Non Inverting opamp
Electronics tutorial OP-AMP tutorial Operational amplifier basics

Non Inverting Amplifier(OPAMPs)

A non-inverting amplifier is an OPAMP circuit configuration whose output is in phase with the input signal at the non-inverting input. The input signal is applied at the non-inverting input of the opamp. A non-inverting amplifier also acts as a voltage follower circuit.

The non-inverting amplifiers also have negative feedback which is used to control the gain of the amplifier. Feedback contains a voltage divider circuit that provides a part of the output to the input terminal. This OPAMP has a high input impedance and a low output impedance. This makes it an ideal buffer.

Non-Inverting Amplifier circuit:

 

The non-inverting amplifier is as shown

non inverting amplifier

  • The signal which is to be amplified is applied to the non-inverting +ve input terminal and the inverting input terminal is connected to ground wire resistor R1.
  • The input and output voltages are in phase with each other, their phase difference is 0 or 360 degrees.
  • The feedback resistor Rf introduces negative feedback at the negative inverting input terminal of the opamp. It makes a voltage divider with a ground resistor.

output of non-inverting amplifier

Expression for closed-loop voltage gain:

To derive expression we are using an ideal opamp. Input resistor Ri = ♾️, current internet input terminals will have 0 value, ( I1 = I2 = 0 ).

The voltage across R1 is given by

V2 = R1÷[(Rf + R1)] × Vo

As per the concept of virtual short discussed earlier,

V1 = Vs = V2

Substituting the value of b2 in the above expression we get,

Vs = R1÷[( Rf + R1 )]×Vo

Close loop voltage gain,

Avf = Vo/Vs = ( R1 + Rf )÷R1
Avf = 1 + (Rf÷R1)
Vo = Avf × Vs

Conclusion from the above expression for Avf:

  1. The positive sign of the equation indicates that the input and output are in phase with each other.
  2. The closed-loop voltage gain is always greater than unity 1.
  3. The value of Avf is adjustable it can be adjusted by varying the value of Rf and R1. Generally, a variable resistor is present in place of Arif to adjust the closed-loop gain to its desired value.
  4. Avf is independent of the open-loop gain of the opamp. It depends only on the value of RF and R1.

Difference between Inverting amplifier & Non-inverting amplifier:

Sr, No. parameters Inverting Amplifier Non-inverting Amplifier
1 Voltage gain Avf = – Rf/R1 Avf = 1 + Rf/R1
2 Phase difference between input and output Voltages 180° out of phase In phase
3 Value of Voltage gain Can be greater than, less than or equal to unity Always greater than or equal to unity
4 Input resistance Equal to R1 Very large

 

Concept of virtual short:

In a non-inverting amplifier, there is a virtual shot between two input terminals

It means that there is a short circuit for voltage but an open circuit for current. It is due to the two reasons,

  • The value of Rin is infinite there for current at both the input terminals is zero.
  • The difference between input voltages ( V1 – V2 ) is zero hence Aol is infinite

Virtual short is an ideal concept. If we operate the opamp in the linear region, virtual short can give accurate values with -Ve feedback.

As the voltage gain reaches infinity there is a virtual short between the input terminal

Because of virtual short inverting voltage follows non-inverting input voltage. If the input signal at non-inverting terminal changes then the voltage at inverting terminal also changes by the same amount. This action is called “Bootstrapping“.

 

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