by Octopart · Wed 6 June 2018

Operational amplifiers (op-amps) are one of the primary building blocks of analog circuitry. They are used to amplify voltage, current, and impedance relative to the needs of your circuit. They are called operational because they employ mathematical operations to condition signals. These operations are accomplished with external biasing applying signals to the op-amp inputs and outputs for conditioning and for communication to other circuit blocks within the design.

The op-amp inputs and outputs are part of an integrated configuration that consists of three sections: input, gain, and output stages. Each stage is built with transistors. This arrangement of transistors into circuit stages for amplification of electrical signals becomes an integrated circuit known as the op-amp. Uses of the op-amp are numerous but include voltage sensing, current sensing, and current-to-voltage amplification which we will discuss below.

Characteristics and Functions of Three Operational Amplifiers

A difference amplifier is the first known application of the op-amp. The original function of the op-amp was to take two small voltage inputs, in the presence of large common-mode voltage. The difference between the two small inputs was amplified when going through the three stages of the op-amp.

Being able to detect and amplify small differences in signals has many uses within analog circuit blocks. Vital signals in your design may be small and you may want to amplify them so you can send their information to a controller or some other circuit block that requires a larger signal. A difference op-amp can be a good choice for this application.

A current-sense amplifier measures and amplifies a very small voltage at its inputs. The small voltage is developed across a current-sense resistor, external and upstream of the op-amp. The amplifier outputs a voltage proportional to the current flowing in the sense resistor. Amplifying a small voltage on its inputs is done in the presence of very high common-mode voltages.

A current sense op-amp is characterized by its ability to withstand high input voltages that are higher than the op-amp rails. When the input voltages present on the op-amp inputs is greater than the op-amp rails, the amplifier powers itself from the input common-mode voltages present. To allow high voltage on its inputs, this type of op-amp employs specialized ESD structures to prevent damage to the part.

Amplifiers are building blocks for circuits

A transimpedance amplifier is used as a current-to-voltage converter. It is useful when wanting to amplify signals from upstream circuits or components whose current is more linear than its voltage. Devices whose current is more linear than their source voltage are typically diodes. Sensing devices commonly use diodes to detect ambient light or to sense a collision. The current signal of these sensors is linear and easier to process, so a transimpedance amplifier is used to amplify and deliver its signal downstream in the system.

A few common terms encountered when using op-amps are:

Bandwidth: The band of frequencies over which the gain of the amplifier is almost constant.

CMRR: Common-mode rejection ratio is the ability of the amplifier to reject common signals present on both inputs of the amplifier.

Gain: The amplitude of magnification to the input signal.

Linearity: When the output signal is directly proportional to the input signal.

Precision Resistor: An actual part that doesn’t drift much from its nominal value.

Quiescent Current: Quiescent current is the average current the amplifier will use when off.

Reference Voltage: A constant voltage provided to the amplifier which doesn’t change when loaded, under temperature changes, the passage of time, and/or power supply variations.

Slew Rate: The maximum rate of time it takes for a signal to become present on the output of an op-amp.

Parameters to Consider When Choosing an Op-Amp

Op-amps are useful as building blocks in analog circuit design. They take signals from upstream blocks and amplify them for use downstream in your design. Op-amps use voltage, current, or impedance to amplify signals. Knowing the signals available in your design and how you would like to move them along to the next circuit block will help you choose which type of op-amp you’ll need. Below we discuss three examples of op-amps to help narrow your search.

Difference Amplifier

Differential amplifiers have been around for a long time and in that long history, they’ve been developed and perfected. Development of precision resistors and resistor networks to integrate within the IC has refined giving way to nearly ideal resistors. The resistors are developed alongside the transistor stages within the material. This produces accurate trimming for your application. Some applications include use as a differential input amplifier, instrumentation amplifier building block, differential current receiver, voltage-controlled current source, ground loop eliminator, or a current shunt monitor.

Difference Amplifier: A differential (or difference) amplifier is a type of electronic amplifier that amplifies the difference between two input voltages but suppresses any voltage common to the two inputs.

TCR: Temperature coefficient of a resistor tells how its value changes as its temperature changes.

If you are looking for a low-cost, precision difference amplifier with an input range capability somewhere between 4V and 18V, take a look at Texas Instruments INA157.

The INA157 is a high slew rate, G = ½ or G = 2 difference amplifier consisting of a precision op amp with a precision resistor network. The on-chip resistors are laser trimmed for accurate gain and high common-mode rejection. Excellent TCR tracking of the resistors maintains gain accuracy and common-mode rejection over temperature. The input common-mode voltage range extends beyond the positive and negative supply rails. It operates on +4V to +18V supplies.

