Lab:Summing and difference Amplifiers(April 6, 2017)

Date: April 6, 2017

Non inverting op Amp; Summing Amplifier; Difference Amplifier

Activity 1: Inverting op amp graphs

            The first activity done today involved revisiting inverting op amps to determine practice our knowledge of the op amp.

The above picture shows the equivalent circuit of a basic inverting op amplifier. Since  we assume the op amp is ideal we can assume infinite open loop gain, A, infinite resistance at Ri, and zero resistance at, Ro. Therefore, we only look at R1 and R2. Since the R2 is connected to the Vo section of the op amp it will act as the feedback resistor. The R1 will be the initial resistor. We follow inverting op amp equation to obtain a gain of -3. The graphs display input voltage on top and output voltage on the bottom.

Activity 2: Noninverting op amp example

            The second activity is an example of how to solve a noninverting op amp circuit.

            The above picture shows the circuit we are working with along with the incorrect approach we took to solving it on the left of the picture. The correct method of solving it included doing nodal analysis at Va giving us the first equation. The second thing was realizing that Vs is equal to Vb which is equal to zero for this example. Then doing nodal analysis at Vb giving the second equation. We simplified both equations and got the Vo to be -1.64. An important observation of this example is that because Vs was equal to zero the circuit acted like an inverting amplifier. If Vs had any voltage the results would not have been a negative number rather a positive Vo.

Lab 1: Summing Amplifier

Pre-lab:

            In order to get this summing circuit to preform the addition of the two incoming voltages, Va and Vb, the three resistance must all be equal to each other, R1=R2=R3.     

The picture above is the summing circuit we set up using all three resistors that are equal to each other.

In the above picture is the results of the changing the voltage at Va and keeping Vb constant. Since all three resistors equal each other that means that this circuit follows the equation, Vo=-(Va+Vb).

Va	Vb	Vo theory	Vo exper	percent error
-4	1	3	2.96	1.33
-2	1	1	0.98	2.00
-1	1	0	0	0.00
0	1	-1	-0.97	3.00
1	1	-2	-1.96	2.00
2	1	-3	-2.05	31.67
3	1	-4	-3.43	14.25
5	1	-6	-3.43	42.83

The table above shows that for the most part the circuit acts the way we want predicted except at one point where the data is skewed, Va=3. However, the rest makes sense because we have observed that op amps saturate at high voltages and the saturation is higher when the Vo is negative.

Lab 2: Difference Amplifier

Pre-lab: First we determined the relationship between Va and Vb which is that they equal each other because they are both connected to the op amp from the positive and negative terminals and we assume ideal op amp; therefore, the equal each other and they both equal zero. But both of them equally zero does not go into deriving the difference amp equation.

In the Picture above we derived the relation ship between Vo and V1 and V2. The circuit is called a difference amplifier because if the all the resistances equal the same value then relationship becomes: Vo=(V2-V1). Resistances are on the bottom left of the board. We let R1=R3 and R2=R4. Furthermore, the results of when Vb=1 are on the bottom right and is the left of the two sets of data. The one on the very bottom right is when we changed the Vb to -1V.

V1	V2	Vo theory	Vo experiment	percent error
-4	1	10	4.27	57.3
-2	1	6	3.28	45.3
-1	1	4	3.27	18.3
0	1	2	2.01	-0.5
1	1	0	0	0.0
3	1	-4	-3.97	0.7
5	1	-8	-4.54	43.3

The table above( is for the results of when Vb=1 or V2=1) shows that high saturation occurs at higher voltages but this is to be expected. However, within smaller ranges of it does what we expect; it follows the equation, Vo=R2/R1(V2-V1) or Vo=2(V2-V1).

The graph above shows the relationship between the input and outputs of this circuit.

The above picture is our difference circuit.

In conclusion:

            We went over inverting op amps again. We continued our study of op amps by covering: non-inverting, summer, and difference op amps. In the experiments, we tested out the summing op amps and were able to determine That if all three resistors are the same the input voltages are added up then made negative. The Difference op amp circuit showed us that if the resistors R2/R1=R4/R3 we get a gain of the ratio of the two different resistors we also determined that if all the resistors were the same the input voltages would subtract which is why this is called a difference op amp. In both experiments we saw that at higher voltages there is a large saturation that occurs. There also appears to be more saturation when the output is negative.