Is there anybody built a non feedback amplifier??

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#201
H & H say that emiter degeneration is a FORM of NFB

OK - we can see where thet are coming from but...

This FORM of NFB is very different from the classic op to input NFB

emitter degeration does increase linearity same as classic NFB

but there are some crucial fundemental differences.

ED will reduce open loop gain and increase stabilty.

classic NFB will not affect OLG and will always reduce stability to some extent.

also ED happens in real time, NFB always has a time lag - hence stability issues

They are completely different animals. Calling these two animals by the same name is obviously a source of confusion.

For this reason I suggest that from now on that we refrain from calling emitter degeneration feedback and just call it ED.

agreeing terms like this means we can get on with creative and intelligent discussion to further out mutual goals without constant misunderstanding.

regards

mike
 
#203
Steve Eddy said:


Well, no. Followers don't have voltage gain. The reason for that, according to a number of sources I've read, is because of feedback.



It's not? Why not?

Take a grounded emitter common-emitter stage. What happens to its voltage gain as you add some emitter resistance? The gain drops. Why? Again, according to a number of sources I've read, it's because of feedback.

Horowitz & Hill for example, in the second edition of The Art of Electronics, chapter 2, p. 84, "Emitter resistor as feedback."

Adding an external series resistor to the intrinsic emitter resistance re (emitter degeneration) improvesd many properties of the common-emitter amplifier, at the expense of gain. You will see the same thing happening in Chapters 4 and 5, when we discuess negative feedback, an important technique for improving amplifier characteristics by feeding back some of the output signal to reduce the effective input signal. The similarity here is no coincidence; the emitter-degenerated amplifier itself uses a form of negative feedback. Think of the transistor as a transconductance device, determining collector current (and therefore output voltage) according to the voltage applied between the base and emitter; but the input to the amplifier is the voltage from base to ground. So the voltage from base to emitter is the input voltage, minus a sample of the output (IERE). That's negative feedback, and that's why emitter degeneration improves most properties of the amplifier (improved linearity and stability and increased input impedance; also the output impedance would be reduced if the feedback were taken directly from the collector).

So why is the emitter resistor providing negative feedback in the case of the common-emitter amplifier but suddenly it stops providing negative feedback just because you've removed the collector resistor and take your output across that same emitter resistor?

Unless Horowitz & Hill are just talking out of their collective *****, it seems to me that removing the collector resistor simply results in the amount of feedback going from something less than 100% to 100%.

se
Click to expand...


Dear SE,

I have Horowitz & Hill The Art of Electronics second edition in front of me, what your are citating is 2.12 The common-emitter amplifier revisited starting on page 82, if you look at picture 2.37 you can see that an common emitter output is NOT at the emitter, it's at the collector, ok?!!
Of course an emitter resistor adds a feedback in this case, I hope this is fundamental common knowledge!

An example, let us add a very big resistor on the collector in an emitter follower, guess what happens with the transistor if you force the base up in an order to drive a very heavy load.... give me an answer on this SE!

So, when you now have Horowitz & Hill in front of you, why don't you go to page 94 2.16 Darlington connection'


Edit:

For some 17 years ago I built an ingnition coil amplifier for cars with BU 208, that transistor have an Hfe of approx 1 (yes, you red i right, ONE!), you can also tell what feedback I have in such a transistor as an emitter follower... tell me that!
 
#204
I will state the obvious again- both ED and EF use negative feedback. Not a "special kind of something which looks like a negative feedback but requires a different understanding because of the ... mmm... quantum" 🙂, but a very simple plain classic example of a proper negative feedback.

I think the confusion happens here mostly because it is not clear what is an open loop gain in this case. However the answer is simple - an open loop gain in EF is exactly the same as the gain of CE amplifier with the same value load resistor.

Imagine an op-amp set up by a couple of feedback resistors for a gain of 10. How we will measure the open loop gain for it? It will be the ratio of the output voltage change to the voltage change between the inputs of the opamp - i.e. control voltage. Control voltage for a transistor, doesn't matter in what connection, is always Vbe. You would not take the full input voltage in the opamp example to calculate an open loop gain figure, why should you do it for an EF?

The only difference between EF and CE is the way we look at the groung connection. From AC point of view, the input voltage source for EF is connected between collector and base , providing for 100% negative feedback through the low impedance of the source, the load resistor sits in series with the power supply so from AC point it is load for the collector current, same as in CE.

x-pro.
 
