How to find the right op amp
Op amps are the most versatile of all electronic components.
There are many parameters to consider when choosing the right device for your design.
Op amps came from a time before digital electronics and formed the building blocks of analog computers. With a ubiquitous presence in signal conditioning, they remain the bedrock of many designs.
The natural world is made of physical elements with analog characteristics. These include size, mass and temperature. We can measure these characteristics and use mathematical operations like addition, subtraction and multiplication to modify them. This creates entirely new and valuable information.
Long before digital computing, analog computers were used to process this information. This led to the birth of the operational amplifier, or op amp. Op amps are high-gain voltage amplifiers with differential inputs and a single output. The earliest examples were made using vacuum tubes (U.S. English) or thermionic valves (U.K. English). They were big, slow, complicated and expensive, but they were uniquely suited to mathematical tasks.
In 1943 an American engineer, Loebe Julie, developed a small, modular op amp. Julie simplified the design down to two 6SL7GT dual-triode tubes and a few passive components.
After World War II, an analog computing researcher, George A. Philbrick, further developed Julie’s design at his own company, creatively named George A. Philbrick Researches Inc. In 1952, the company introduced the GAP/R Model K2-W op amp. The device was small and cheap. This saw its widespread adoption in analog computers.
The GAP/R used two 12AX7 dual-triode tubes. The same basic type is used for preamplification in many of the world’s most famous hi-fi and guitar amplifiers. While no new devices were made after 1971, used examples are available on eBay. The data sheet for the GAP/R K2-W can still be found online.
The data sheet describes the device as being capable of performing addition, subtraction, integration, differentiation, multiplication and division operations. It also states that the GAP/R K2-W can be used for impedance conversion and current injection. From the outset, the versatility of the op-amp was clear.
It was 10 years before Philbrick’s “hollow state” op amps gave way to solid state versions when the first germanium and then silicon transistors were developed. Just another year or two later the first integrated circuit op amps began to appear: the Fairchild uA702 in 1964, the uA709 a year later, and the most popular op amp of all time, the uA741 (generically, just the “741”) in 1968. The 741 is still manufactured widely today.
How op amps work
Op amps are high-gain voltage amplifiers with differential inputs and a single output. Let’s look at how they work and the key parameters you need to consider in selecting an op amp for your application. We’ll discuss the typical performance of 741 op amps, but we’ll also describe some of the characteristics of today’s more specialized devices.
A circuit diagram of the ST UA741 general purpose op amp shows 20 transistors, 12 resistors and a capacitor.
The ST UA741 is a typical generic op amp that can be used as the basis for summing amplifiers, voltage followers, integrators, active filters and function generators. The circuit diagram shows the differential inputs. One input is inverting and the other is non-inverting. It has a single output.
The device normally operates with positive and negative power rails and there are two offset null pins, which are described below. The output signal is directly proportional to the difference between the inputs. Schematically, the op amp is simplified to the symbol shown below.
See the simplified schematic diagram symbol for an operational amplifier.
A positive voltage applied to the inverting input will appear as a negative voltage at the output. A positive signal applied to the non-inverting input will cause a positive swing at the output. Because the op amp is differential, if identical signals are applied to both inputs, the output will ideally be zero. The “offset null” connections can be used to compensate for any DC offsets in the circuit. In most applications these are not required because the offsets are too small to cause significant problems.
Op amp specifications
The perfect op amp would have infinite open-loop gain, infinitely high input impedance and zero output impedance. The reality is that open-loop gain is frequency-dependent, declining with increasing frequency. At low frequencies, in the kHz region, it will often be in the 100,000 to 500,000 range but can be even higher.
Negative feedback is used to control the amplifier’s gain, but the high open-loop gain helps minimize distortion. At 1kHz, total harmonic distortion (THD) will typically be below 0.1%. Datasheets usually specify input resistance, which is consistent, rather than impedance, which varies with frequency.
Resistance is the dominant impedance vector. For the ST UA741 this is around 2 MΩ. Similarly, the ideal output impedance is zero but hard to achieve. Output impedance is dependent on frequency and the feedback used to control the gain. In practice, the output impedance will be between a few Ohms and a few tens of Ohms. Again, resistance is the dominant part of the impedance, but reactance may need to be considered in high-frequency applications.
