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Wien bridge oscillator

Wien bridge oscillator is a engineering topic covered in the lgStudy science library. This page brings together a partial reference excerpt, illustrations, worked examples, real-world applications and a short study plan, so you can understand Wien bridge oscillator rather than just read about it. In short: A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies.

Wien bridge oscillator — main illustration
Wien bridge oscillator — illustration

Key takeaways

  • Wien bridge oscillator belongs to engineering; place it in that map before memorising details.
  • Learn the definition first, then one example that makes the definition concrete.
  • Connect Wien bridge oscillator to a quantity you can measure, compute or draw — that is where exam questions come from.
  • Reproduce the core statement of Wien bridge oscillator from memory before moving on to harder problems.

Reference excerpt

A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies. The oscillator is based on a bridge circuit originally developed by Max Wien in 1891 for the measurement of impedances. The bridge comprises four resistors and two capacitors. The oscillator can also be viewed as a positive gain amplifier combined with a bandpass filter that provides positive feedback. Automatic gain control, intentional non-linearity, and incidental non-linearity limit the output amplitude in various implementations of the oscillator. The circuit shown to the right depicts a once-common implementation of the oscillator, with automatic gain control using an incandescent lamp. Under the condition that R1=R2=R and C1=C2=C, the frequency of oscillation is given by:

f h z = 1 2 π R C {\displaystyle f_{hz}={\frac {1}{2\pi RC}}}

and the condition of stable oscillation is given by

R b = R f 2 {\displaystyle R_{b}={\frac {R_{f}}{2}}}

Background There were several efforts to improve oscillators in the 1930s. Linearity was recognized as important. The "resistance-stabilized oscillator" had an adjustable feedback resistor; that resistor would be set so the oscillator just started (thus setting the loop gain to just over unity). The oscillations would build until the vacuum tube's grid would start conducting current, which would increase losses and limit the output amplitude. Automatic amplitude control was investigated. Frederick Terman states, "The frequency stability and wave-shape form of any common oscillator can be improved by using an automatic-amplitude-control arrangement to maintain the amplitude of oscillations constant under all conditions." In 1937, Larned Meacham described using a filament lamp for automatic gain control in bridge oscillators. Also in 1937, Hermon Hosmer Scott described audio oscillators based on various bridges including the Wien bridge. Terman, at Stanford University, was interested in Harold Stephen Black's work on negative feedback, so he held a graduate seminar on negative feedback. Bill Hewlett attended the seminar. Scott's February 1938 oscillator paper came out during the seminar. Here is a recollection by Terman:

Fred Terman explains: "To complete the requirements for an Engineer's degree at Stanford, Bill had to prepare a thesis. At that time I had decided to devote an entire quarter of my graduate seminar to the subject of 'negative feedback' I had become interested in this then new technique because it seemed to have great potential for doing many useful things. I would report on some applications I had thought up on negative feedback, and the boys would read recent articles and report to each other on current developments. This seminar was just well started when a paper came out that looked interesting to me. It was by a man from General Radio and dealt with a fixed-frequency audio oscillator in which the frequency was controlled by a resistance-capacitance network, and was changed by means of push-buttons. Oscillations were obtained by an ingenious application of negative feedback." In June 1938, Terman, R. R. Buss, Hewlett and F. C. Cahill gave a presentation about negative feedback at the IRE Convention in New York; in August 1938, there was a second presentation at the IRE Pacific Coast Convention in Portland, OR; the presentation became an IRE paper. One topic was amplitude control in a Wien bridge oscillator. The oscillator was demonstrated in Portland. Hewlett, along with David Packard, co-founded Hewlett-Packard, and Hewlett-Packard's first product was the HP200A, a precision Wien bridge oscillator. The first sale was in January 1939. Hewlett's June 1939 engineer's degree thesis used a lamp to control the amplitude of a Wien bridge oscillator. Hewlett's oscillator produced a sinusoidal output with a stable amplitude and low distortion.

Oscillators without automatic gain control

… excerpt ends here. Continue reading the full article.

