An operational amplifier is an electronic device with feedback with the task of repeatedly increasing the signal difference between its two inputs. Initially, the design was used by Bell Labs to drive an anti-aircraft gun within the T9 system. Hence the name. In English, operational is translated in two ways: operator and at the same time successful( working, effective).And the effectiveness of the T9 system today is beyond doubt.
How the operational amplifier
was created The development of electronic lamps
The story begins with the birth of the twentieth century( 1904), when Fleming improved the Edison electronic lamp( see incandescent lamp), having received the first vacuum diode. In detail, the patent of 1883 did not become the first mention of thermionic emission. A decade of the date previously noted, Frederic Guthrie already mentioned this phenomenon( see Magnetism and Electricity, 1873).In 1906, thanks to Lee De Forest, the first vacuum triode appeared - a device that is structurally part of the first operational amplifiers. Between the existing filament( cathode) and a positive potential disk( anode), a grid was added, the signal on which actively influenced the passage of electrons through space.
Differential Amplifiers and Feedback
A new step was the development in the early 30s of Bell Labs technology of amplifiers with feedback, which leads directly to the patent of US 2401779 Karl Schwartzel Jr., declared May 1, 1941.
In 1928, the feedback so prevalent today was not known. And when Bell Labs employee Harold Black posted the patent for the first time, it turned out to be of little use. It took as many as 9 years( US Patent 2,102,671) to bring the invention to mind. As often happens with great inventions, many people in different parts of the Earth worked on the same subject. Scientists include:
- Paul Voight( UK Patent 231792, 1924).
- A.D. Blumlein( UK Patent 425553, 1933).
- Known company N.V.Philips.
When Black developed his idea, he tried to solve the situation with signal repeaters in communication lines. One vacuum triode gave a gain of a maximum of 1 dB exclusively in favorable conditions. It required hundreds, thousands, and this band needed energy and demanded the attention of the attendants. An amplifier with feedback has become a stunning invention - the gain has increased many times while at the same time increasing stability( Nyquist criteria).The company Bell Labs literally pissed off on the said idea.
The development of the idea of Black are the patents US Patent 1915440( Harry Nyquist) and US Patent 2123178( Hendrick Bode).Nyquist threw the idea of the work of vacuum tubes with direct current, which again expanded the scope of applicability of feedback. In parallel, there was the development of differential amplifiers - the working signal for them is the difference between the two inputs. The stages of development are noted:
- B.H.K. Matthews invented a differential input for an amplifier in 1934.Disadvantage: in the circuit with common cathodes they are directly connected to the negative pole of the power source, which directly reduces the gain.
- Alan Blumlein went a little further in UK Patent 482470( 1936).Separated common cathodes from the ground resistor.
- In 1937, Franklin Offner introduced feedback into the design, which slightly reduced the gain, but increased the stability of the system. In the aforementioned year, Otto Schmitt came up with a scheme for pentodes, where this deficiency was absent.
- In 1938, JF Tonnis introduced the concept of a long-tail differential pair for vacuum tubes. In this case, a power source is added between the ground and the common high-resistance resistor( in Tonnis - minus 90 V), which additionally reduces the cathode potential.
- Otto Schmitt in 1938 also discusses the long-tail differential pair, but already as a phase inverter( one input is grounded).
- Harold Goldberg in 1940 invents the scheme of low-noise( about 2 µV) multi-stage differential amplifier. Later introduces a pentode into the circuit to provide the desired bias current.
Operational Amplifiers: The First Birdies of the
The focus of the development of operational amplifiers in the 30s was in the field of analog computing devices. Similar constructions were discussed in the late 30s and in 1940 by George Philbrick and Per Holst, lacking the last step - a large gain. The use of bipolar power allowed to work out the mismatch signals in both directions for accurate aiming. The M9 system served as an operating unit of a computer system that calculates the trajectory of the projectiles to hit air targets.
Research details are described in Higgins' Defense Research at Bell Labs: Electrical Computers for Fire Control.
So, Karl Schwartzel in the 1941 patent discusses the first opamps. In the documentation, the invention is called summing. The origin of the name is trivial. The inventor himself writes that the device is designed to add the n-th number of voltages and it is possible in a similar way to modify existing computers. A feature of the novelty was the introduction of feedback to reduce the input resistance of the system( which will simplify its coordination with other parts of the electrical circuit and increase the gain).
Before the summation occurred gradually, the only voltage had two poles, which greatly complicated the coordination. This patent deals with a device where everything is simplified. Each of the summed voltages at one of the poles acquires a common wire, and the transmission coefficient of the system can be adjusted by adjusting the depth of the feedback. The only limitation is the direct current, not always able to overcome the vacuum.
