Am I confusing input & output impedance?

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Discussion Overview

The discussion revolves around the design of a common-emitter amplifier, specifically focusing on the concepts of input and output impedance, as well as the implications of transistor operation modes on these parameters. The context includes theoretical considerations and practical design aspects related to amplifier circuits.

Discussion Character

  • Homework-related
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant expresses confusion about why the collector resistor is set equal to the specified output impedance, suggesting a misunderstanding of how output impedance is defined in this context.
  • Another participant clarifies that the output impedance of a common-emitter amplifier is approximately equal to the collector resistor, linking this to the requirement of 4.7kOhms.
  • A further contribution notes that the exact output resistance is a parallel combination of the transistor's internal output resistance and the collector resistor, indicating that the internal resistance is typically much larger.
  • Questions arise regarding the implications of the transistor being in saturation mode, with one participant questioning whether the internal output resistance should be considered in that scenario.
  • Responses indicate that the transistor should ideally operate in linear mode for amplification purposes, with concerns raised about the effects of saturation on output behavior.

Areas of Agreement / Disagreement

Participants express differing views on the operation mode of the transistor in the context of amplifier design, particularly regarding saturation. While some assert that the transistor should remain in linear mode, others acknowledge scenarios where saturation might occur, leading to unresolved discussions about its implications.

Contextual Notes

There are unresolved assumptions regarding the definitions of input and output impedance, as well as the conditions under which the transistor operates (linear vs. saturation mode). The discussion does not reach a consensus on how these factors influence the design of the amplifier.

iScience
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Homework Statement



design a common-emitter amplifier with an output impedance of 4.7kOhms and again of-10 using a transistor with a beta=200 powered by a 12V DC supply.

Homework Equations



v=ir

The Attempt at a Solution



------------------------------------------------------
rest of the problem i get; just this one thing bugging me.
the solution starts out with solving for the collector current. I understand why the output is taken at the collector, but i don't get why they set R_c equal to the given output impedance. i thought the output impedance was the equivalent resistance as seen from where ever the "output" was being taken (in this case the collector).
 
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Perhaps think of a amplifier as a voltage source that has some internal resistance (capacitance or inductance). The output impedance is a measure of how near the output is to an ideal voltage source. If the output impedance is very low then the load (the input impedance of the next stage) will have little effect.

The output impedance of a common emitter amp is roughly equal to the collector resistor. The required output resistance was specified as 4.7K so they set the collector resistor at that value.
 
CWatters said:
The output impedance of a common emitter amp is roughly equal to the collector resistor. ...

...because the exact output resistance is ro||Rc. And the internal BJT output resistance ro is very large (20...50 kohms).
 
but if we were in saturation mode would we take into account r0?
 
Why would the transistor be saturated? It should be in a linear mode.
 
high power switch?
 
Well yes in a switch the transistor might be saturated but normally you would hope it's not saturated in an amplifier. What happens to the output if it does?
 
iScience said:
but if we were in saturation mode would we take into account r0?
In your question, you spoke about a common-emitter amplifer, didn`t you?
 

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