#206
Yes, the transconductance of the transistor is unchanged. But the transconductance of a circuit with emitter degeneration is measured by taking the voltage between base and the lower end of the resistor. So the Ic/Vb decreases as the degeneration resistance increases.

The upshot is that the contribution of the transistor's gm to the circuit's gm is reduced, much like a potential divider, by the emitter resistor. Can this effect be meaningfully called negative feedback? Not in my opinion.
 
#207
x-pro said:
I will state the obvious again- both ED and EF use negative feedback. Not a "special kind of something which looks like a negative feedback but requires a different understanding because of the ... mmm... quantum" 🙂, but a very simple plain classic example of a proper negative feedback.

I think the confusion happens here mostly because it is not clear what is an open loop gain in this case. However the answer is simple - an open loop gain in EF is exactly the same as the gain of CE amplifier with the same value load resistor.

Imagine an op-amp set up by a couple of feedback resistors for a gain of 10. How we will measure the open loop gain for it? It will be the ratio of the output voltage change to the voltage change between the inputs of the opamp - i.e. control voltage. Control voltage for a transistor, doesn't matter in what connection, is always Vbe. You would not take the full input voltage in the opamp example to calculate an open loop gain figure, why should you do it for an EF?

The only difference between EF and CE is the way we look at the groung connection. From AC point of view, the input voltage source for EF is connected between collector and base , providing for 100% negative feedback through the low impedance of the source, the load resistor sits in series with the power supply so from AC point it is load for the collector current, same as in CE.

x-pro.
Click to expand...


Hi x-pro

I would really like get this things sorted out with all interested parties so that we can actually get on and make some progress here.

I get the feeling that we are not disagreeing here but seem to be caught up in a crazy discussion about what names we give things.

in my post 201 do you accept lines 6 - 8 i.e. is that there is a difference between FB as in ED and global FB that goes from o/p back to i/p.

If so then do you see what charles and I are trying to achieve here ?

we just want give these things different names so that we can discuss things without confusion.

If you have some fixed ideas on this, OK, just let us know and we can all adopt your terminology but please lets come to some common agreement before I go compltely mad !!!!!

or perhaps we should just call every cct element or feature "Brian"
that would be simple. What do you think ? Good idea ?
 
#208
traderbam said:
The upshot is that the contribution of the transistor's gm to the circuit's gm is reduced, much like a potential divider, by the emitter resistor. Can this effect be meaningfully called negative feedback? Not in my opinion.
Click to expand...

OK. Let's refrase the same for the opamp circuit:

The contribution of the opamp gain to the circuit gain is reduced, much like a potential divider, by feedback resistors. Can this effect be meaningfully called negative feedback?

🙂

x-pro
 
#210
x-pro said:


No. In my view there is no difference in essence. Check in a good reference book - there are more than one type of negative feedback, but they all still do the same thing.

x-pro.
Click to expand...

but ED does not just do one thing. the features of increased ED are

increased linearity
decreased open loop gain
no change in closed loop gain
increased stability

likewise global FB does not do just one thing. the features of increased GFB are

increased linearity
no change in open loop gain
reduced closed loop gain
decreased stability

so the only 'same thing' is linearity - the other effects are different
and crucially to the sound of the amp stability is affected differently.


anyway if you think Charles, Traderbam and I are are totally deluded, misguided or just simply insane, perhaps you will just take pity on us and leave us to have our weird and deluded conversation in peace.

chou
 
#212
mikelm said:
anyway if you think Charles, Traderbam and I are are totally deluded, misguided or just simply insane, perhaps you will just take pity on us and leave us to have our weird and deluded conversation in peace.
Click to expand...

What's with all the drama?

Why do some people seem to have to personalize everything?

What's at issue here is whether a certain thing is or is not a certain way. That's it. It's not about who's deluded, misguided or just simply insane.

*sigh*

se
 
#213
Steve Eddy said:


What's with all the drama?

Why do some people seem to have to personalize everything?

What's at issue here is whether a certain thing is or is not a certain way. That's it. It's not about who's deluded, misguided or just simply insane.

*sigh*
Click to expand...

Wow you are really deluded, Steve Eddy. Literally 10 pages ago I posted that there are more types of feedback than we have terms for. Furthermore that in the field of high performance audio that it is useful to distinguish between these different types of feedback. I went on to explain why it was useful to do so. Mike Elm has also give reasons why it is useful to do so.

Nobody has argued that these are incorrect reasons. Instead they just keep insisting that "feedback is feedback" on and on and on like broken record. It got so bad that the moderator split off the "semantics" part into a different thread.

