Operational amplifiers, also known as “op amps,” are commonly used in analog electronic circuit designs. Designers of all skill levels frequently incorporate these workhorse components into their electronic designs.
Karl D. Swartzel Jr. of Bell Labs filed a patent for the “summing amplifier” in 1941. Originally, the device was used to perform mathematical operations in analog computers. Thus, the component earned the “operational” part of its name.
Today, op amps are the building blocks of many modern analog electronic circuits. The devices still perform complex mathematical operations like integration and differentiation. However, thanks to their low cost, optimal performance, and widespread availability, these versatile components are used in a myriad of consumer, industrial, and scientific applications.
For example, op amps are used in audio amplifiers, video applications, medical sensor interfaces, voltage regulators, baseband receivers, analog to digital converters, and so much more.
Technically, an op amp is a high-gain voltage amplifier with differential inputs. Commonly found as integrated circuits (ICs), their input and output terminals can produce voltage signals that are larger than what is being passed through them. The op amp essentially helps boost signals that typically decrease as they pass through discrete elements in an analog circuit. Ultimately, the devices produce a useful output signal.
An op amp’s extremely high gain can’t be changed. However, applying feedback loops to an op amp circuit can control gain. Ultimately, adding resistors, capacitors, and inductors creates different configurations on an op amp circuit that will produce widely different results.
Overall, the adaptability and versatility of this three-terminal device makes it a popular component in many circuit designs.
Selecting the Proper Op Amp
An ideal op amp would have infinite open loop gain, infinite input impedance, zero output impedance, and infinite frequency response with zero noise and no distortion. Realistically, no op amp can satisfy all these requirements.
General purpose op amps will work well in many circuits. However, some applications may require higher performance from specialty op amps.
To meet the varying design requirements of different applications, designers should consider several factors to make sure they are selecting the proper op amp. These key attributes are listed below.
One of the most distinctive characteristics of an op amp is its high gain.
Gain measures an op amp’s amplification factor —or, how much larger the magnitude of its output is compared to its input. It’s typically referred to as an “open loop gain” or a “large signal voltage gain.”
An open loop gain measures the op amp’s gain without positive or negative feedback. When open loop gain is quoted, it represents the maximum AC gain at very low frequencies. Ideally, op amp gain should be infinite. However, real values usually range from 20,000 to 200,000 ohms.
By comparison, a large signal voltage gain (aka AVD), depicts the ratio of the change in the output to the differential voltage change in the input. It is measured at DC – at low frequency – and the amplifier produces a large voltage output. It is typically quoted in preference to an open loop gain, usually as V/mV. This gain is notably measured without an output load. As such, it accounts for loading effects.
Number of Channels
The most common number of channels for an op amp is 1, 2, or 4. However, they can have as many as 1 to 8 channels.
Input Impedance and Output Impedance
In general, input and output impedance of an op amp show a relationship between voltage and current.
Input impedance shows how much more current will flow as voltage is increased or how much less current flows when voltage is decreased.
Ideally, this value should be infinite. Actual op amps have very high input impedance (typical values can be in hundreds of millions of ohms). A prime advantage of high input impedance is that very little current is needed from the source to produce voltage.
Output impedance, on the other hand, is ideally zero. However, most actual IC-based op amps have very small output impedance values of less than a hundredth of an ohm.
Bandwidth and Gain Bandwidth Product
An ideal op amp would have infinite bandwidth and be able to amplify any frequency signal from DC to the highest AC frequencies. Bandwidth for actual op amps, however, can only amplify sound through a certain range of frequencies. Once it goes above its frequency limit, the op amp can’t produce sound.
The Gain Bandwidth Product (GBP) describes the frequency at which the op amp’s amplifier becomes unity. The factor allows designers to achieve the maximum gain an op amp can produce for a given frequency.
The slew rate of an op amp is the rate of change in the output voltage caused by a step change on the input. It is measured as the ability for an op amp to change its output voltage by a certain amount in a given amount of time.
Ideally, an op amp should have an infinite slew rate. This would mean output from the amplifier would be a distortion-free amplified copy of the input.
In actual designs, the higher the slew rate is, the faster the output can change. Designers should also be aware that slew rate change values vary according to the type of op amp being used. So, low power op amps might yield a slew rate value of one volt per microsecond while faster op amps can provide rates of 1,000v/microsecond.
The ideal operational amplifier would have zero noise. Unfortunately, all op amps contain several internal noise sources such as current noise, resistor noise, and more. They are measured at the output and refer back to the inputs. The Equivalent Input Noise Voltage is the most significant noise type. It is also bandwidth dependent. Overall, designers should seek for this noise value to be as small as possible.
