Non-Inverting Amplifier

OBJECTIVE:

To measure the gain and verify the non-inverted output waveform from non-inverting amplifier circuit design.

THEORY :

The operational amplifier can also be used to construct a non-inverting amplifier with the circuit indicated below. The input signal is applied to the positive or non-inverting input terminal of the operational amplifier, and a portion of the output signal is fed back to the negative input terminal. Analysis of the circuit is performed by relating the voltage at V₂ to both the input voltage V_in and the output voltage V_o.

Figure 1: Non-inverting amplifier configuration of op-amp

Non-Inverting Amplifier Voltage Gain

The output is applied back to the inverting (-) input through the feedback circuit (closed loop) formed by the input resistor R₁ and the feedback resistor R₂ . This creates �ve feedback as follows. Resistors R₁ and R₂ form a voltage-divider circuit, which reduces V_o and connects the reduced voltage V₂ to the inverting input. The feedback is expressed as
The difference of the input voltage, V_in and the feedback voltage, V₂ is the differential input of the op-amp. This differential voltage is amplified by the gain of the op-amp and produces an output voltage expressed as
The closed-loop gain of the non-inverting amplifier is, thus

Notice that the closed loop gain is

independent of open-loop gain of op-amp.

set by selecting values of R₁and R₂.

Input and Output Resistance of Inverting Amplifier

An expression for the input impedance of a non-inverting amplifier can be written as

Where A_ol= open-loop voltage gain of op-amp,
Z_in= internal input impedance of op-amp (without feedback)
beta = attenuation of the feedback circuit


Equation above shows that the input impedance of the non-inverting amplifier configuration with feedback is much greater than the internal output impedance of the op-amp itself. The output impedance of a NI amplifier can be written as


This equation shows that the output impedance of non-inverting amplifier is much less than the internal output impedance, Z_oof the op-amp.