Noninverting Amplifier

A circuit diagram of an opamp in a noninverting amplifier configuration is shown in Figure 13.

**Figure 13:** Noninverting amplifier.
$\begin{figure} \begin{center} \epsfig{file=images/opampimg1.eps}\end{center}\end{figure}$

The input-output formula for the noninverting amplifier is

$\begin{displaymath}v_{out} = (1+\frac{R_2}{R_1}) v_{in} , \end{displaymath}$

(12)

as we saw earlier.

Let's now derive this using the virtual model.

The key step is to apply KCL at the input nodes (we never apply KCL at the opamp output). The + node is connected to v_in, so v_in=v₊. The virtual model says i₊=0.

The - node is connected to two resistors, so we apply KCL:

$\begin{displaymath}\frac{v_- - 0}{R_1} + \frac{v_- - v_{out}}{R_2} +i_- = 0 . \end{displaymath}$

The virtual model says i_- = 0, and also v_- = v₊ = v_in, so we get

$\begin{displaymath}\frac{v_{in}}{R_1} + \frac{v_{in} - v_{out}}{R_2} = 0 . \end{displaymath}$

Solving for v_out we get (12).

PSPICE file: noninv1.sch.

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