Maximum Input Offset Voltage
When both op amp inputs are zero, ideally, the op amp would produce zero output. However, due to manufacturing imperfections, differential input transistors in actual op amps may not be exactly matched. Thus, a small differential input voltage must be applied at the input to force a zero output. This is input offset voltage, and it should be a small value.
Maximum Supply Voltage
Each op amp has an allowable operating voltage range. Thus, designers should not exceed an op amp’s maximum supply voltage in any specific application.
Overall, designers should consider these factors and how they impact the specific design requirements for each application when they decide which op amp to use for a project.
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A brief description of 10 of the most popular op amps follows below.
1. LM358ADT STMicroelectronics
Like the LM358DT, this part consists of two high gain, independent, internally frequency compensated op amps. The dual operational amplifier is designed to function across a wide range of voltages from a single power supply. Available in a SOIC-8 package, the device has 1.1 MHz Wide bandwidth (unity gain). The part features a typical input offset voltage of 1mV, a maximum input offset current of 10nA, and a maximum input bias current of 50nA (temperature compensated).
2. LMV358IDGKR Texas Instruments Inc.
This dual low voltage rail-to-rail output op amp is well-suited for applications that require low-cost, space saving, and low voltage operation. The part comes in a VSSOP-8 package and is designed to operate at voltage between 2.7V to 5V with a low supply current of 210 μA. These devices are used in a variety of applications including pro audio mixers, HVAC systems, desktop computers, and more.
3. LM358AN ON Semiconductor
This single supply dual op amp comes in a DIP-8 package type. It’s the same as the LM358N. However, it has a maximum input offset voltage of 3mV and a maximum input offset current of 30nA. It operates with atypical input bias current of 45nA and a maximum input bias current of 100nA. These values are lower than the LM358N.
4. LM358N/NOPB Texas Instruments Inc.
The LM358N is internally frequency compensated and comes in a DIP-8 package. It is a dual input operational amplifier that has a typical input offset voltage of 2.9mV and a typical input offset current of 5nA. The typical input bias current of the device is 45nA and the maximum is 250nA.
5. UA741CP Texas Instruments
The UA741CP is a single, general purpose op amp with a7500uV offset max. It comes in a PDIP8 tube and offers short-circuit protection and offset voltage null capability. The device features internal frequency compensation which helps ensure stability without external components. Its supply voltage range is ±15V with a large signal differential voltage gain of 200V/mV. The amplifier’s high common-mode input voltage range and absence of latch-up make it well-suited for voltage follower applications.
6. LM358DT STMicroelectronics
Like the LM358ADT, the LM358DT is a dual function operational amplifier that’s designed to function over a wide range of voltages from a single power supply. The two independent, high gain, internally frequency compensated op amps come in a SOIC8 package. The device features atypical input offset voltage of 2mV and an input offset current of 2nA. The part’s wide gain-bandwidth is 1.1MHz and it can support dual power supplies with a range of ±16 V. Its large signal voltage gain is 100v/mV.
7. LM324N Texas Instruments Inc.
This low power quad op amp has a wide gain bandwidth of 1.3MHz, a maximum input offset voltage of 5mV and a low input bias current of 20nA. It comes in a PDIP14 package type with a 9000uV offset max.
8. MCP6001T-I/OT Microchip Technology Inc.
The MCP6001T-I/OT is designed for low-power, low-cost, general purpose applications. The single op amp operates in an industrial temperature range of -40°C to +85°C. Typical gain bandwidth product for the device is 1 MHz and it comes in a SOT-23 package type. This family of operational amplifiers can operate from a single supply with voltage as low as 1.8V while drawing 100µA (typical) quiescent current. Common uses include analog filters, notebooks and PDAs, automotive applications, and more.
9. LM324AMX/NOPB Texas Instruments Inc.
The LM324AMX/NOPB comes as four internally compensated op amps that come in a single SOIC14 package. It operates from a single power supply across a wide voltage range for 3V to 32V. The low-power, general purpose op amp carries a large signal voltage gain of about 100V/mV. It also features a wide gain-bandwidth of 1MHz with a low input bias current of 45nA. Its low input offset voltage of 2V and offset current of 5nA.
10. LM741H Texas Instruments Inc.
The LM741H is a single function operational amplifier that features overload protection on the input and output. There is notably no latch-up when the common mode range is exceeded. The general-purpose op amp operates with a supply voltage range of ±15V and a maximum supply voltage range of ±22V. Its bandwidth is up to 1.5MHz and it has a large signal voltage gain of 200V/mV. The device comes in a TO-99-8 package and is a direct, plug-in replacement for other op amps such as 709C, MC1439, LM201 and more.
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