# Current measurement circuit

#### Akshatha Venkatesh

Jan 14, 2017
145
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.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What are the characteristics of a voltage follower? Why might some of those characteristics be useful?

#### Akshatha Venkatesh

Jan 14, 2017
145
What are the characteristics of a voltage follower? Why might some of those characteristics be useful?
the input has high impedance and the output has low impedance.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Ok, that's the first part.

Why could that be useful?

#### Akshatha Venkatesh

Jan 14, 2017
145
Ok, that's the first part.

Why could that be useful?
To isolate the input circuit from changes in the output circuit , but what could change in the output circuit ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What would happen if the output circuit had a low input impedance? Consider the case with and without the voltage follower.

#### Akshatha Venkatesh

Jan 14, 2017
145
What would happen if the output circuit had a low input impedance? Consider the case with and without the voltage follower.
Sorry I don't understand , which is the output circuit ? Please can you explain

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
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?

#### Akshatha Venkatesh

Jan 14, 2017
145
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?
Yeah ok , a high impedance circuit would then be connected to a low impedance , but what happens when this occurs ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
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?

#### Akshatha Venkatesh

Jan 14, 2017
145
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?
I'm trying , but not able to recall . Would you help me by explaining please ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
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?

#### Akshatha Venkatesh

Jan 14, 2017
145
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?
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.

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
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.

#### Akshatha Venkatesh

Jan 14, 2017
145
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.
The voltage would increase ?

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
Do you honestly think that if you tried to draw several hundred amps from a tiny battery that the voltage would increase?

#### Akshatha Venkatesh

Jan 14, 2017
145
Do you honestly think that if you tried to draw several hundred amps from a tiny battery that the voltage would increase?

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

#### (*steve*)

##### ¡sǝpodᴉʇuɐ ǝɥʇ ɹɐǝɥd
Moderator
Jan 21, 2010
25,505
What is your textbook? Perhaps someone here is familiar with it and can refer you to a section.

#### Akshatha Venkatesh

Jan 14, 2017
145
What is your textbook? Perhaps someone here is familiar with it and can refer you to a section.
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

Jan 14, 2017
145
What is your textbook? Perhaps someone here is familiar with it and can refer you to a section.
Can you please suggest me what to study to understand this

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