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What Does an Op Amp Do?
October 1969 Popular Electronics

October 1969 Popular Electronics

Popular Electronics Cover, October 1969 - RF CafeTable of Contents

Wax nostalgic about and learn from the history of early electronics. See articles from Popular Electronics, published October 1954 - April 1985. All copyrights are hereby acknowledged.

GAP/R K2-W vacuum tube operational amplifier - RF Cafe

GAP/R K2-W vacuum tube operational amplifier.

Here is an 10-question quiz on the basics of operational amplifiers (aka op amps, or opamps, compliments of Mr. John Seginski, in the October 1969 issue of Popular Electronics magazine. The first commercial opamp, the μA709, was introduced by Fairchild Semiconductor in 1963 (designed by Bob Widlar). Five years later they released the μA741, which was the most famous opamp of the era; it is still widely used today. One of my electronics circuit courses in college (c1987) used the 741 as the basis for analyzing integrated circuits, and in particular, opamps. A couple companies offered vacuum tube operational amplifiers which combined two or more tubes along with some integrated leaded biasing and compensating components, and had a tube sock pinout on the bottom for plugging into a standard socket. The GAP/R K2-W, produced by George A. Philbrick Researches (hence the GAP/R prefix), shown at the left is an example. It was commonly advertised in electronics magazines of the era. Today, just about everyone having any involvement knows the basic equations for calculating opamp gain for both inverting (Rfeedback/Rinput) and non-inverting (Rfeedback/Rgnd +1) configurations.

Op Amp Quiz

What Does an Op Amp Do?, October 1969 Popular Electronics - RF CafeBy John Seginski

That versatile building block, the operational amplifier, is the mindspring of much of today's most advanced electronic equipment. In computers and automatic control systems, for example, they are indispensable, and they can be made to function in a number of different ways - precision voltage sources, current sources, and voltage adders, to name only a few.

The operational amplifier, or op amp, is an extremely high -gain amplifier with a very high input impedance. The actual gain of a specific circuit is determined by feedback resistors connected around the amplifier. Various characteristics can be achieved is generated. by connecting other components and other op amps in the circuit. The numbered sentences and equations that follow refer to the circuits below. Test your knowledge of op amps by filling in the blanks.

1. Eout =  ______

2. Gain =  ______

3. Eout = ______

4. This is a ______ generator. 

5. Gain =  ______

6. This is a ______ voltage amplifier. 

7. This is an ______

8. This is a ______

9. When switches are closed one by one, ______ are generated.

10. The outputs are ______.

See answers below.

Quizzes from vintage electronics magazines such as Popular Electronics, Electronics-World, QST, and Radio News were published over the years - some really simple and others not so simple. Robert P. Balin created most of the quizzes for Popular Electronics. This is a listing of all I have posted thus far.

RF Cafe Quizzes

Vintage Electronics Magazine Quizzes

Vintage Electronics Magazine Quizzes

Op Amp Quiz Answers

1. The gain of an operational amplifier as shown can be determined by dividing the value of the feedback R1 by the value of the input resistor R15. Therefore the gain of the circuit is 1000/500=2. Note that the input connects to the input terminal marked (-) and the output connects to the output terminal marked (+). The circuit is thus an inverting amplifier. Since the input is -5 volts, Ecl is +10 volts.

2. The gain of this inverting amplifier is also determined by dividing the feedback resistance 'by the input resistance. Thus, gain=100/ 1000=0.1. Since the amplifier inverts, the gain is actually -0.1.

3. Operational amplifiers can be used to "sum" or algebraically add the voltages at their inputs. In this circuit, the gain of each input leg is one. The output voltage is the sum of the amplified inputs and is inverted. Thus Eour is +6 volts.

4. Operational amplifiers can be used as current generators if the current in the feedback loop is utilized. The load must be connected into the feedback loop as shown in the circuit.

5. The gain of the voltage follower is one. The input impedance of this circuit is very high and the output impedance is very low. The circuit is noninverting.

6. Notice that E15 connects to the input terminal marked (+) and EvT connects to the output terminal marked (+). Therefore this is a non -inverting amplifier.

7. The circuit is an integrator because the effective output is that which appears across the capacitor.

8. The circuit is a differentiator because the effective output is that which appears across the resistor.

9. As the switches are closed one by one, the inputs are inverted and summed, one on top of the other in sequence, generating a positive staircase waveform.

10. The first of these two unity -gain amplifiers inverts the input signal. The second inverts the output of the first. Therefore, the outputs are balanced.

 

 

Posted September 25, 2023

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RF Cafe began life in 1996 as "RF Tools" in an AOL screen name web space totaling 2 MB. Its primary purpose was to provide me with ready access to commonly needed formulas and reference material while performing my work as an RF system and circuit design engineer. The World Wide Web (Internet) was largely an unknown entity at the time and bandwidth was a scarce commodity. Dial-up modems blazed along at 14.4 kbps while tying up your telephone line, and a nice lady's voice announced "You've Got Mail" when a new message arrived...

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