The development of Bell Labs leads to the creation of a prototype of the aiming computer system, code-named T10.The system( US Patent 2493183) was successfully tested in December 1941 and was actively developed in the future. The range of use of operational amplifiers in it has been significantly expanded. It should be noted that the publication of the patent Shvarttsel, declared May 1, 1941, occurred only after the end of the Second World War( 1946).So allies considered this innovation important. Moreover, the specification for the guidance system itself was practically in open access( for enemy agents).
The question arises: why does the Shvartzel operational amplifier reverse the signal? We believe - although it is not directly stated anywhere - that this is done for the convenience of the pilots. In aviation, it is customary to use the inversion of the aircraft in pitch angle. Thus, the designers wanted to simplify the electrical circuit and in the future to use operational amplifiers as part of the onboard electronics. Pitch angle inversion is adopted to compensate for the physiological characteristics of the pilot as a representative of Homo Sapiens. If you do the opposite, at low altitudes, the plane will fall to the ground. This was discussed at the lessons of physics in the middle classes of secondary schools.
The next question: why does the Schwartzel operational amplifier add up to three signals at the input? We believe the answer lies in the area of functional limitations of automation. The initial guidance to the target is done manually by the operator, then the optical system provides the computing device with information on which the refinement is performed. Perhaps with a lead on the speed and range of the target. As a result, the signal from the control handle should be summed up with the commands of the transmitter. The third input is needed for feedback, which will give the movement of the trunk the desired smoothness and eliminate various excesses.
As a result, the operational amplifier solved tasks and at the same time did gain 95 dB( 65,000 times) and carried an incredible load of 6 kΩ( the input of a modern loudspeaker is a hundred Ohm - for comparison).And in April 1947, the main designers of the T9 guidance system, Lovell, Parkinson and Kun, received a medal for services to the fatherland, established by the president for the period of the Second World War( from September 8, 1939) to 1952 inclusive. This is the highest award for civilians who contributed to the victory over the enemy.
The probability of hitting the T9 target was 90%.So the thought of the existence of such a computer for a long time discouraged the enemy from attacking the United States and allies. The tool was immediately declared the most important means of protecting freedom and democracy in the world.
Development of operational amplifiers
Further work in the field of development of operational amplifiers moved( 1947) to Columbia University in New York. Activities were monitored and directed by Professor John Ragazzini. In the course of development, a scheme of two triodes was found( there were three before them), but for understandable reasons, there is little information about the design to this day. The author is called Julie Loeb. The load of the circuit has increased several times and amounted to 300 kΩ.
It is in the scheme of Julie Loeb that two inputs appear instead of one inverting: inverting and not inverting. It is still possible to add stress on each. This quality is used today - no new one was invented. The differential input compensates for the noise drift, but they remain large in case of need to amplify sub-millivolt signals. Inaccuracies in thermal loss of the operating point and long-term fluctuations are introduced. The difficulty is solved by the use of a chopper( cutting the voltage into high-frequency pulses).Scheme proposed in 1949 by Edwin Goldberg.
Drift is reduced by chopper transfer ratio. A side advantage is the possibility of using low frequencies, including constant voltage. Due to the presence of feedback, the chopper can give a gain up to 100 dB, and in total, the Goldberg circuit provides 163( 150,000,000 times).The novelty had several limitations:
- The first schemes with choppers worked only in the inverting mode. The implementation of the usual required inclusion in the scheme too many cascades.
- At the time of 1949 there was no concept of power switches. The cutting was done with mechanical devices. The situation has already been solved in semiconductor technology, and today each switching power supply includes a chopper on a thyristor( triac).
Semiconductors in operational amplifiers
In the second half of the 40s bipolar and field-effect transistors appeared on the scene, and in 1958, Jack Kilby from Texas Instruments invented integrated circuits. The planar process of mounting on a crystal of various configurations has revolutionized the field of operational amplifiers. As a result, the beginning of the 60s gives new devices with power supply of the order of 10-15 V instead of 350 that existed before. The first integrated circuits turned out to be clumsy and represented a small board with mounted elements( and transistors) flooded with compound. Suffered gain, load resistance barely reached 500 ohms.
But the equipment did not stand still. For example, the varactor bridge made it possible to amplify very small DC signals to a large value. What made it possible to control various mechanisms directly. Today, most operational amplifiers are semiconductor crystals with active and passive elements formed on them.