Question

What is an operational amplifier ? Explain its working as a non-inverting amplifier.

Answer 1

OP-AMP: OP-AMP is a solid state device capable of sensing and amplifying dc and ac input signals. OP-AMP is an amplifier with two inputs (different inputs) and a single output. OP-AMP consists of $20$ transistors, $11$ resistors and one capacitor. It usually requires a positive and negative power supply (dual power supply). This allows the output voltage to swing positive and negative with respect to ground.

Non-inverting amplifier:

The basic OP-AMP non-inverting amplifier is shown in Figure. The input signal $V_{in}$ is applied to the non-inverting input terminal. The resistor $R_{in}$ is connected from the inverting input to ground. The feedback resistor $R_f$ is connected between the output and the inverting input.

Resistors $R_f$ and $R_{in}$ form a resistive ratio network to produce the feedback voltage $\left(V_A\right)$ needed at the inverting input. Feedback voltage $\left(V_A\right)$ is developed across $R_{in}$. Since the potential at the inverting input tends to be the same as the non-inverting input (as pointed out with the description of virtual ground), $V_{in}=V_A$.

Since $V_A=V_{in}$, the gain of the amplifier can be expressed as

$A_v=\frac{V_{out}}{V_A}$

Howeverm, $V_A$ is determined by the resistance ratio of $R_{in}$ and $R_f$, thus,

$V_A=\frac{R_{in}}{R_f+R_{in}}{V}_{out}$

(or) $\frac{V_{out}}{V_A}=\frac{R_f+R_{in}}{R_{in}}$

$\frac{V_{out}}{V_A}=1+\frac{R_f}{R_{in}}$

$\therefore A_v=1+\frac{R_f}{R_{in}}$

Finally, the output voltage can be found by,

$V_{out}=(1+\frac{R_f}{R_{in}})V_{in}$

It is seen that the input and output voltages are in phase.