- #1
physicist 12345
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i want to ask why the feed back at multivibrator must be positive feedback ?
Feedback needed for an oscillator or multivibrator
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Discussion Overview
The discussion revolves around the necessity of positive feedback in multivibrators and oscillators, exploring the underlying principles and mechanisms that enable these circuits to function. Participants reference theoretical concepts, circuit designs, and educational materials related to switching circuits and feedback systems.
Discussion Character
- Exploratory
- Technical explanation
- Conceptual clarification
- Debate/contested
- Mathematical reasoning
Main Points Raised
- Some participants inquire about the fundamental reasons for requiring positive feedback in multivibrators, questioning the stability and self-excitation mechanisms involved.
- One participant references a textbook chapter that describes multivibrators as two-stage amplifiers that operate with positive feedback, indicating a prior understanding of the topic.
- Another participant emphasizes the need for a part of the output voltage to be coupled back to the input without phase shift to achieve positive feedback, linking this to the concept of stability.
- Concerns are raised about how capacitors and resistors contribute to positive feedback in astable multivibrators, prompting requests for circuit diagrams for clarity.
- A participant discusses the implications of infinite gain in amplifiers and how feedback from the output is necessary to maintain oscillation, contrasting positive feedback with negative feedback.
- Mathematical relationships involving feedback and gain are introduced, illustrating how feedback affects the overall gain of an amplifier and its role in oscillation.
Areas of Agreement / Disagreement
Participants express varying levels of understanding and inquiry regarding the mechanisms of positive feedback in multivibrators. There is no consensus on the explanations provided, and multiple viewpoints regarding the role of components in achieving positive feedback remain evident.
Contextual Notes
Some participants reference specific stability criteria such as Nyquist and Barkhausen, indicating a reliance on these concepts for understanding feedback mechanisms. The discussion includes assumptions about prior knowledge of circuit theory and feedback principles.
Who May Find This Useful
This discussion may be useful for students and practitioners interested in electronics, particularly those studying oscillators, multivibrators, and feedback systems in circuit design.
- #2
berkeman
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Can you post links to the reading you've been doing? Why fundamentally do you need positive feedback to form an oscillator?physicist 12345 said:
- #3
physicist 12345
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berkeman said:
i read chapter 18 (switching circuits ) at (principles of electronics by v.k mehta) ..
section 18:10
he mention that the multivibrator is a two stage amplifier that work with a positive feed back .. i can take a screenshot if you need ..
the use of positive feed back at oscillators i already understand ..
- #4
berkeman
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That would help, thank you.physicist 12345 said:
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- #7
LvW
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physicist 12345 said:
To answer this question it is necessary for you to know the meaning of the term "stability" and to be familiar with stability criteria (Nyquist, Barkhausen,).
Intuitively, each harmonic oscillator and each multivibrator needs a kind of self-excitement (autonomous build-up signal amplitudes). For this purpose, a part of the output voltage must be coupled back to the input - without any phase shift: That`s positive feedback.
- #8
physicist 12345
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LvW said:
it seems helpful .. but at multivibrator say- astable- how the capacitor and resistor make positive feed back
- #9
LvW
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physicist 12345 said:
Which circuit are you referring to? With opamp or with transistors?
Please, show the circuit. Otherwise, explanations are problematic.
- #10
Svein
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Here is the simplest possible circuit for an astable multivibrator:
- #11
jim hardy
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Hmmmm
since we're dealing with an introductory textbook
maybe a boiled down answer is best for right now ?
The multivibrator is described as an amplifier
but it has no input
meaning
its gain, output/input, must be infinite.
Since no amplifier can have truly infinite gain , in order to have an output it must get an input from someplace.
That someplace is from its own output.
It must get its input from a fraction of its output and that input must be in a direction to reinforce output not oppose it. And, that is the definition of positive feedback.
Were feedback in a direction to oppose further output it'd soon drive output to zero which is what negative feedback does.
When you study feedback they will explain that
An amplifier with gain A
and feedback B
has gain A/(AB-1)
note that when product AB = +1 the denominator becomes zero so gain becomes infinite and it'll make an output with no input and that's an oscillator..
but if product AB is negative , which is negative feedback, gain is always less than A.
For simplicity set A = 1 and plot gain for B = -1 , -0.5, 0, +0.5 and +1
since we're dealing with an introductory textbook
maybe a boiled down answer is best for right now ?
The multivibrator is described as an amplifier
but it has no input
meaning
its gain, output/input, must be infinite.
Since no amplifier can have truly infinite gain , in order to have an output it must get an input from someplace.
That someplace is from its own output.
It must get its input from a fraction of its output and that input must be in a direction to reinforce output not oppose it. And, that is the definition of positive feedback.
Were feedback in a direction to oppose further output it'd soon drive output to zero which is what negative feedback does.
When you study feedback they will explain that
An amplifier with gain A
and feedback B
has gain A/(AB-1)
note that when product AB = +1 the denominator becomes zero so gain becomes infinite and it'll make an output with no input and that's an oscillator..
but if product AB is negative , which is negative feedback, gain is always less than A.
For simplicity set A = 1 and plot gain for B = -1 , -0.5, 0, +0.5 and +1