Operational Amplifiers (Op-Amps)

Op-amps and transistors are the staples of analog circuit design. Internally, op-amps consist of many transistors, capacitors, and resistors; all crammed onto a small integrated circuit.

The symbol for an op-amp:

Each op-amp has an inverting input, a non-inverting input, and an output. In practice, op-amps also must be powered by a positive and negative voltage, but since the power and ground won't affect our schematic design, the leads are usually omitted in the drawings.

Ideal Op-Amps

Op-amps are generally very well described the their ideal model.There are several basic rules for ideal op-amps:

1. There is no voltage difference across the inputs. Although the open-loop gain of an ideal op-amp is considered to be infinitely large, the op-amp will try to keep the inputs at the same voltage if it can somehow send the output signal back to the input to push or pull the input voltages to the same level.

2. The input current draw is zero—no current can flow in or out the input terminals.

3. The output impedance is zero. This means that the output current does not affect the output voltage.

Real Op-Amps

There are a few differences between real and ideal op-amps:

Saturation: The output voltage is bounded by the positive and negative power supplies, known as rails. In fact, many op-amps will only go up to a few volts short of rails. If we look at the Output Voltage Swing in the data sheet for the LM411, we see that under a ±15V power source, the output will will generally be able to swing between ±13.5V—but you just might get a chip that can only output ±12V. Op-amps that are designed to be able to output voltages very close thier rails are refered to as being "rail to rail".

Slew Rate:The output voltage cannot change instantaneously; the maxiumum rate of change possible for the output voltage is known as the "slew rate".

$SR={\frac {\Delta V}{\Delta t}}$

In real op-amps, there will be a small current draw because the op-amp does not have infinite resistance (maybe $10^{1}2\Omega$ for FETs and $10^{8}\Omega$ for BJTs).

The 741 and 411 Type Op-Amps

The 741 (bipolar) and 411 (FET) families of op-amps are popular chips made by many manufacturers. The model number is often preceded by a modifier/identifier, such as "LM741" or "LF411". These op-amps also come in varieties where you get multiple op-amps on a single package.

[Click here for the data sheet of the LM471]

[Click here for the data sheet of the LM148 quad 471]

[Click here for the data sheet of the LF411]

For now, we can ignore the "offset" pins. These are used to make very fine adjustments in the reference voltages.

For more information on how to read the op-amp spec sheets, try going to National Semiconductor's Knowledge Base