Discussion in 'General Electronics Chat' started by Aleph(0), Mar 29, 2015.

1. ### Aleph(0) Thread Starter Member

Mar 14, 2015
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Op Amp output is difference of inputs multiplied by gain yes? For unity gain, output is 0v when inputs equal if offset ignored yes? But in unity gain follower inverting input is connected to output and output voltage is always non inverting input voltage yes? Now I am confused because now both inputs equal but output not 0v I understand each step of way but can't get around contradiction please help get my mind right thankyou!

2. ### Papabravo Expert

Feb 24, 2006
11,065
2,152
Your first sentence is true for an opamp in the open loop configuration.

A unity gain follower is connected in a closed loop configuration. When the difference between the inputs is 0, the output is equal to the value of the non-inverting input making the gain, in the closed loop configuration, unity. The only way to make a zero output with a unity gain follower is to have a zero input on the non-inverting pin.

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3. ### Aleph(0) Thread Starter Member

Mar 14, 2015
423
591
Thanks for reply! how about then for closed loop with nonunity gain? I've seen linear regulator for dc to dc converter with reference voltage on noninverting input and voltage divided converter output on inverting input and op amp output to bias of control transistor but I have same confusion because feedback loop system will seek state where divided converter output equal reference input but then seems like op amp output will be zero and shut converter off? I leave out compensation network because not important to question. Plz help me to understand Sorry to sound dumb

Last edited: Mar 29, 2015
4. ### Papabravo Expert

Feb 24, 2006
11,065
2,152
In the case of a gain that is not unity, the output of the opamp goes to the voltage required to make the difference between the inputs equal to zero. In your the divided output and the reference voltage should be equal and the output goes to whatever voltage is required to make that happen. The output cannot go beyond the limits of the supply voltage, If that is required, the opamp saturates and things become ..well non-linear.

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5. ### Aleph(0) Thread Starter Member

Mar 14, 2015
423
591
Thanks! Now I think I unserstand that direct algebraic dependance of output on inputs not apply to closed loop? I want to study more could you plz show me where are some online resource to study on this aspect of op amp theory?

6. ### Papabravo Expert

Feb 24, 2006
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Mar 14, 2015
423
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8. ### Papabravo Expert

Feb 24, 2006
11,065
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You're welcome. Good luck and have fun.