Operational amplifier models

While mention of operational amplifiers typically provokes visions of = semiconductor devices built as integrated circuits on a miniature = silicon chip,=20 the first op-amps were actually vacuum tube circuits. The first = commercial,=20 general purpose operational amplifier was manufactured by the George A.=20 Philbrick Researches, Incorporated, in 1952. Designated the K2-W, it = was built=20 around two twin-triode tubes mounted in an assembly with an octal = (8-pin) socket=20 for easy installation and servicing in electronic equipment chassis of = that era.=20 The assembly looked something like this:

The schematic diagram shows the two tubes, along with ten resistors = and two=20 capacitors, a fairly simple circuit design even by 1952 standards:

=20

In case you're unfamiliar with the operation of vacuum tubes, they = operate=20 similarly to N-channel depletion-type IGFET transistors: that is, they = conduct=20 more current when the control grid (the dashed line) is made more = positive with=20 respect to the cathode (the bent line near the bottom of the tube = symbol), and=20 conduct less current when the control grid is made less positive (or = more=20 negative) than the cathode. The twin triode tube on the left functions = as a=20 differential pair, converting the differential inputs (inverting = and=20 noninverting input voltage signals) into a single, amplified voltage = signal=20 which is then fed to the control grid of the left triode of the second = triode=20 pair through a voltage divider (1 MΩ -- 2.2 MΩ). That triode = amplifies and=20 inverts the output of the differential pair for a larger voltage gain, = then the=20 amplified signal is coupled to the second triode of the same dual-triode = tube in=20 a noninverting amplifier configuration for a larger current gain. The = two neon=20 "glow tubes" act as voltage regulators, similar to the behavior of = semiconductor=20 zener diodes, to provide a bias voltage in the coupling between the two=20 single-ended amplifier triodes.

With a dual-supply voltage of +300/-300 volts, this op-amp could only = swing=20 its output +/- 50 volts, which is very poor by today's standards. It = had an=20 open-loop voltage gain of 15,000 to 20,000, a slew rate of +/- 12 = volts/=B5second,=20 a maximum output current of 1 mA, a quiescent power consumption of over = 3 watts=20 (not including power for the tubes' filaments!), and cost about $24 in = 1952=20 dollars. Better performance could have been attained using a more = sophisticated=20 circuit design, but only at the expense of greater power consumption, = greater=20 cost, and decreased reliability.

With the advent of solid-state transistors, op-amps with far less = quiescent=20 power consumption and increased reliability became feasible, but many of = the=20 other performance parameters remained about the same. Take for instance = Philbrick's model P55A, a general-purpose solid-state op-amp circa 1966. = The=20 P55A sported an open-loop gain of 40,000, a slew rate of 1.5 = volt/=B5second and an=20 output swing of +/- 11 volts (at a power supply voltage of +/- 15 = volts), a=20 maximum output current of 2.2 mA, and a cost of $49 (or about $21 for = the=20 "utility grade" version). The P55A, as well as other op-amps in = Philbrick's=20 lineup of the time, was of discrete-component construction, its = constituent=20 transistors, resistors, and capacitors housed in a solid "brick" = resembling a=20 large integrated circuit package.

It isn't very difficult to build a crude operational amplifier using = discrete=20 components. A schematic of one such circuit is shown in Figure = below.

=20

A simple operational amplifier made from discrete = components.

=20

While its performance is rather dismal by modern standards, it = demonstrates=20 that complexity is not necessary to create a minimally functional = op-amp. =20 Transistors Q₃ and Q₄ form the heart of another=20 differential pair circuit, the semiconductor equivalent of the first = triode tube=20 in the K2-W schematic. As it was in the vacuum tube circuit, the = purpose of a=20 differential pair is to amplify and convert a differential voltage = between the=20 two input terminals to a single-ended output voltage.

