circuit-fantasia > circuit stories > building circuits > op-amp ammeter (short)

Building Circuits on the Whiteboard...

Op-amp Ammeter (short version)

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

What is the basic idea behind this circuit? "An imperfect ammeter is connected in the negative feedback loop. The input current flows through the meter coil. The op-amp strives to make the voltage difference between the inputs zero; so, the op-amp ammeter has zero input resistance..."

But yet, what is the basic idea behind this circuit? Let's build it on the whiteboard to reveal the truth...

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

Imagine we have a real voltage source with a voltage V and an internal resistance Ri.

A real voltage source with a voltage V and an internal resistance Ri. Click to view full-size picture.

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

Then suppose that you have to measure the current Ish = V/Ri that flows if you short the real voltage source.

A shorted real voltage source. Click to view full-size picture.

 

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

Problem. For this purpose, break the circuit and connect an ammeter. Only, this ammeter is imperfect - it has some internal resistance RA, which affects the current. How do we solve the problem?

In order to measure the current, we break the circuit and connect an ammeter. Click to view full-size picture.

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

Remedy: removing a voltage by an "antivoltage". The voltage drop VRA across the ammeter is harmful; so, we have to remove it by an "antivoltage" -VRA. That means to connect an additional supplementary battery and adjust its voltage so that VRA = -VRA. As a result, the "harmful" voltage VRA disappears.

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 - next step >

Then, replace the "op-amp man" with a real op-amp. It compensates the "harmful" voltage drop across the capacitor by copying and adding it to the voltage of the input source (in other words, it "helps" the input source).

Finally, replace the 'op-amp man' with a real op-amp. Click to view full-size picture.

< prev step - 0 - 1 - 2 - 3 - 4 - 5 - 6 -  next step >

A simple, small but... nonunderstandable circuit.Cleaning" the circuit diagram (don't do it, if you are a real teacher!) Finally, remove all the "unnecessary" components of the picture to get the classic circuit diagram of the op-amp ammeter:)

Put this humble circuit diagram into the latest recipe book on electronics and serve it to the poor students :).

circuit-fantasia > circuit stories > building circuits > current source (short version)