# Current measurement circuit

Discussion in 'Electronics Homework Help' started by Akshatha Venkatesh, Nov 6, 2017.

1. ### Akshatha Venkatesh

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The below circuit is a current measurement circuit , the mosfet is always on in my application and the volatge across the shunt 100k is measured and given to ADC pin of uC.can anyone explain the function of the Opamp as voltage follower in the circuit ? What would happen if the opamp wasn't present ? The current at IDE- is 45V/45Mohm= 10 micro amp.

Akshatha Venkatesh, Nov 6, 2017
2. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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What are the characteristics of a voltage follower? Why might some of those characteristics be useful?

(*steve*), Nov 7, 2017
3. ### Akshatha Venkatesh

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the input has high impedance and the output has low impedance.

Akshatha Venkatesh, Nov 7, 2017
4. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Ok, that's the first part.

Why could that be useful?

(*steve*), Nov 7, 2017
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5. ### Akshatha Venkatesh

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To isolate the input circuit from changes in the output circuit , but what could change in the output circuit ?

Akshatha Venkatesh, Nov 8, 2017
6. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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What would happen if the output circuit had a low input impedance? Consider the case with and without the voltage follower.

(*steve*), Nov 8, 2017
7. ### Akshatha Venkatesh

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Sorry I don't understand , which is the output circuit ? Please can you explain

Akshatha Venkatesh, Nov 8, 2017
8. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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I thought I was using the terms you were using

The output of your voltage follower is obviously there to be used by something. What happens if that something has a low impedance.

Would the effect be different if the voltage follower was not there, and that low impedance was connected to the signal that is currently buggered by the voltage follower?

(*steve*), Nov 8, 2017
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9. ### Akshatha Venkatesh

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Yeah ok , a high impedance circuit would then be connected to a low impedance , but what happens when this occurs ?

Akshatha Venkatesh, Nov 8, 2017
10. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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You need to research that. It is something fundamental that you've missed somewhere.

If you've been given this homework then you're expected to know this. Can you go back and check your notes?

(*steve*), Nov 8, 2017
11. ### Akshatha Venkatesh

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I'm trying , but not able to recall . Would you help me by explaining please ?

Akshatha Venkatesh, Nov 8, 2017
12. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Imagine you have a 12Vcar battery and a 12V battery from a remote control (the ones about 2/3 the size of a AAA cell)

You have to start a car, which one will work? Why won't the other one work?

Can you apply this to the voltage signal from your circuit, with and without the voltage follower?

(*steve*), Nov 9, 2017
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13. ### Akshatha Venkatesh

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The current is amplified , but since this is a current measurement circuit , why do I need to amplify the current , the voltage is anyway not changing since it is a voltage follower.

Akshatha Venkatesh, Nov 9, 2017
14. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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What would happen to the voltage across a tiny battery if you tried to use it to start your car?

The tiny battery has a high impedance.

(*steve*), Nov 9, 2017
15. ### Akshatha Venkatesh

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The voltage would increase ?

Akshatha Venkatesh, Nov 9, 2017
16. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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Do you honestly think that if you tried to draw several hundred amps from a tiny battery that the voltage would increase?

(*steve*), Nov 9, 2017
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17. ### Akshatha Venkatesh

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Could you please tell me what I need to read to understand this concept?

Akshatha Venkatesh, Nov 9, 2017
18. ### (*steve*)¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥdModerator

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What is your textbook? Perhaps someone here is familiar with it and can refer you to a section.

(*steve*), Nov 10, 2017
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19. ### Akshatha Venkatesh

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Millmans electronic devices and circuits , any other textbook which you think talks about this topic is also fine. I just need to understand the function of the Opamp in the circuit and what would happen if it is not used, at the earliest, please help .

Akshatha Venkatesh, Nov 10, 2017

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