With the advent of integrated-circuit (IC) technology, op-amp designs = experienced a dramatic increase in performance, reliability, density, = and=20 economy. Between the years of 1964 and 1968, the Fairchild corporation=20 introduced three models of IC op-amps: the 702, 709, and the = still-popular 741. =20 While the 741 is now considered outdated in terms of performance, it is = still a=20 favorite among hobbyists for its simplicity and fault tolerance = (short-circuit=20 protection on the output, for instance). Personal experience abusing = many 741=20 op-amps has led me to the conclusion that it is a hard chip to kill . . = .

The internal schematic diagram for a model 741 op-amp is shown in = Figure = =20 below.

=20

Schematic diagram of a model 741 op-amp.

=20 =20

By integrated circuit standards, the 741 is a very simple device: an = example=20 of small-scale integration, or SSI technology. It would = be no=20 small matter to build this circuit using discrete components, so you can = see the=20 advantages of even the most primitive integrated circuit technology over = discrete components where high parts counts are involved.

For the hobbyist, student, or engineer desiring greater performance, = there=20 are literally hundreds of op-amp models to choose from. Many sell for = less than=20 a dollar apiece, even retail! Special-purpose instrumentation and=20 radio-frequency (RF) op-amps may be quite a bit more expensive. In this = section=20 I will showcase several popular and affordable op-amps, comparing and=20 contrasting their performance specifications. The venerable 741 is = included as=20 a "benchmark" for comparison, although it is, as I said before, = considered an=20 obsolete design.

Widely used operational amplifiers

=20

Model	Devices/ package	Power supply	Bandwidth	Bias current	Slew rate	Output current
number	(count)	(V)	(MHz)	(nA)	(V/=B5S)	(mA)
TL082	2	12 / 36	4	8	13	17
LM301A	1	10 / 36	1	250	0.5	25
LM318	1	10 / 40	15	500	70	20
LM324	4	3 / 32	1	45	0.25	20
LF353	2	12 / 36	4	8	13	20
LF356	1	10 / 36	5	8	12	25
LF411	1	10 / 36	4	20	15	25
741C	1	10 / 36	1	500	0.5	25
LM833	2	10 / 36	15	1050	7	40
LM1458	2	6 / 36	1	800	10	45
CA3130	1	5 / 16	15	0.05	10	20

Listed in Table = =20 above are but a few of the low-cost operational amplifier models = widely=20 available from electronics suppliers. Most of them are available = through retail=20 supply stores such as Radio Shack. All are under $1.00 cost direct from = the=20 manufacturer (year 2001 prices). As you can see, there is substantial = variation=20 in performance between some of these units. Take for instance the = parameter of=20 input bias current: the CA3130 wins the prize for lowest, at 0.05 nA (or = 50 pA),=20 and the LM833 has the highest at slightly over 1 =B5A. The model CA3130 = achieves=20 its incredibly low bias current through the use of MOSFET transistors in = its=20 input stage. One manufacturer advertises the 3130's input impedance as = 1.5=20 tera-ohms, or 1.5 x 10¹² Ω! Other op-amps shown here = with low bias=20 current figures use JFET input transistors, while the high bias current = models=20 use bipolar input transistors.

While the 741 is specified in many electronic project schematics and=20 showcased in many textbooks, its performance has long been surpassed by = other=20 designs in every measure. Even some designs originally based on the 741 = have=20 been improved over the years to far surpass original design = specifications. One=20 such example is the model 1458, two op-amps in an 8-pin DIP package, = which at=20 one time had the exact same performance specifications as the single = 741. In=20 its latest incarnation it boasts a wider power supply voltage range, a = slew rate=20 50 times as great, and almost twice the output current capability of a = 741,=20 while still retaining the output short-circuit protection feature of the = 741. =20 Op-amps with JFET and MOSFET input transistors far exceed the = 741's=20 performance in terms of bias current, and generally manage to beat the = 741 in=20 terms of bandwidth and slew rate as well.

