Understanding KCL and the In=0 Principle in OMP Amplifiers

[Saeed.z]

i came across an example in my textbook as shown :

http://img593.imageshack.us/img593/8444/img1314i.jpg

if we used KCL and In = 0 :

In = I_100 + I_25

=> I_25 = -I_100

but as being written in the answer , I_25 = I_100 !

could anyone explain it for me ? thanks

 

[technician]

No current flows into or out of the + or - inputs of an ideal op amp.
Hope this helps

 

[sandy.bridge]

Wouldn't the technical answer be [itex]i_{25}=-i_{100}[/itex] since the direction in the diagram is opposite to the actual direction? My textbooks take into account the assumed direction, and if the final answer is supposed to be opposite to the assumed direction, you merely place a minus sign in front of it to indicate that it is in the opposite direction to the supposed direction.

 

[technician]

Completely agree with Sandy Bridge

 

[DeeNos]

KCL (Kirchhoff's Current Law) and the In=0 principle are fundamental principles in circuit analysis. KCL states that the sum of currents entering and exiting a node in a circuit must be equal. This means that the current flowing into a node must be equal to the current flowing out of that node.

In the example shown in the textbook, there are two currents entering the node where I_25 and I_100 meet. According to KCL, these two currents must be equal in magnitude but opposite in direction. This is where the In=0 principle comes in. It states that the current entering a non-inverting input of an operational amplifier (such as in this circuit) is equal to the current exiting the inverting input. This is because the input impedance of an operational amplifier is assumed to be infinite, meaning no current can flow into or out of the input terminals.

Applying KCL and the In=0 principle, we can set up the following equation:

In = I_100 + I_25 = 0

Since the input current In is equal to zero, we can rearrange the equation to solve for I_25:

I_25 = -I_100

This means that the current flowing into the inverting input (I_25) is equal in magnitude but opposite in direction to the current flowing into the non-inverting input (I_100). This is why the answer in the textbook states that I_25 = I_100.

In summary, KCL and the In=0 principle are important tools in understanding the behavior of operational amplifiers. They allow us to analyze complex circuits and determine the relationships between different currents and voltages.