Feedback in Amplifiers

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Basics of Op Amps

Operational amplifiers are a very common component in electrical systems. The term Op-Amp specifically refers to an amplifier with two inputs, a positive input and an inverted input, and one amplified output, produced from the difference between the inputs Vout=G*(V+-V-), where G is the gain from the amp.


This negative feedback Op Amp setup will amplify and invert Vin on its way to Vout. When Vin increases, the output from the Op Amp becomes negative. With the appropriate value for R1 (a very large resistance since gain will be high), VToAmp will be pulled down until equilibrium is reached and VToAmp=0.

Since the gain in an op amp is extremely high, usually hundreds of thousands in magnitude<ref>"Op Amps." Williams Labs, 2007. Web. 10 Jan 2010.</ref>, very small differences in input voltages can cause the output to reach the amp's maximum gain. This makes op amps useful as voltage comparators when no feedback is present. But for most practical uses, feedback from the output into the inverted input is needed in order to control the input voltage difference and keep the output from reaching its maximum. When the positive input gets higher, the output goes up, causing the inverted input to rise, thus re-lowering the output. Output and input change for a while as they stabilize at their new values, at which point the positive and inverted inputs are very nearly equal. Since the gain is extremely large, this minute difference in input voltages, which we approximate to be 0, amplifies to give a voltage at Vout.




--Michael Vier (Author)


Greg Fong

  • V_{out}=G(V_+-V_-)\,
  • < ref>[URL:URL caption] Notes </ref>
    • URL Caption: Op Amps
    • Notes: Williams Labs, 2007. Wed. 10 Jan 2010.
  • "The output is 0" - Feedback paragraph, last line. Are you sure?
  • Henry,Ben