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  • Suppose we have a voltage that we refer to as the input.

  • Suppose we want to multiply this input by a number

  • to produce another voltage that we will refer to as the output.

  • Or, suppose that we have several input voltages,

  • and we wish to add them together,

  • and for the output voltage to represent their sum.

  • Operations such as these, as well as many far more advanced calculations

  • can be performed with circuits that have what we call an

  • operational amplifier, typically referred to as anop amp.”

  • In this video, we will discuss circuits that have anideal op amp.”

  • An op amp has two input terminals and one output terminal.

  • The op amp also has two terminals for providing power to the device.

  • Current can never flow into or out of the input terminals.

  • Current can flow into and out of the output terminal.

  • The current that flows into or out of the output is supplied

  • by the two terminals providing power to the op amp.

  • The two terminals providing power to the op amp are typically not shown.

  • But, it is important to keep in mind that the op amp

  • only has the ability to produce output voltages that are

  • in between the voltages of the two power terminals.

  • One of the two inputs has a plus sign next to it,

  • and the other input has a minus sign next to it.

  • The op amp takes the voltage value of theplusinput,

  • and subtracts from it the voltage value of theminusinput.

  • The op amp then takes this difference between the two input voltages,

  • and multiplies it by a very large number.

  • The op amp then tries to make the output voltage

  • equal to the value of this result.

  • This means that if theplusinput is even slightly lower than theminusinput,

  • the op amp will try to make the output voltage equal to

  • the largest negative number it is capable of producing.

  • And if theplusinput is slightly higher than theminusinput,

  • the op amp will try to make the output voltage equal to

  • the largest positive number it is capable of producing.

  • Now, suppose that we do something that we call providing negative feedback.

  • These two resistors cause a portion of the output voltage to be added

  • to the value of the op amp's “minusinput terminal.

  • Suppose that the voltage of the op amp's “plusinput terminal

  • is lower than the value of the op amp's “minusinput terminal.

  • This will cause the op amp to want to decrease the voltage value

  • of the output terminal, which will then also end up decreasing

  • the voltage of the op amp's “minusinput terminal.

  • The output voltage will stop decreasing when the op amp's “minusinput terminal

  • is almost exactly equal to the value of the op amp's “plusinput terminal.

  • Now, let us consider a new scenario.

  • Suppose that the voltage of the op amp's “plusinput terminal

  • is higher than the value of the op amp's “minusinput terminal.

  • This will cause the op amp to increase the voltage value of

  • the output terminal, which will then also end up increasing

  • the voltage of the op amp's “minusinput terminal.

  • The output voltage will stop increasing when the op amp's “minusinput terminal is

  • almost exactly equal to the value of the op amp's “plusinput terminal.

  • Therefore, the presence of negative feedback always forces

  • the op amp's two input terminals to always be at almost the same voltage value.

  • Once the voltages of the op amp's two input terminals are almost at the same value,

  • the difference between them is a very small number.

  • The output voltage is then this very small number

  • multiplied by a very large number.

  • For this example, let us suppose that the value of the

  • op amp's “plusinput terminal is always set to zero volts.

  • The negative feedback will force the op amp's “minusinput terminal

  • to also always be set to close to zero volts.

  • Let us define this point here as the input of the circuit.

  • If we apply a voltage to this input, a current will flow.

  • Since current is unable to flow into or out of the op amp's input terminals,

  • the value of current that flows through the first resistor

  • must also be the exact same value of current that flow through the second resistor.

  • The voltage drop across each resistor is the value of

  • this current multiplied by the resistor's resistance.

  • Now, let us suppose that one of the resistors has a larger value,

  • which we will represent as two resistors in series.

  • Now, the magnitude of output voltage is changed as shown.

  • If we want the output voltage to be positive when the input voltage is positive,

  • then we can change the location of the input voltage as shown.

  • Our circuit can have several different inputs.

  • All resistance values are the same.

  • Since current can't flow into the op amp's input terminals,

  • this means that the current through the resistor on the right

  • is equal to the sum of the current flowing through the other three resistors.

  • The voltage drop of the resistor on the right is the sum of all these currents

  • multiplied by the resistor's resistance.

  • The output voltage therefore represents the sum of all the input voltages.

  • Suppose we use a capacitor in the circuit as shown.

  • The output voltage represents

  • the integral of the input voltage waveform with respect to time.

  • Suppose we swap the positions of the resistor and the capacitor.

  • The output voltage represents

  • derivative of the input voltage waveform with respect to time.

  • Another important use of op amps is to act as a buffer.

  • In this case, the output voltage is exactly equal to the input voltage.

  • Although no current is ever drawn from the input,

  • the output terminal supplies a current coming from the op amp's power terminals.

  • Much more information about electric circuits

  • is available in the other videos on this channel.

  • Please subscribe for notifications when new videos are ready.

Suppose we have a voltage that we refer to as the input.

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B1 US input amp voltage output terminal resistor

Op Amp Circuits - Analog Computers from operational amplifiers

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    Amy.Lin posted on 2021/01/06
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