But certain people keep insisting on arguing about whether we should call it "feedback" or "degeneration" or "no feedback" or whatever. You are one of those people Steve Eddy. You have helped turn this thread that was originally started by someone looking for circuit ideas for a no feedback amp into a cesspool.

The issue CANNOT be about "whether a certain thing is or is not a certain way" unless we can agree on our definitions. You love to argue, and this time you are arguing about definitions of words.

And *then* you have the audacity to pretend like you have no idea what is going on. (*sigh*)

I think that is "deluded, misguided or just simply insane". I would even add "disgusting" to that list.

SY asked me to refrain from personal comments, and I failed to do so. I'll probably be banned. But you know what Steve? I don't care. It's simply not worth it to me to hang around a place that tolerates your BS.

Charles Hansen
 
#214
My last word on the subject :angel: :whazzat:

A system is "open-loop" if the output quanity does not affect the input quantity.

You will see what I mean with regard to either degeneration or a simple resistor divider chain if you first identify what the input is, then the output, then see whether the output changes the input.
 
#216
traderbam said:
My last word on the subject :angel: :whazzat:

A system is "open-loop" if the output quanity does not affect the input quantity.

You will see what I mean with regard to either degeneration or a simple resistor divider chain if you first identify what the input is, then the output, then see whether the output changes the input.
Click to expand...

But in a case of a EF if you make a disturbance in the emmiter of the output transistor...this disturbance change the conditions of the input transistor of the EF.

I have made many experiences driving a amp via a 8 Ohms resistor from a channel in the other (for to see the influence of the EMF in the amp)

And if you do that you will see that a disturbance in the output of a EF will change the conditions of the input.

Anyway the some thing happen in a CFP.
 
#217
Jorge,
The thing is to clearly identify input and output. I'm not sure what you are using.
For example: voltages are always relative to two points. So if you change the voltage at the emitter (the voltage measured between emitter and some point X) then does this affect the input voltage (the voltage between base and point X)?
 
#219
traderbam said:
My last word on the subject :angel: :whazzat:

A system is "open-loop" if the output quanity does not affect the input quantity.
Click to expand...

Sure. And a grounded-emitter common-emitter stage (assuming no shunt feedback) would be an "open loop" amplifier.

Add an emitter resistor and you have a closed loop amplifier whose closed loop gain is the ratio of the collector resistor and the emitter resistor by way of RC/RE.

Which is not unlike an opamp configured as an inverting amplifier.

With the emitter-degenerated common-emitter, gain is:

-RC/RE.

With an inverting opamp it's:

-RF/RIN.

With the common-emitter, the output point and the feedback point are two different nodes. And so it is with the inverting opamp.

Now make a common-collector amplifier by removing the collector resistor and leaving the emitter resistor and taking your output from the emitter rather than the collector.

Now the gain is 1. VOUT = VIN

Also, the output point and the feedback point are no longer at two different nodes but now they are the same node.

But if you take an opamp and feed its output straight into its inverting input, you also end up with an amplifier whose gain is 1, VOUT = VIN. Further, output point and the feedback point are the same node, just as with the common-collector amplifier.

Now, I don't think anyone would argue that an opamp configured as a buffer is an open loop amplifier. Nor do I think anyone would argue that the gain within the loop is the open loop gain of the opamp. That the amount of negative feedback euqals the open loop gain of the opamp. So if the opamp's open loop gain is 100dB, you have 100dB of negative feedback.

I fail to see how this concept is fundamentally any different with regard to a transistor configured as an emitter-follower.

se

Correction: When I said "Nor do I think anyone would argue that the gain within the loop is the open loop gain of the opamp" I meant to say "Nor do I think anyone would argue that the gain within the loop is NOT the open loop gain of the opamp."
 
#220
traderbam said:
Jorge,
The thing is to clearly identify input and output. I'm not sure what you are using.
For example: voltages are always relative to two points. So if you change the voltage at the emitter (the voltage measured between emitter and some point X) then does this affect the input voltage (the voltage between base and point X)?
Click to expand...

Yes..i see...but we usually in amp design use a EF as the output stage...and what i have said in my previus post...really happen.

If you disconnect the overall feedback and put the scop prob at the colector of the VAS transistor...if you provock a disturbance at the output...you will see the some disturbance (minus the current gain) reflected in the collector of the VAS...a form of feedback!

Sorry for my English ...😉
 
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