# which opamp is this ?

Discussion in 'Homework Help' started by Xufyan, Aug 21, 2011.

1. ### Xufyan Thread Starter Member

Aug 3, 2010
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0
please look at the attachment , i am totally confused about this opamp, the negative of opamp is feedback and the input is applied to positive

is it non-inverting ???
is it a voltage follower ?? i guess this isn't a voltage follower.

i want to find out the output of that opamp, for this purpose i applied Voltage divider rule to the non-inverting side which gives V=10.2volts.

am i right ?

how to solve the circuit.

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2. ### hgmjr Moderator

Jan 28, 2005
9,030
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What you have there is a voltage-follower configured opamp. The gain of the opamp stage should be +1.

hgmjr

3. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
how this is a voltage follower :/ what about the + input ?? :/

4. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Here is an experiment with such an opamp configuration.

hgmjr

5. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
multisim also shows output voltage equals to 10.2 volts :/

the gain is completely dependent on non-inverting input as the inverting side is voltage follower who gain is 1
is it so ??

Last edited: Aug 21, 2011
6. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Millman's Theorem allows me to calculate that the output should be +10.2V.

$V_{in}\ =\ \frac{\frac{15}{4700}}{\frac{1}{4700}\ +\ \frac{1}{10000}}\ =\ 10.2\ volts$

hgmjr

7. ### praondevou AAC Fanatic!

Jul 9, 2011
2,939
489
Always remember one basic (simplified) rule: The opamp will try to adjust the voltage difference between it's inputs to ZERO.

So if you have 10.2 at the + input and the the opamps "wants" to have the same voltage at it's - input the only way to do this is by increasing it's output voltage to 10.2V too. (in this circuit of course)

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8. ### Xufyan Thread Starter Member

Aug 3, 2010
114
0
thats the point !!! thanks alot !!!! !!!

9. ### hgmjr Moderator

Jan 28, 2005
9,030
214
Keep in mind that the output can only take on the value of 10.2V only if the power supply for the opamp is greater than 10.2 volts. The simulation software may not show it but the output of the 741 opamp cannot excursion very close to the power rails. In other words, a real 741 would probably need a positive power supply voltage of 15V to faithfully respond to 10.2 volts on its input.

hgmjr