Operational amplifiers (op amps) continue to serve an important role in circuits that interface with systems processing analog signals. These voltage amplifying devices are designed to be used with external feedback components such as resistors and capacitors between their output and input terminals.
Op amps are among the most widely utilized electronic devices today, being used in a vast array of consumer, industrial, and scientific devices. There is a very large number of operational amplifier ICs available to suit every possible application from standard bipolar, precision, high-speed, low-noise, high-voltage, etc., in either standard configuration or with internal Junction Field Effect Transistors (JFETs).
Since the first op amp—having a vacuum tube design—was invented by Karl D. Swartzel, Jr. of Bell Labs in 1941, manufacturers have been striving to design a better op amp. Characteristics of the “ideal” or perfect op amp include infinite open-loop gain Ao, infinite input resistance Rin, zero output resistance Rout, infinite bandwidth 0 to ∞ and zero offset (the output is exactly zero when the input is zero).
In reality, physical and electrical design and cost constraints have forced op amp manufacturers to build op amps that juggle performance and design tradeoffs.
For instance, op amps do not have infinite gain or bandwidth but have a typical “open loop gain,” which is defined as the amplifier’s output amplification without any external feedback signals connected to it, and for a typical operational amplifier is about 100dB at DC (zero Hz). This output gain decreases linearly with frequency down to “unity gain” or 1, at about 1 MHz.
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With applications continuing to push performance limits or requiring additional functionality, many newer amplifiers are highly optimized circuits for a specific purpose or class of performance needs, according to Brian Black, marketing manager at Linear Technology. Black gives the example of an op amp configured as a transimpedance amplifier in a photodiode application. Linear Technology has developed two unity gain stable op amps—the LTC6268 (see Figure 1) and the LTC6269—that are optimized for high impedance circuit applications.
Figure 1: Linear Technology’s LTC6268 op amp. (Source: Linear Technology)
Linear Technology designed the op amps with a low input capacitance of 0.45 pF, as well as input referred voltage and current noise of 4.3 nV/√Hz @ 1 MHz and 5.5 fA√Hz @ 100 kHz, respectively. The op amps achieve a 4 GHz gain bandwidth.
With op amps offering performance above more integrated analog solutions, they are being used more in high-performance systems and less in consumer-grade electronics, according to John Caldwell, Systems Engineer of Audio Operational Amplifiers for Texas Instruments. “Companies that build data acquisition systems for test and measurement applications or industrial process control are very interested in op-amps, while a company that builds Bluetooth speakers may not need op-amps because their product can be built much faster and cheaper with an integrated solution.”
A continuing challenge in op-amp design is trying to achieve low noise with moderate power consumption, according to Dwight Byrd, Marketing Manager of Op Amps for Texas Instruments.
“A major issue we face at TI is building low-power output stages that maintain stability into capacitive loads. Op amps can be made to consume extremely low amounts of power if they never interact with a capacitive load. But, this is not the case in the real world. The application circuit, PC board, and IC package itself can all contribute capacitance that may destabilize the op amp if it is not properly designed. Solving this issue without consuming excessive power is always a design hurdle.”
Better design techniques have improved op amp performance. Texas Instruments’ Byrd gives the example of the company’s TL072 (Figure 2) that—with a broadband voltage noise spec of 18 nV/√Hz—was once considered a low-noise amp.
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According to Byrd, Texas Instruments now offers the OPA170, whose broadband noise is 18 nV√Hz but consumes one-tenth the power (110 µA compared to 1.4 mA). JFET-input op amps like the OPA827 are now pushing the sub-4 nV/√Hz noise region, and bipolar op amps like the OPA211 and LME49990 are near 1 nV√Hz.
Figure 2: Texas Instruments’ TL072 op amp. (Source: Texas Instruments)
Modern trim technologies and chopper amplifier technologies are also achieving reductions in DC errors due to offset voltages and temperature drift, according to Texas Instruments’ Byrd.
With supply voltages in analog circuits continuing to decrease below 5 V to 3.3 V and sometimes 1.8 V, op amps capable of lower voltage operation continue to emerge.
“To compensate for the reduced input voltage and provide even better harmonic distortion performance, many analog-to-digital converter drivers now feature differential inputs,” said Linear Technology’s Brian Black.
Fully differential amplifiers such as Linear Technology’s LTC6363 include differential inputs and can accept a single-ended or differential input. The LTC6363 is also representative of the industry trend for op amps to have a rail-to-rail output (the output signal can range from the lowest supply voltage to the highest). The op amp has a supply voltage range of 2.8 to 11 V. It exhibits low distortion of 115 dB at 2 kHz and a fast settling time of 780 ns to 18-bit, 8 V peak-to-peak output.
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Reaching performance goals in op amps is becoming easier due to the greater use of IC device modeling and software tools for IC layout, according to Texas Instruments’ Dwight Byrd.
“Now the design process includes an examination of how parasitics in the IC layout affect the functionality and performance of the circuit. By examining the effects of parasitics, we can reduce the chance for surprises when we received our silicon, such as degradation in common-mode rejection due to mismatch in the input capacitances of an amplifier.”