Opamps

Operational amplifiers, or opamps for short, are common electronic device modules. They can easily be used to create a variety of useful electronic circuits, and save designers time by reducing the need to design transistor amplifiers.

The 741 is a standard opamp module, available in an integrated circuit package. The detailed circuit diagram (no, you don't need to understand it in detail, at least not in this course) is shown in Figure 6.

**Figure 6:** LM741 circuit diagram.
$\begin{figure} \begin{center} \epsfig{file=images/LM741.eps}\end{center}\end{figure}$

The pinout, defining the external connections, is shown in Table 1.

Table 1: 741 pinouts.

Pin No.	Function
1	offset null
2	inverting input v_-
3	non-inverting input v₊
4	negative power supply V_CC-
5	offset null
6	output v_out
7	positive power supply V_CC+

We won't be using the OFFSET features.

A block diagram of the opamp, showing inputs, outputs, and power supply connections, is shown in Figure 7.

**Figure 7:** Opamp block diagram.
$\begin{figure} \begin{center} \epsfig{file=images/opaimg3.eps}\end{center}\end{figure}$

The opamp amplifies the difference input v_in=v₊-v_-, so that

v_out = A( v₊ - v_- )

(5)

The gain A is very large, typically 10⁴ - 10⁶.

Power is supplied to the opamp via V_cc+ and V_cc-. For example, V_cc+=+15V, V_cc-=-15V, where all voltages are measured relative to a common ground reference point. When v₊=v_-, we will have v_out=0V. If we use V_cc+=10V, V_cc-=0V, then v₊=v_- will give v_out=5V, the voltage midway between. The power supply connections are often omitted in opamp circuit diagrams, and their presence is implicitly understood.

To summarise, an opamp simply amplifies the difference between its inputs.

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