Author Topic: Quick question... can I make the following assumption?  (Read 1565 times)

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ilbs

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Quick question... can I make the following assumption?
« on: October 17, 2013, 01:49:14 am »
Hello,

I could totally wrong about this, so guys go easy on me if I am completely out in left field.

But, lately by experimenting with op amps I have come across some observations and I would like to know if what I observed is right. The configuration is where the inverting input
is connected with the typical rf and ra resistors and the non inverting
input is connected with the typical voltage divider formed by r1 and r2 resistors.

So, when dealing with traditional op amps in this type of configuration, can
one make the following statements:

Quote
When the voltage of the non-Inverting pin rises, the output will rise as well no matter what the inverting voltage is.
When the voltage of the non-Inverting pin lowers, the output will lower as well no matter what the inverting voltage is.

When the voltage of the Inverting pin rises, the output will lower no matter what the Non-Inverting voltage is.
When the voltage of the Inverting pin lowers, the output will rise no matter what the inverting voltage is.

HOWEVER:
When the voltage of the non-Inverting pin rises, the output can't lower no matter what the inverting voltage is.
When the voltage of the non-Inverting pin lowers, the output can't rise no matter what the inverting voltage is.

When the voltage of the Inverting pin rises, the output can't rise no matter what the Non-Inverting voltage is.
When the voltage of the Inverting pin lowers, the output can't lower no matter what the inverting voltage is.

Please note these statements are in reference to op amp configurations where we have an input voltage
ramp at one of the two op amp inputs while the other input is at a steady voltage.

« Last Edit: October 17, 2013, 03:41:29 am by ilbs »

w2aew

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Re: Quick question... can I make the following assumption?
« Reply #1 on: October 17, 2013, 02:46:16 am »
In general, yes, those statements are true - given the assumption that the op amp is sitting in an active state where the output isn't jammed to its min or max value, the inputs are well within their common mode range, and the negative feedback loop is closed properly such that the voltage difference between the inverting and non-inverting inputs is effectively zero.

Have you watched my video on a no-nonsense way of understanding the operation of most op amp circuits?
FAE for Tektronix
Technical Coordinator for the ARRL Northern NJ Section

ilbs

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Re: Quick question... can I make the following assumption?
« Reply #2 on: October 17, 2013, 03:01:14 am »
Hi w2aew,

yes I did .... thanks for a very insightful and useful
video.