational Amplifiers I

In this lab we examined the inner workings of a LM741 Op-amp when put in a circuit. The function of it is to amplify the voltage from a voltage source. In order for the op-amp to be functional, it must be fed both positive and negative voltage.

We started by building this circuit layout:

We are given the task to come up with a circuit that ranges between 0 and -10V which has to consume 1mA or less of current from a sensor that ranges from 0 to +1V. In other words, we have to build an inverting amplifier with a gain of -10.

Step 1A:
We need to first calculate Ri ( input resistance ), given that the voltage in the sensor ranges from 0 - 1V

V = iR, R = V/i = 1/0.001 = 1000 ohms

Step 1B:
Next we need to assign an Rf (feedback resistor),
R = V/i = 10/0.001 = 10,000 ohms

When we introduce our 'sensor', the circuit schematics will be slightly changed;

Step 1C:
Disconnecting the inverting amplifier, we can obtain Rx by assuming we want to operate it at half of its standard 1/4W power rating;

1/8 = 12^2/Rx, Rx = 1152ohms

Although we could not get an exact 1152 ohm resistor, we obtained a 1300 ohm resistor

Step 1D:
To find Ry, we use voltage divider in series for both Rx and Ry, provided that there's no loading in Ry and that there's 1V across Ry;

1 = 12Ry/ (1152 + Ry), and Ry = 104.72, using 130 ohm resistor

Step 1E:
Next, we need to find Rth for the circuit
1/1152 + 1/104.72 = 1/Rth
Rth = 96 ohms

Step 1F:
We found out that the Thevenin Resistance is NOT above at least a factor of 20 times smaller than Ri. This means that we'll need to choose a considerably larger value for Ri. Ri = 3,000 ohms  and Rf = 30,000 ohms.

Step 2: Measuring components

Component	Nominal	Measured	Rating
Ri	3000 ohms	2999+/-2 ohms	1/8
Rf	30000ohms	29998+/-2 ohms	1/8
Rx	1300 ohms	1299 +/-1 ohms	1/8
Ry	130 ohms	129+/-1 ohms	1/8
V1	12V	12.27+/- .1 V	2 amps
V2	12V	12.27+/-.1 V	2amps

Step 3: Circuit overview

Step 4: Performing experiment

Vin (V)	Vout (V)	Gain	Vri (V)	Iri (mA)	Vrf (V)
0.00	0	0	0	0	0
0.25	2.79+/-.01	0.0896+/-.00032	0.251+/-.001	0.0837+/-.00034	2.79+/-.02
0.50	5.55+/-.02	0.0901+/-.00032	0.494+/-.001	0.1647+/-.00035	5.49+/-.01
0.75	8.29+/-.02	0.0905+/-.00022	0.745+/-.002	0.2483+/-.00069	8.29+/-.01
1.00	10.71+/-.01	0.09337+/-.000087	.968+/-.001	0.3228+/-.00040	10.77+/-.02

We measured the current in V1 to be 0.96+/-.01mA and the current in V2 to be -1.29+/-.02mA.