Illustrations

Wien bridge oscillator: In this version of the oscillator, Rb is a small incandescent lamp. Usually R1 = R2 = R and C1 = C2 = C. In normal operation, Rb self-heats to the point where its resistance is Rf/2
In this version of the oscillator, Rb is a small incandescent lamp. Usually R1 = R2 = R and C1 = C2 = C. In normal operation, Rb self-heats to the point where its resistance is Rf/2
Wien bridge oscillator: Schematic of a Wien bridge oscillator that uses diodes to control amplitude. This circuit typically produces total harmonic distortion in the range of 1–5% depending on how carefully it is trimmed
Schematic of a Wien bridge oscillator that uses diodes to control amplitude. This circuit typically produces total harmonic distortion in the range of 1–5% depending on how carefully it is trimmed
Wien bridge oscillator: Simplified schematic of a Meacham's bridge oscillator published in Bell System Technical Journal, Oct 1938. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors and inductors are considered to be appropriate values for biasing and loading the vacuum tube. Node labels in this figure are not present in the publication.
Simplified schematic of a Meacham's bridge oscillator published in Bell System Technical Journal, Oct 1938. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors and inductors are considered to be appropriate values for biasing and loading the vacuum tube. Node labels in this figure are not present in the publication.
Wien bridge oscillator: Simplified schematic of a Wien bridge oscillator from Hewlett's US patent 2,268,872. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors are considered to be appropriate values for biasing and loading the vacuum tubes. Node labels and reference designators in this figure are not the same as used in the patent. The vacuum tubes indicated in Hewlett's patent were pentodes rather than the triodes shown here.
Simplified schematic of a Wien bridge oscillator from Hewlett's US patent 2,268,872. Unmarked capacitors have enough capacitance to be considered short circuits at signal frequency. Unmarked resistors are considered to be appropriate values for biasing and loading the vacuum tubes. Node labels and reference designators in this figure are not the same as used in the patent. The vacuum tubes indicated in Hewlett's patent were pentodes rather than the triodes shown here.
Wien bridge oscillator: Root locus plot of Wien bridge oscillator pole positions for R1 = R2 = 1 and C1 = C2 =1 versus K = (Rb + Rf)/Rb. The numerical values of K are shown in a purple font. The trajectory of the poles for K=3 is perpendicular to the imaginary (β) axis. For K >> 5, one pole approaches the origin and the other approaches K.[34]
Root locus plot of Wien bridge oscillator pole positions for R1 = R2 = 1 and C1 = C2 =1 versus K = (Rb + Rf)/Rb. The numerical values of K are shown in a purple font. The trajectory of the poles for K=3 is perpendicular to the imaginary (β) axis. For K >> 5, one pole approaches the origin and the other approaches K.[34]

Worked examples

Example 1 — a first encounter with Wien bridge oscillator

Start with the simplest possible case. Write down what Wien bridge oscillator claims or describes in one sentence, then invent the smallest concrete situation in which that sentence is true. In engineering, the smallest case is usually a single object, a single equation or a single measurement. Check that every symbol or term in your sentence has a meaning in that case.

Example 2 — changing one variable

Take the situation from Example 1 and change exactly one quantity: double it, halve it, or set it to zero. Predict what should happen to Wien bridge oscillator before you calculate. Comparing your prediction with the result is the fastest way to find out whether you understand the idea or only the words.

Example 3 — an exam-style question

Typical questions about Wien bridge oscillator ask you to (a) state it precisely, (b) apply it to given data, and (c) explain a limitation. Practise writing all three answers in under five minutes; the third part is what separates a full-mark answer from an average one.

Applications of Wien bridge oscillator

In research
Wien bridge oscillator appears in engineering research whenever the underlying quantities have to be modelled precisely. Papers usually cite it as a starting assumption and then explore where it breaks down.
In technology and industry
Engineering practice reuses Wien bridge oscillator in design rules, simulations and safety margins. Knowing the idea lets you read a specification sheet and understand why the numbers look the way they do.
In the classroom
Wien bridge oscillator is common in secondary-school and first-year university syllabi. It links to neighbouring topics Analog circuits, Electronic oscillators, Electronic test equipment, so understanding it makes those chapters shorter.
In everyday life
Look for Wien bridge oscillator outside the textbook — in sport, cooking, traffic, electronics or the sky above you. An example you found yourself is remembered far longer than one you were given.
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How to study Wien bridge oscillator in 20 minutes

  1. Read the reference excerpt below once, without taking notes.
  2. Close the page and write down what Wien bridge oscillator means in your own words.
  3. Compare your version with the excerpt and mark what you missed.
  4. Work through the three examples above with pen and paper.
  5. Explain Wien bridge oscillator out loud to somebody else — or to Teacher Smith in the lgStudy chat.

Frequently asked questions

What is Wien bridge oscillator in simple terms?

A Wien bridge oscillator is a type of electronic oscillator that generates sine waves. It can generate a large range of frequencies.

Why does Wien bridge oscillator matter?

Because it connects several engineering ideas at once: it gives you a definition you can apply, a quantity you can calculate, and a way to check whether a result is plausible.

How should I study Wien bridge oscillator?

Read the excerpt, restate it from memory, then work through the examples and applications listed on this page. The five-step study plan above takes about twenty minutes.

What does this page cover?

It gives you a compact reference excerpt plus original lgStudy explanations, examples, applications and study material on Wien bridge oscillator.

Tags

  • Analog circuits
  • Electronic oscillators
  • Electronic test equipment

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