My own personal recommendations for op-amps are as such: when low = bias=20 current is a priority (such as in low-speed integrator circuits), choose = the=20 3130. For general-purpose DC amplifier work, the 1458 offers good = performance=20 (and you get two op-amps in the space of one package). For an upgrade = in=20 performance, choose the model 353, as it is a pin-compatible replacement = for the=20 1458. The 353 is designed with JFET input circuitry for very low bias = current,=20 and has a bandwidth 4 times are great as the 1458, although its output = current=20 limit is lower (but still short-circuit protected). It may be more = difficult to=20 find on the shelf of your local electronics supply house, but it is just = as=20 reasonably priced as the 1458.

If low power supply voltage is a requirement, I recommend the model = 324, as=20 it functions on as low as 3 volts DC. Its input bias current = requirements are=20 also low, and it provides four op-amps in a single 14-pin chip. Its = major=20 weakness is speed, limited to 1 MHz bandwidth and an output slew rate of = only=20 0.25 volts per =B5s. For high-frequency AC amplifier circuits, the 318 = is a very=20 good "general purpose" model.

Special-purpose op-amps are available for modest cost which provide = better=20 performance specifications. Many of these are tailored for a specific = type of=20 performance advantage, such as maximum bandwidth or minimum bias = current. Take=20 for instance the op-amps, both designed for high bandwidth in Table = =20 below.

High bandwidth operational amplifiers

=20

Model	Devices/ package	Power supply	Bandwidth	Bias current	Slew rate	Output current
number	(count)	(V)	(MHz)	(nA)	(V/=B5S)	(mA)
CLC404	1	10 / 14	232	44,000	2600	70
CLC425	1	5 / 14	1900	40,000	350	90

The CLC404 lists at $21.80 (almost as much as George Philbrick's = first=20 commercial op-amp, albeit without correction for inflation), while the = CLC425 is=20 quite a bit less expensive at $3.23 per unit. In both cases high speed = is=20 achieved at the expense of high bias currents and restrictive power = supply=20 voltage ranges. Some op-amps, designed for high power output are = listed in=20 Table = =20 below.

High current operational amplifiers

=20

Model	Devices/ package	Power supply	Bandwidth	Bias current	Slew rate	Output current
number	(count)	(V)	(MHz)	(nA)	(V/=B5S)	(mA)
LM12CL	1	15 / 80	0.7	1000	9	13,000
LM7171	1	5.5 / 36	200	12,000	4100	100

Yes, the LM12CL actually has an output current rating of 13 = amps=20 (13,000 milliamps)! It lists at $14.40, which is not a lot of money,=20 considering the raw power of the device. The LM7171, on the other hand, = trades=20 high current output ability for fast voltage output ability (a high slew = rate). =20 It lists at $1.19, about as low as some "general purpose" op-amps.

Amplifier packages may also be purchased as complete application = circuits as=20 opposed to bare operational amplifiers. The Burr-Brown and Analog = Devices=20 corporations, for example, both long known for their precision amplifier = product=20 lines, offer instrumentation amplifiers in pre-designed packages as well = as=20 other specialized amplifier devices. In designs where high precision = and=20 repeatability after repair is important, it might be advantageous for = the=20 circuit designer to choose such a pre-engineered amplifier "block" = rather than=20 build the circuit from individual op-amps. Of course, these units = typically=20 cost quite a bit more than individual op-amps.

Discuss this = topic | Feedba= ck =ABPrevious= =20 Page | Next=20 Page=BB

Related Links

Worksheet: Perfor= mance-based=20 assessments for analog integrated circuit competencies
Worksheet: AC = negative=20 feedback opamp circuits
Worksheet: Linear=20 computational circuitry
Worksheet: Negative = feedback opamp circuits
Worksheet: Basic=20 operational amplifiers
Video Lecture: Op Amps = Characteristics=20 (Part 4) - Common vs. Differential - Op Amps and Op Amp = Circuits
Forum: Need = Opamp recommendations
Forum: Low=20 bias current bipolar op amps?
Forum: Help = me=20 understand this example- colpitts oscillator
Forum: Probl= em with=20 measuring the Slew Rate of an LM741CH Op-Amp on=20 Multisim