**In a linear IC, the relative output is a linear (straight-line) function of the relative input. The solid lines show examples of linear IC characteristics. The dashed curves show functions not characteristic of properly operating linear ICs.**

A linear IC is used to process analog signals such as voices, music, and radio transmissions.

The term linear arises from the fact that, in general, the amplification factor is constant as the input amplitude varies. That is, the output signal strength is a linear function of the input signal strength (above figure).

#### Operational Amplifier

**Schematic symbol for an op amp. Connections are discussed in the text.**

An operational amplifier, or op amp, is a specialized linear IC that consists of several bipolar transistors, resistors, diodes, and capacitors, interconnected so that high gain is possible over a wide range of frequencies. An op amp might comprise an entire IC. Or, an IC might consist of two or more op amps. Thus, you’ll sometimes hear of dual op amps or quad op amps. Some ICs have op amps in addition to other circuits.

An op amp has two inputs, one noninverting and one inverting, and one output. When a signal goes into the noninverting input, the output is in phase with the input; when a signal goes into the inverting input, the output is 180° out of phase with the input. An op amp has two power supply connections, one for the emitters of the transistors (Vee) and one for the collectors (Vcc). The usual schematic symbol for an op amp is a triangle. The inputs, output, and power-supply connections are drawn as lines emerging from it (above figure).

**A closed-loop op amp circuit with negative feedback. If the feedback resistor is removed, it becomes an open-loop circuit.**

The gain of an op amp is determined by one or more external resistors. Normally, a resistor is connected between the output and the inverting input. This is called the closed-loop configuration. The feedback is negative (out of phase), causing the gain of the op amp to be less than it would be if there were no feedback (the open-loop configuration). Above figure is a schematic diagram of a noninverting closed-loop amplifier.

The reason for providing negative feedback in an op amp circuit is the fact that without it, the gain may be too great. Excessive amplifier gain can cause problems. Open-loop op amps are prone to instability, especially at low frequencies. They also generate a lot of internal noise.

**Gain-versus-frequency response curves. At A, lowpass; at B, highpass; at C, resonant peak; at D, resonant notch.**

When an RC combination is used in the feedback loop of an op amp, the gain depends on the input-signal frequency. It is possible to get a lowpass response, a highpass response, a resonant peak, or a resonant notch using an op amp and various RC feedback arrangements. These four types of responses are shown in above figure.

#### Op Amp Differentiator

**A differentiator circuit that uses an op amp.**

A differentiator is an electronic circuit whose instantaneous output amplitude is proportional to the rate at which the input amplitude changes. The circuit mathematically differentiates the input signal. Op amps can be used as differentiator circuits. An example is shown in above figure.

When the input to a differentiator is a constant dc voltage, the output is zero (no signal). When the input amplitude is increasing, the output is a positive dc voltage. When the input decreases, the output is a negative dc voltage. If the input waveform fluctuates periodically (the usual case), the output varies according to the instantaneous rate of change of the input amplitude. This results in an output signal with the same frequency as that of the input signal, although the waveform is often quite different. A pure sine wave input produces a pure sine wave at the output, but the phase is shifted 90° to the left (that is, 1⁄ 4 cycle earlier in time). Complex input waveforms can produce a wide variety of outputs from a differentiator.

#### Op Amp Integrator

**An integrator circuit that uses an op amp.**

An integrator is an electronic circuit whose instantaneous output amplitude is proportional to the accumulated input signal amplitude as a function of time. The circuit mathematically integrates the input signal. The function of an integrator is basically the inverse, or opposite, of the function of a differentiator circuit. Above figure shows how an op amp can be connected to obtain an integrator.

If an integrator circuit is supplied with an input signal waveform that fluctuates periodically (the usual case), the output voltage varies according to the integral, or antiderivative, of the input voltage. This results in an output signal with the same frequency as that of the input signal, although the waveform is likely to be different. A pure sine wave input produces a pure sine wave output, but the phase is shifted 90° to the right (that is, 1⁄ 4 cycle later in time). Complex input waveforms can produce many types of output waveforms.

An indefinite rise, either negatively or positively, in output voltage cannot occur in a practical integrator. If the mathematical integral (in pure theory) of an input function is an endlessly increasing output function, the actual output voltage rises to a certain maximum, either positive or negative, and stays there. This maximum is less than or equal to the power supply or battery voltage.

#### Voltage Regulator

A voltage regulator IC acts to control the output voltage of a power supply. This is important with precision electronic equipment. These ICs are available with various voltage and current ratings. Typical voltage regulator ICs have three terminals, and because of this, they are sometimes mistaken for power transistors.

#### Timer

A timer IC is a specialized oscillator that produces a delayed output. The delay is adjustable to suit the needs of a particular device. The delay is generated by counting the number of oscillator pulses; the length of the delay can be adjusted by means of external resistors and capacitors. Timers are commonly used in circuits such as digital frequency counters, where a precise time interval or window must be provided.

#### Multiplexer

A multiplexer IC allows several different signals to be combined in a single channel by means of a process called multiplexing. An analog multiplexer can also be used in reverse; then it works as a demultiplexer. Thus, you’ll sometimes hear or read about a multiplexer/demultiplexer IC.

#### Comparator

A comparator IC has two inputs. It compares the voltages at the two inputs, which are called A and B. If the voltage at input A is significantly higher than the voltage at input B, the output is about +5 V. This is logic 1, or high. If the voltage at input A is lower than or equal to the voltage at input B, the output voltage is about +2 V. This is designated as logic 0, or low.

Voltage comparators are available for a variety of applications. Some can switch between low and high states at a rapid rate of speed, while others are slow. Some have low input impedance, and others have high impedance. Some are intended for AF or low-frequency RF use; others are fabricated for video or high-frequency RF applications. Voltage comparators can be used to actuate, or trigger, other devices such as relays and electronic switching circuits.