# An operational amplifier

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### Introduction:

Anoperational amplifier, which is often called anop-amp, is aDC-coupledhigh-gainelectronic voltageamplifierwith a differential input and, usually, a single-ended output.An op-amp produces an output voltage that is typically millions of times larger than the voltagedifferencebetween its input terminals. Typically uses of the operational amplifier are to provide voltage amplitude changes (amplitude and polarity), oscillators, filter circuits, and many types of instrumentation circuits. An op-amp contains a number of differential amplifier stages to achieve a very high voltage gain.

Typically the op-amp's very large gain is controlled bynegative feedback, which largely determines the magnitude of its output voltage gain in amplifier applications, or thetransfer functionrequired. Without negative feedback, and possibly withpositive feedbackforregeneration, an op-amp essentially acts as acomparator. High inputimpedanceat the input terminals and low output impedance at the output terminals (ideally zero) are important typical characteristics.

### Full-power response:

This, fmax is called the full-power response. It is the maximum frequency of a large-amplitude sinusoidal wave that the op-amp can amplify without distortion. The worthnoting point is that it is entirely separate from GBW product (which limits the output frequency because of a drop in gain). Above the full-power response frequency, the op-amp cannot charge the compensation capacitor fast enough to cause the output signal to swing to Vp out.

### Comparator

Compares two voltages and switches its output to indicate which voltage is larger.

(whereVsis the supply voltage and the opamp is powered by+Vsand-Vs.)

### Inverting amplifier

An inverting amplifier uses negative feedback to invert andamplifya voltage. The Rfresistor allows some of the output signal to be returned to the input. Since the output is 180&deg; out of phase, this amount is effectively subtracted from the input, thereby reducing the input into the operational amplifier. This reduces the overall gain of the amplifier and is dubbed negative feedback.

• The input impedance isat leastthe impedance between non-inverting (+) and inverting (-) inputs, which is typically 1 MO to 10 TO, plus the impedance of the path from the inverting (-) input to ground (i.e.,R1in parallel withR2).
• Because negative feedback ensures that the non-inverting and inverting inputs match, the input impedance is actuallymuch higher.
• Although this circuit has a large input impedance, it suffers from error of input bias current.
• The non-inverting (+) and inverting (-) inputs draw small leakage currents into the operational amplifier.
• These input currents generate voltages that act like unmodelled input offsets. These unmodelled effects can lead to noise on the output (e.g., offsets or drift).
• Assuming that the two leaking currents arematched,their effect can be mitigated by ensuring the DC impedance lookingoutof each input is the same.
• The voltage produced by each bias current is equal to the product of the bias current with the equivalent DC impedance looking out of each input. Making those impedances equal makes the offset voltage at each input equal, and so the non-zero bias currents will have no impact on thedifferencebetween the two inputs.
• A resistor of value
• which is the equivalent resistance ofR1in parallel withR2, between theVinsource and the non-inverting (+) input will ensure the impedances lookingoutof each input will be matched.
• The matched bias currents will then generate matched offset voltages, and their effect will be hidden to the operational amplifier (which acts on the difference between its inputs) so long as theCMRRis good.
• Very often, the input currents arenotmatched.
• Most operational amplifiers provide some method of balancing the two input currents (e.g., by way of an externalpotentiometer).
• Alternatively, an external offset can be added to the operational amplifier input to nullify the effect.
• Another solution is to insert a variable resistor between theVinsource and the non-inverting (+) input. The resistance can be tuned until the offset voltages at each input are matched.
• Operational amplifiers withMOSFET-based input stages have input currents that are so small that they often can be neglected.

### References:

1. http://en.wikipedia.org/wiki/Operational_amplifier
2. http://williamson-labs.com/480_opam.htm
3. http://en.wikipedia.org/wiki/Gain-bandwidth_product
4. http://www.ecircuitcenter.com/Circuits/op_bandwidth1/op_bandwidth1.htm
5. http://en.wikipedia.org/wiki/Rise_time
6. http://www.amplifier.cd/Tutorial/Slew_Rate/SlewRate.htm
7. http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/a741p3.html
8. http://books.google.co.in/books?id=2k8jCwgx7gIC&pg=PA44&lpg=PA44&dq=full+power+response&source=bl&ots=uMWzzcdEuq&sig=WtVFVGTS2crRP
9. http://en.wikipedia.org/wiki/Operational_amplifier_applications
10. electronic devices and circuits by J.B. Gupta, chapter 27, page nos. 604, 614, 616, 617

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