# Linear Devices in Electronics Keep Your Designs Running Straight

July 19, 2018

Many cultures view time as linear or as something that moves along a straight line. The flow of time moves from past to present to future.

Other cultures, however, have a different perspective about time. Those see time moving in cycles. History repeats itself and all human events occur in cycles.

## Basic Linear Device Operation

In electronics, we can separate devices according to linear and non-linear operation. Just as the term, linear, describes time as moving in a straight line, linear devices also operate in a straight line. Circuit parameters—such as resistance, inductance, capacitance, waveform, and frequency—do not change with respect to current and voltage. Current flowing through a linear circuit remains directly proportional to the applied voltage.

Just as a straight line represents linear time, the characteristic curve for a linear device is a straight line. We can go all the way back to Ohm’s Law to find that any increase in applied voltage leads to an increase in current if the resistance remains the same.

Sometimes all you need is a meter for your voltage, but often your designs require much more

Linear devices include resistors, capacitors, most inductors, and linear integrated circuits such as operational amplifiers. Diodes, transistors, transformers, and iron core inductors or inductors that have a saturated core act as non-linear devices.

## Straight Talk about Linear Integrated Circuits

Swiss Army Knives gained fame because all the tools included with a Swiss Army Knife allow the owner to accomplish several different tasks. Linear integrated circuits serve as the Swiss Army Knife of electronics because the analog devices have a seemingly infinite number of possible operating states and regions.

As a result, a linear IC operates over a continuous range of input levels and has an output proportional to its inputs. When we look at the circuits used for any consumer product, we will see linear ICs working as DC amplifiers, audio amplifiers, IF amplifiers, power amplifiers, operational amplifiers, differential amplifiers, multiplexers, and comparators.

## Home on the Linear Range

Basic operational amplifiers function as a three-stage high gain amplifier that amplifies, filters, sums, and buffers analog signals. A differential amplifier usually works as the first stage of the amplifier while the next stage develops a large voltage gain. With the final stage working as a class B voltage follower, an op-amp has a high input impedance, a low output impedance, and a high voltage gain. Op-amps use negative feedback to eliminate distortion caused by non-linear transistors.

Referring to the basic diagram, op-amps have a minimum five terminals with the positive and negative supply voltage terminals located at +V and –V.  One input produces an inverted output signal while the other input produces a non-inverted output signal. The inverting input terminal has a negative designation while the non-inverting input terminal has a positive designation. The point of triangle points in the direction of the signal flow and connects to the output terminal.

We can use a basic operational amplifier to explore the linear range of linear integrated circuits. When an op-amp operates in the linear region, the -mode input voltage reaches its maximum value. The positive and negative supply voltages define the input mode and output mode swing ranges and the output voltage of the amplifier. In the linear range, the input and output signals remain within the input -mode and output -mode swing ranges.

When we look at the voltage transfer curve for an op-amp, two regions of operation become apparent. In the linear region, the output changes linearly with respect to the input. The large slope of the line indicates that the relationship between the output voltage and input voltage. In the non-linear or saturation region, the input voltage has very little impact on the output voltage.

## Altium Designer® Has Linear Devices Covered

Altium includes libraries cover linear devices such as general purpose operational amplifiers. You can browse the libraries by topic or by type by displaying and accessing the Libraries workspace panel, selecting Tools, and selecting Find Component.

In addition, you can use Altium to define and run circuit simulation analyses. The software allows you to create a new schematic sheet, draw a schematic that contains components with attached simulation models from manufacturer websites, set voltage sources, ground references, and net labels for viewing waveforms. After completing the circuit, you can run circuit simulations directly from the schematic.

If this interests you, then let me be the first to tell you: sign up and talk to an expert at Altium today.

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