bigdogguru
Administrator
- Joined
- Mar 12, 2010
- Messages
- 9,821
- Helped
- 2,350
- Reputation
- 4,694
- Reaction score
- 2,272
- Trophy points
- 1,413
- Location
- Southwest, USA
- Activity points
- 62,386
There is a big difference with the concept of FEEDBACK and NEGATIVE FEEDBACK. We are discussing NEGATIVE FEEDBACK
It would be interesting to hear about his approach to explain this effect.
Well, if you concede that feedback is involved, but still insist that there's no negative feedback, then what are you proposing? That it's positive feedback?:shock:
I didn't bring it up, LvW did. I just asked you to clariy what you wrote in post 19....so the term FEEDBACK should not be used.
AND IT IS NOT USED. It is only you that has brought up this absurdity.
Feedback implies a signal taken from a section of a circuit is fed back to a previous section and this is not the case. It is just a relationship-law or a determination that involves the gain of the transistor.
And it is obviously not a NEGATIVE FEEDBACK relationship as found in the common emitter configuration nor is it a positive feedback so the term FEEDBACK should not be used.
AND IT IS NOT USED. It is only you that has brought up this absurdity.
Show me one book where it states the emitter resistor provides NEGATIVE FEEDBACK.
You are getting confused with the common emitter stage where the emitter resistor provides EMITTER DEGENERATION.
That's specifically referring to emitter degeneration in a common emitter amplifier, though. I don't think Colin has any argument with that, only with applying the same logic to common collector amps...."The art of Electronics" (Horowitz/Hill).
Here is what they say in chapt. 2-12...
That's specifically referring to emitter degeneration in a common emitter amplifier, though. I don't think Colin has any argument with that, only with applying the same logic to common collector amps.
IMHO, it's interesting to consider the question of feedback in a phase splitter circuit where outputs are taken from the emitter and the collector of the same transistor. i.e. the transistor is acting as common-emitter and emitter-follower at the same time. I don't think one could reasonably claim that one output uses feedback but the other doesn't. If one did, one would be hard pressed to explain how only the output at the collector has feedback, when it is the output voltage at the emitter which is fed back to the input.
:fight:
Yes, agreed. That's pretty much the point I was trying to make, in a round about way, and apparently with a notable lack of success. :smile:It also does not matter if we use Ve as an output or not or together with a collector voltage. In any case, an output quantity (current Ic) acts back to the input with a negative sign. And that´s exactly what we call "negative feedback".
It should be noted that a CE amplifier with emitter degeneration is implementing a different kind of feedback than the CC stage.
The former is current controlled voltage ("series-series") feedback while the latter is voltage controlled voltage ("parallel-series") feedback.
Yes- of course 100% agreement.The original post asks about the reason for the high input impedance of a transistor amplifier in CC configuration. The answer is voltage or xx-series feedback and applies also to CE stage with emitter degeneration. In so far it's O.K. to refer to the latter in many previous posts. Generally (negative) voltage feedback increases the input impedance.
Again agreed.The difference, voltage versus current controlled (parallel-xx versus series-xx) feedback matters for the output impedance.
Emitter degeneration increases the CE output impedance while voltage controlled feedback causes the low output impedance of the CC stage.
Agreed.I would say: ...causes the lowering of the output impedance.
I prefer a different viewpoint. The feedback is not provided by Re, it also works with a current source load. The feedback is constituted by the way how the transistor is connected to the load, which results in Vbe = Vin-Vout. In so far, a CC stage without feedback isn't possible.I would say: ...causes the lowering of the output impedance.
What is the output impedance of the CC stage without voltage controlled feedback (Re=0)? Difficult to answer, I think, because in this case, the emitter is grounded.
Yes - the same applies to another unity-gain circuit: An opamp with unity gain (at low frequencies) without negative feedback is also not possible.Agreed.
I prefer a different viewpoint. The feedback is not provided by Re, it also works with a current source load. The feedback is constituted by the way how the transistor is connected to the load, which results in Vbe = Vin-Vout. In so far, a CC stage without feedback isn't possible.
But at which node is this CC output impedance (without feedback) defined ?In my view, the CC output impedance "without feedback" would be equal to the output impedance of a transistor in CE configuration.
I think the question is in so far hypothectical as a CC stage without voltage feedback can't be realized. In the equivalent circuit of a CC stage, we have the BJT model comprised of rbe, gm and rce. If the input voltage is applied without feedback, directly to rbe, the output impedance is rce, or infinite if rce is ignored.But at which node is this CC output impedance (without feedback) defined ?
You are completely wrong in your reasoning.
I have been teaching electronics for 40 years and written 25 books with sales of over 750,000 copies. My website has reached 22,000,000 visitors.
No-one has yet said I have been wrong.
Hi Colin55, since I am interested in some of the mentioned 25 books, I started a search on Amazon´s site - but I failed.
Not a single publication from you could be found.
Please, can you tell me where I can find these books? Are they in printed form? Which publisher?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?
We use cookies and similar technologies for the following purposes:
Do you accept cookies and these technologies?