Other key parameters to understand include:
- Common-mode rejection ratio
The ratio between common-mode and differential-mode gain is called the common-mode rejection ratio, or CMRR. CMRR (sometimes abbreviated to CMR). It is usually in the range of 70 dB to 120 dB. Common mode means the same signal is applied to both the inverting and non-inverting inputs. With a common mode input the output should be zero. Even the best op amp has some imbalance, so with gain there will be at least a small signal at the output. - Gain bandwidth product
Open-loop gain falls with increasing frequency, typically starting to decline at just a few Hz in general-purpose op amps. The gain-bandwidth product (GBP) is defined as the open-loop voltage gain multiplied by the frequency of operation. For example, if the amplifier has an open-loop gain of 10 at 100 kHz, the GBP will be 1 MHz. - Unity gain amplifiers
Op amps can be configured for unity gain by connecting the output directly to the inverting input. The unity gain amplifier is also known as a voltage follower, a buffer or isolation amplifier. Its primary purpose is to connect one part of a circuit to another while preserving the voltage level. By using a resistive divider on the non-inverting input of the op amp, this circuit can be used as a level shifter, for example, to convert 5.5 V logic to 3.3 V. - Slew rate
The speed of an op amp is expressed as its slew rate. Slew is the maximum rate of voltage change that can be achieved at the output for a change at the input. It is stated as Volts/µS and is usually specified at unity gain. The UA741 has a typical slew rate of 0.5 V/µS. By contrast, the onsemi NCS2552 750 MHz op amp achieves 1700 V/µS, demonstrating just how far op amp technology has advanced in recent years. In fact, op amps with slew rates up to 5500 V/µS are now possible with very high frequency devices. - Output voltage swing
This defines the maximum voltage swing that can be accommodated at the output of the amplifier for specified load conditions. For general-purpose amplifiers like the UA741, it’s around 2 V below the rail-to-rail voltage, which is the difference between the positive and negative power supply rails. For many precision amplifiers, the output voltage swing can be the same as the rail-to-rail voltage.
Op amp functions and types
Additional components are used to define op amp behavior. Op amps can be configured to operate in various modes. This includes inverting or non-inverting amplifiers, comparators, oscillators, filters, multi-vibrators, audio mixers, and many other circuit functions. It’s this incredible versatility that makes them so popular as the core building blocks of analog designs.
Showing circuit diagrams for each function is beyond the scope of this article but to illustrate the point, the diagram below shows a circuit that can function as an inverting or non-inverting amplifier, an inverting integrator, a subtractor or a buffer amplifier, simply by changing four resistor values and configuring the inputs appropriately.
Simply by changing resistor values and how the inputs are configured, this op amp circuit can perform any one of five circuit functions.
An op amp has differential inputs, but a differential amplifier is a specific function. Referring to the diagram above, using resistors of equal value creates a voltage subtractor. This configuration produces an output voltage proportional to the voltage difference between the two input signals. This is the differential amplifier configuration.
The comparator is another useful op amp implementation. The output simply indicates whether the value on one input is larger or smaller than the value on the other. One value is typically held constant and used as the reference. The circuit can be configured as inverting or non-inverting.
In this mode the op amp is not functioning as a linear device. The output level will be equal to either the positive supply rail or the negative rail, or as close as the output voltage swing allows.
The general-purpose op amp is very versatile. However, specialized parts optimized for specific characteristics can simplify and improve a design.
Precision op amps
These amplifiers are most frequently used in sensors and instrumentation applications. They are optimized to minimize the input bias current or input offset voltage, both of which can contribute to errors at the output.
They may also be designed to minimize changes in performance over time (aging), or temperature-related variations (drift). So-called “zero-drift” op amps are precision devices.
The onsemi NCS233 is a good example. Drift is not quite zero but is only 0.07 µV/ °C, and its input offset voltage is a maximum of 30 µV, compared with 5 mV (at 25 °C) for the general-purpose UA741. This low offset voltage means that the NCS233 and similar devices amplify small differential voltages with greater output accuracy.
When working with precision op amps, good board layout is critical. Power rails need to be decoupled for high and low frequencies as close as possible to the supply pins on the amplifier. To avoid instability, it’s also important to design the board for minimal stray capacitance between input and output.
Instrumentation amplifiers
Instrumentation amplifiers usually comprise three op amps, configured as shown below. This elegant design has several advantages over a single op amp. A1 and A2 form a buffer with extremely high, balanced input impedances because no feedback is applied to them. A3 is configured as a difference amplifier. This combination delivers significantly higher CMRR than is possible with a single op amp. Furthermore, the amplifier’s gain is simply controlled by adjusting RG, without the need for closely matched resistors, which would be required in a single op amp implementation.
The gain of this instrumentation amplifier is simply adjusted by varying the resistor labeled RG.
Conclusion
The range of op amps is diverse. This short introduction touches on a few of the criteria that are important in component selection. It describes some of the most popular op amp configurations. Package size and pin-outs, operating voltage and the number of op amps available within each package are also important design considerations.
Today, you can choose op amps that operate from DC into the GHz region. They can be configured to perform many more functions than amplification. The op amp is arguably the most versatile analog component ever invented.
Type “operational amplifier” into the search box at Avnet.com to see nearly 5,000 devices to choose from. The choice includes a wide selection from Microchip (the ultimate home of George A. Philbrick Researches Inc.), onsemi, Renesas and STMicroelectronics.