The difference amplifier is the foundation of many commonly used circuits. The INA157 provides this circuit function without using an expensive precision resistor network. The INA157 is available in a SO-8 surface-mount package and is specified for operation over the extended industrial temperature range, -40℃ to +85℃.

A functional block diagram of the INA157 difference amplifier

Current Sensing Amplifier

Many applications have a need for sensing current within the circuit of interest. This becomes necessary to determine if overcurrent conditions exist that may damage hardware. An op-amp may be used along with a sense resistor to measure current. The op-amp inputs are tied on either side of the sense resistor to provide voltage readings for input to the op-amp. The op-amp discerns the voltage sense input difference and produces the amplified signal on its output. This amplified signal is used to send information downstream. Current sensing amplifiers may be configured in two ways. Some of their characteristics are defined below:

Unidirectional: Unidirectional operation is where the load current only flows in one direction. Application examples are PA monitoring, non-inductive load monitoring, and laser or LED drivers.

Bidirectional: Bidirectional operation is where the load current can flow in both directions. Application examples are battery-charging or regenerative motor monitoring.

Sense Resistor: A resistor placed in a current path to allow the current to be measured. Load Current: The amount of current consumed by the circuit.

Low-Dropout Regulator: A low-dropout (LDO) regulator is a DC linear voltage regulator that can regulate the output voltage even when the supply voltage is very close to the output voltage.

If you are looking for a current-sense amplifier with common-mode input range capability somewhere between 4V and 76V, take a look at Texas Instruments LMP8480MME-S/NOPB.

The LMP8480 and LMP8481 are precision high-side current sense amplifiers that amplify a small differential voltage developed across a current sense resistor in the presence of high input common-mode voltages. These amplifiers are designed for bidirectional (LMP8481) or unidirectional (LMP8480) current applications and accept input signals with common-mode voltage range from 4V to 76V with a bandwidth of 270 kHz. Because the operating power supply range overlaps the input common-mode voltage range, the LMP848x can be powered by the same voltage that is being monitored. This benefit eliminates the need for an intermediate supply voltage to be routed to the point of load where the current is being monitored, resulting in reduced component count and board space.

The LMP848x family consists of fixed gains of 20, 60, and 100 for applications that demand high accuracy over temperature. The low-input offset voltage allows the use of smaller sense resistors without sacrificing system error. The wide operating temperature range of -40℃ to 125℃ makes the LMP848x an ideal choice for automotive, telecommunications, industrial, and consumer applications.

This part comes with evaluation module (EVM) for use during development of your circuit.

LMP8480 Evaluation Board offered by Texas Instruments

Transimpedance Amplifier

Transimpedance amplifiers are current-to-voltage converters. This is helpful when diodes are used upstream in a circuit with a need for amplification downstream. As diode current is linear while diode voltage is not, use of a transimpedance amplifier is the way to go. They are ideal for use with circuits that use diodes or other nonlinear devices in their design.

FDDI receivers: Fibre Distributed Data Interface

SONET/SDH: Synchronous Optical Network / Synchronous Digital Hierarchy

ECL: Emitter-Coupled Logic

PECL: Positive Emitter-Coupled Logic

If you are looking for a low-cost transimpedance amplifier with the capability to support high bandwidth data rates beyond 300 Mbps, take a look at Analog Devices AD8015.

The AD8015 is a wide bandwidth, single supply transimpedance amplifier optimized for use in a fiber optic receiver circuit. It is a complete, single-chip solution for converting photodiode current into a differential voltage output. The 240 MHz bandwidth enables AD8015 application in FDDI receivers and SONET/SDH receivers with data rates up to 155 Mbps. This high bandwidth supports data rates beyond 300 Mbps. The differential outputs drive ECL directly or can drive a comparator/fiber optic post amplifier.

In addition to fiber optic applications, this low cost, silicon alternative to GaAs-based transimpedance amplifiers is ideal for systems requiring a wide dynamic range preamplifier or single-ended to differential conversion. The IC can be used with a standard ECL power supply (-5.2V) or a PECL (+5V) power supply; the common-mode at the output is ECL compatible. The AD8015 is available in die form, or in an 8-pin SOIC package.

Fiber Optic Receiver Application: Photodiode Referred to Positive Supply

Operational amplifiers are building blocks in analog circuit design and may be a great choice for your circuit design. Octopart has a powerful search engine that can assist you in narrowing your choice. While narrowing your choice relative to the designs needs, you can also look at availability and pricing.

If you have comments or suggestions on op-amp part selection, drop us a note in our Slack chat room or in comments below.

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