Operational Amplifiers demystified – starting with the uA741
As a writer of some of the articles here we do try to keep the topic off common ground, throw in a bit of humour but give the reader some insight into components and circuit design. Op-amps we treat no differently.
Today we look at op-amps in general and specifically look at the brain child in 1968 of British Fairchild engineer, Dave Fullagar, the incredible 741 op-amp.
(note the charity work done by Mr. and Mrs. Fullager in South Africa)
In simple terms an operational amplifier is a DC coupled voltage amplifier which has a differential input and a single ended output.
GAP/R George A Philbrick Researches – currently Microchip
The world’s first commercial op-amp was known as the K2-W, manufactured from 1952 until 1971.
The first tube op-amps were known as general purpose, high gain, DC coupled, inverting feedback amplifiers. Although many experimenters see these devices as being rather simple it’s only when one looks at the GAP/R P65, an early discrete component solid state op-amp designed and built by Alan Pearlman, co founder Nexus, that we note the complexity of the circuit. Pricing for the later P45 was in the region of U$120.00 each.
The company veered clear of Germanium transistors and only started pushing solid state once silicon transistors proved reliable.
About the time of GAP/R startup Robert Page Burr and Thomas Brown started a company, Burr-Brown (of great audio fame) which was purchased out by Texas Instruments in 2000. Burr-Brown is known to manufacture some of the best quality operational amplifiers used in analog circuitry and DSP / electronic signal processing.
So what’s the interest in Op-Amps?
Well, recently we ran an article on the not-so-dreaded 4558. I mean what’s the point of shooting down a 1970s chip in 2017 for poor performance? An Op-Amp has made out life easier, it’s made designing cheaper, it’s made audio simpler and there’s very little one cannot do with it. And it’s analogue. Well mostly, of course.
We can multi-vibrate, add, subtract, differentiate (or), compare (comparator), integrate (or), filter, chop, amplify, invert and …. the list goes on. Op-amps are used in digital circuits as well as analogue. Op amps are temp stable. Gain calculations are simple, in the op-amp (741) image above the gain is equal to R2/R1. The output is in phase. Input to the (-) negative terminal inverts the phase.
What more can one want from a device which in open loop mode (without feedback resistors, gain can be anything from 10 000 to 100 000 (or higher).
Many manufacturers now give open loop gain for an op-amp as V/mV. E.g. 10V/mV = 10 * 1000 or 10 000. The 741 shows a typical open loop gain of 200 000.
Some historical and other facts
Tube negative feedback devices were in use pre 1940s.
The op-amp 709 was plagued by issues, instability, latching and self-destruction was not uncommon. The 741 was the fix.
The uA741 was commercialised in 1968, possibly the largest selling op-amp. They are also amongst the the cheapest – about R8.00 at Yebo.
The opamp cannot live in a world without feedback.
The input (voltages) will try to remain the same through the feedback path, if not, the output will swing towards the polarity of the most positive input e.g. if invert is high, then the output goes negative and vice versa. This forms the basis of many configurations.
Walt Jung – Chapter H, Op Amp History
Philbrick Archives – GAP/R and the commercialisation of opamps