Operational amplifier

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Operational amplifier

  1. 1. An operational amplifier (Op-Amp) is a differential amplifierthat amplifies the difference of voltages applied to its twoinput terminals (differential input), and provides a single-ended output. The basic circuit of op-amp IC 741
  2. 2. IC Op-amp comes so close to ideal performance that it is useful to state the characteristics of anideal amplifier without regard to what is inside the package. 1.Infinite voltage gain 2.Infinite input impedance 3.Zero output impedance 4.Infinite bandwidth 5.Zero input offset voltage (i.e., exactly zero out if zero in). These characteristics lead to the golden rules for op-amps. They allow you to logically deduce the operation of any op-amp circuit.
  3. 3. • Inverting amplifier• Non-inverting amplifier• Summing amplifier• Substractor• Differentiation• Integrator• Comparator
  4. 4. Opposite is a diagram of an INVERTINGAMPLIFIER. This means that if the voltagegoing into the 741 chip is positive, it is negativewhen it comes out of the 741. In other words itreverses polarity (inverts polarity).Two resistors are needed to make the 741work as an amplifier, R1 and R2. In most textbooks diagrams like this are used to representthe 741. GAIN (AV) = -R2 / R1 Example : if R2 is 100 kilo-ohm and R1 is 10 kilo-ohm the gain would be : -100 / 10 = -10 (Gain AV) If the input voltage is 0.5v the output voltage would be : 0.5v X -10 = -5v
  5. 5. In this circuit the signal is applied to the non- inverting input of the op-amp. However the feedback is taken from the output of the op- amp via a resistor to the inverting input of the operational amplifier where another resistor is taken to ground. It is the value of these two resistors that govern the gain of the operational amplifier circuit.Av = 1 + R2 / R1
  6. 6. The summing amplifier is a handy circuit enabling you to add several signals together. What are some examples? If youre measuring temperature, you can add a negative offset to make the display read "0" at the freezing point. On a precision amplifier, you may need to add a small voltage to cancel the offset error of the op amp itself. An audio mixer is another good example of adding waveforms (sounds) from different channels (vocals, instruments) together before sending the combined signal to a recorder.Vo = - RF ( V1 / R1 + V2 / R2 + V3 / R3) = - ( V1 · RF / R1 + V2 · RF / R2 + V3 · RF / R3 )
  7. 7. The Subtractor also called a difference amplifier, uses both the inverting and non-invertinginputs to produce an output signal which is proportional to the difference between thetwo input voltages V1 and V2. More inputs can be subtracted. Resistances are equal (R = Rand RA = RA) then the output voltage is as given and the gain is +1. If the input resistanceare unequal the circuit becomes a differential amplifier.
  8. 8. The right-hand side of the capacitor is held to a voltage of 0 volts, due tothe "virtual ground" effect. Therefore, current "through" the capacitor issolely due to change in the input voltage. A steady input voltage wontcause a current through C, but a changing input voltage will.
  9. 9. Continue…Capacitor current moves through the feedback resistor, producing a dropacross it, which is the same as the output voltage. A linear, positive rate ofinput voltage change will result in a steady negative voltage at the output ofthe op-amp. Conversely, a linear, negative rate of input voltage change willresult in a steady positive voltage at the output of the op-amp. This polarityinversion from input to output is due to the fact that the input signal is beingsent (essentially) to the inverting input of the op-amp, so it acts like theinverting amplifier mentioned previously. The faster the rate of voltage changeat the input (either positive or negative), the greater the voltage at the output.The formula for determining voltage output for the differentiator is as follows:
  10. 10. As before, the negative feedback of the op-amp ensures that the invertinginput will be held at 0 volts (the virtual ground). If the input voltage is exactly0 volts, there will be no current through the resistor, therefore no charging ofthe capacitor, and therefore the output voltage will not change. We cannotguarantee what voltage will be at the output with respect to ground in thiscondition, but we can say that the output voltage will be constant.
  11. 11. The Comparator has many uses but the most common is to compare the input voltage toa reference voltage and switch the output if the input voltage is above the referencevoltage. If the input goes more positive than the reference voltage set by the voltagedivider, Vin > Vref, the output changes state. When the input voltage drops below thepreset reference voltage and Vin < Vref, the output switches back. By using negativefeedback the comparator can be converted into a Schmitt Trigger circuit. Here are just some of the more common operational amplifier building block configurations used in electronic circuits. All the above circuits can be constructed using a variety of different op-amps including the famous 741 op-amp.

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