# How does the varying resistor affect the voltage at node 2?

• mathrocks
In summary, the conversation is about solving an op amp circuit for a lab, specifically addressing the placement and role of the 4.7k ohm and 100 ohm resistors. The conversation also touches on nodal analysis and the assumptions of an ideal op-amp. It is explained that the varying resistor acts as a potential divider and its value does not affect the amplifier circuit. The voltage at node 2 can be calculated using the potential divider formula.
mathrocks
I have to solve an op amp circuit for a lab I'm doing but the circuit they have given me looks confusing (it's on page 46, http://filebox.vt.edu/users/oshekari/Manual_Student.pdf). Why does the 4.7k ohm resistor have an arrow pointing to the middle of the 100 ohm resistor? I know the 100ohm is a varying resistor but if I were to actual solve the circuit for output and leave the varying resistor at 100ohms would the 4.7k be connected to node 3 or to node 1? And if it is indeed connected to node 3 then how would I go about doing nodal analysis at that node since I don't know the current to the left of the node, I just have a voltage source.

Correct if I'm wrong but at node 3 the nodal equation is: 5/x+(v3-v1)/4.7k+v3/100. Where x is the resistance, v3 is the voltage at node 3, and v1 is the voltage at node 6.

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Now are we assuming this op-amp is ideal? For laboratory purposes I would suspect that it is not--- however I did not read your lab to find out the Rin, A and Rout values.

Theelectricchild said:
Now are we assuming this op-amp is ideal? For laboratory purposes I would suspect that it is not--- however I did not read your lab to find out the Rin, A and Rout values.

Yes, it's an ideal op-amp.

Hi,

I forgot my nodal analysis learned during 1st year, but I can give you some light on the 100 ohms varying resistor.

Whether the varying resistor is 100 ohms or 1kohms is of no concern in the amplifier circuit because it effectively acts as a potentiometer OR potential divider. Whatever the voltage at node 2 depends on the position of the dial: If it is at node 3, the voltage at node 2 is the full 5V : If it is at node 1, the voltage at node 2 is the full 0V : If it is halfway in between, it is 1/2 * 5V = 2.5 V.
This is because as you turn the dial downwards, the resistance with respect to node 1 decreases, and the resistance with respect to node 3 increases.
The voltage at node 2 can be found using the potential divider formula:
R1/(R1+R2) * Vcc.

## What is an op amp circuit?

An op amp circuit, short for operational amplifier circuit, is a type of electronic circuit that uses an operational amplifier to perform mathematical operations on electrical signals. It is commonly used in analog circuits for amplification, filtering, and other signal processing tasks.

## How do I solve an op amp circuit?

To solve an op amp circuit, you will need to apply the basic principles of circuit analysis, including Kirchhoff's laws and Ohm's law. You will also need to understand the properties and ideal behavior of an op amp, such as infinite input impedance, zero output impedance, and infinite gain. By applying these principles, you can use circuit analysis techniques to determine the output voltage or current of the circuit.

## What are some common problems when solving op amp circuits?

Some common problems when solving op amp circuits include stability issues, non-ideal behavior of the op amp, and incorrect assumptions about the circuit. Stability issues can arise when the circuit is not properly designed or when feedback is not used correctly. Non-ideal behavior of the op amp, such as finite gain and input/output impedance, can also cause errors in the analysis. Additionally, incorrect assumptions about the circuit can lead to incorrect solutions.

## Can I use software to solve op amp circuits?

Yes, there are many software programs available that can help you solve op amp circuits. These programs use simulation techniques to analyze the behavior of the circuit and provide accurate solutions. However, it is important to have a basic understanding of op amp circuits and circuit analysis principles in order to use these software programs effectively.

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