Parallel Resistance into Emitter Follower

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Discussion Overview

The discussion revolves around the behavior of resistances in an emitter follower circuit, particularly focusing on how a capacitor (C1) interacts with resistors (R1 and R2) in terms of parallel resistance when considering AC signals. The participants explore the implications of AC versus DC in this context, seeking clarity on the underlying principles.

Discussion Character

  • Exploratory
  • Technical explanation
  • Debate/contested

Main Points Raised

  • One participant notes that the resistance seen by C1 is the parallel resistance of the resistance looking into the base and the voltage divider, specifically mentioning that the resistance looking into the base is (hfe x re) which is 750k.
  • Another participant explains that to an AC voltage, R1 and R2 appear to be in parallel, as the power supply presents low impedance to AC.
  • A participant questions whether this reasoning holds for DC, expressing confusion about the differences between AC and DC behavior in this context.
  • It is stated that DC does not pass through the capacitor, rendering it irrelevant for the analysis of C1.
  • Another participant seeks clarification on why AC current perceives R1 and R2 as parallel resistance, questioning the equipotential requirement for parallel resistances.
  • One participant suggests drawing a capacitor in the diagram between the collector of Q1 and ground to visualize the low impedance path for AC.
  • A participant describes the voltage source as having very low impedance, acting like a short for both AC and DC, and explains how this affects the voltage divider and coupling capacitor.
  • Despite the explanations, some participants express ongoing confusion and seek alternative ways to understand the concept.

Areas of Agreement / Disagreement

Participants generally agree that the behavior of the circuit differs between AC and DC, but there is no consensus on the specifics of how R1 and R2 are perceived as parallel by C1, and the discussion remains unresolved regarding the underlying principles.

Contextual Notes

Participants express uncertainty about the assumptions regarding AC and DC behavior, and there are unresolved questions about the equipotential condition for parallel resistances in the context of this circuit.

Adder_Noir
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Hi,

Please have a look at the following image:

http://img70.imageshack.us/img70/4008/parallelryl3.jpg"

I have a problem that the book states the resistance seen by C1 is the parallel resistance of the resistance looking into the base and that of the voltage divider. The resistance looking into the base is (hfe x re) in this case 750k. No problems there I understand how that one is derived.

I really don't get however how R1 and R2 are seen by the capacitor as a parallel resistance? The book evaluates R1 and R2 in parallel and then treats them as a single resistance. It then puts said resistance in parallel with the 750k.

I understand all that but from C1's perspective why is R1 included and how can it possibly see R1 and R2 as a parallel part of a parallel load?

Also does anyone know how to post up pictures instead of links?

I'm a bit of a dummy in electronics so please if you wouldn't mind keep the replies simple for my inadequate experience to understand :wink:
 
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To an AC voltage, which is the only thing that matters when considering C1, R1 and R2 appear to be in parallel. The power supply appears to be a low impedance to AC.
 
But this would not hold for DC right? Why does it hold for AC then. Sorry to ask what may sound a stupid question.
 
DC will not make it through the capacitor so it is irrelevant.
 
Averagesupernova, already said that it only matters to AC. What does a cap do to DC?

EDIT: a little slow on the response there.
 
Averagesupernova said:
DC will not make it through the capacitor so it is irrelevant.

I see that point thanks. In fact I just answered someone else's question eleswhere less than a few hours ago and myself pointed that out lol. Crazy how I forget stuff sometimes but I just have to do the best I can with my dodgy mind :smile:

Ok so can you or anyone else explain why an AC current sees it as a parallel resitance? Doesn't a parallel resistance have to be equipotential across both resistances? R1 and R2 don't obey this rule at all.
 
There is a low impedance to AC between power supply + and power supply ground. Draw a capacitor in your diagram between the collector of Q1 and ground. See it now?
 
Averagesupernova said:
There is a low impedance to AC between power supply + and power supply ground. Draw a capacitor in your diagram between the collector of Q1 and ground. See it now?

To be honest no but I'll go and do some work on that concept now and see if I can grasp it. You guys are just too damn clever, how you can interpret these diagrams so easily I just don't know :confused:
 
You guys are just too damn clever, how you can interpret these diagrams so easily I just don't know

Cuz we've been at it for a while. :smile:
 
  • #10
A voltage source has a very low impedance, that means it acts like a short (meaning just a piece of wire) for DC and AC voltages.

That means electrons can flow right through a battery as if it has 0 resistance, only to be pushed again.

Having said that, just look at the voltage divider and the coupling capacitor. First replace the power source of the voltage divider by a short, or a piece of wire. By introducing AC through the capacitor, the AC sees only two resistors R1 and R2 as being tied down together in parallel.

Hope that helps.
 
  • #11
what said:
A voltage source has a very low impedance, that means it acts like a short (meaning just a piece of wire) for DC and AC voltages.

That means electrons can flow right through a battery as if it has 0 resistance, only to be pushed again.

Having said that, just look at the voltage divider and the coupling capacitor. First replace the power source of the voltage divider by a short, or a piece of wire. By introducing AC through the capacitor, the AC sees only two resistors R1 and R2 as being tied down together in parallel.

Hope that helps.

To be honest I still don't get it. I'll have another look today and see if I can get any closer. Thanks for the reply :wink:
 
  • #12
I'm still stuck. Is there any other way I can look at it?
 

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