Q: feedback and input impedance in a emitter follower

[Eeyore]

No, It's not at problem to calculate the inputimpedance in EW9, so what I am
looking for is some algebra to do it by the hand in the designing phase
after I have calculatet the resistorvalues for this kind of circuit. As
Charles Schuler said it must be standard textbook procedure, but I can't
find any algebra and procedures anywhere that goes deeper than describing
the very simple rules about feedback (the four basic methods, etc.).

Unbelievable !

Graham

 

[Peter Andersen]

Your analysis is so far wrong I can't imagine where you got that idea.

R6 is the dominant term in the input impedance in that circuit.

Yes - but instead of writing all these reply's with all that noise you can
answer the basic question: WHY is R6 the dominant input impedance?

Peter

 

[Eeyore]

Yes - but instead of writing all these reply's with all that noise you can
answer the basic question: WHY is R6 the dominant input impedance?

Because it's feeding a 'virtual earth' as a result of the negative feedback
from R5. That's why the figures for input impedance with and without the
feedback network in place are so hugely different.

Basic electronics.

You've ben reading the wrong books or paying too much attention to university
professors and their waffling theory who know bugger all about real circuits. I
don't even need a data sheet to solve this kind of thing although some simple (
valid ) assumptions need to be made.

It should take no more than 5 minutes on a piece of paper.

It's a lousy configuration for noise figure too btw.

Graham

 

[The Phantom]

And you reckon you need these for a simple analysis ?

Where did I say I was doing a *simple* analysis?

The OP asked "...how to do this right and how to
calculate the input impedance, gain and so on...".

My purpose was to show one way to "do this right".

 

[The Phantom]

Yes - but instead of writing all these reply's with all that noise you can
answer the basic question: WHY is R6 the dominant input impedance?

He's pointing that to a certain level of approximation your circuit with
feedback is similar to an inverting op amp with a 10K feedback resistor and
1K input resistor. The base of the transistor is, to some extent, a
"virtual earth". So, as a first approximation, you could expect the input
impedance of the circuit to just be the resistance of R6, 1K ohm. And the
gain of the circuit to be -R5/R6, or -10.

However, a more accurate analysis shows the gain to be about 10% lower
than that, and the input impedance to be about 5 % higher. Your measured
values also show about that level of error in the approximate values.

Without feedback, R6 is not so dominant.

 

[Eeyore]

Where did I say I was doing a *simple* analysis?

The OP asked "...how to do this right and how to
calculate the input impedance, gain and so on...".

My purpose was to show one way to "do this right".

Which you totally failed to do since you clearly can't analyse a simple circuit !

And what's the tolerance on those h parameters btw ?

Graham

 

[Eeyore]

He's pointing that to a certain level of approximation your circuit with
feedback is similar to an inverting op amp with a 10K feedback resistor and
1K input resistor. The base of the transistor is, to some extent, a
"virtual earth".

To 'some extent' ???

Close enough that the answers are in practice swamped by tolerances !

So, as a first approximation, you could expect the input
impedance of the circuit to just be the resistance of R6, 1K ohm. And the
gain of the circuit to be -R5/R6, or -10.

However, a more accurate analysis shows the gain to be about 10% lower
than that, and the input impedance to be about 5 % higher.

He measured 0.9% in fact.

As for the gain, The Aol of that circuit is ~ 100, so with closed loop feedback
targeting a voltage gain of 10 it will indeed be in the region of 9x.

Your measured
values also show about that level of error in the approximate values.

Without feedback, R6 is not so dominant.

Without feedback it's a totally different circuit !


Graham

 

[Charles Schuler]

I dont think so. The input (left end of R6) draws 4,86uA in 20mV which
gived 4,1Kohm input impedance.

But first fo all I much rather want to know the correct formulars for
calculating these things in this kind of circuit, after that we can discus
the errors. I need to know if I'm doing this right.

The correct formulas require information about the transistor which is
usually not known. Approximations commonly used are:

Gain = R3/re

Z = Beta x re in parallel with (R5/Gain)

re is the ac resistance of the emitter and is often estimated by .026/Ie
where Ie is the dc emitter current

The Gain and Z above are from the right end of R6

 

[The Phantom]

To 'some extent' ???

Close enough that the answers are in practice swamped by tolerances !

Without doing an accurate analysis, how would one know whether this is
true or not? How does your "simple" analysis enable a person to determine
the sensitivity of gain and input impedance to transistor parameters?

After doing a number of accurate analyses, a new designer of circuits
will eventually gain the experience to know when approximations are good
enough.

Accurate analysis has its place.

He measured 0.9% in fact.

He says he measured an input impedance of 1.08 Kohm, which is 8% higher
than 1 Kohm, not 0.9%.

As for the gain, The Aol of that circuit is ~ 100, so with closed loop feedback
targeting a voltage gain of 10 it will indeed be in the region of 9x.



Without feedback it's a totally different circuit !

So you agree with my statement?

 

[The Phantom]

Which you totally failed to do since you clearly can't analyse a simple circuit !

I *did* analyse it, so how is it that I failed?

And what's the tolerance on those h parameters btw ?

The Vishay data sheet only gives tolerances for hie (3 times variation)
and HFE (2 times variation) for gain group A.

Given those tolerances, what does your "simple" analysis method say the
variation in input impedance and gain would be?

 

[Charles Schuler]

Hi there

I have a problem with some calculations regarding feedback and input
impedance on a simple one stage transistor circuit here:
http://www.mespilus.dk/Feedback_1.pdf

With the feedback circuit (R5 + C6) connected I can measure a gain of
8.85. The input impedance is 1,08Kohm.

With the feedback circuit disconnected I get a gain of 105 and a input
impedance of 4,11Kohm.

The question is now: how is the right way to calculate these values when
I'm designing? if I use the standard formular for designing input
impedance (Zin= hie//R1//R2) I get a impedance of 1,88Kohm. If the
feedbackcircuit is going to be in parallel with R1 it will decrease to
1,6Kohm - but still not right.

Please.... can anyone please explain how to do this right and how to
calculate the input impedance, gain and so one for this kind of circuits?

Peter

I had no idea when I first responded to your question that this was going to
turn into a pissing contest; and I really did think that it was a homework
problem because that circuit is generally a real loser. It is typical of
academic assignments, where the intent is to teach concepts but not
necessarily specific examples of good design.

Anyway, accurate modeling (for this circuit ilk) is usually done with either
hybrid parameters or scattering parameters. The hybrid parameters are often
not available or are too flaky to be trusted. So, many practicing designers
use circuit approximations ... dig back into today's posts and you will see
how simple they can be. But, they can be the source of errors approaching
100% ... but those kinds of errors are usually easy to deal with in
practical designs ... you know, design for an approximated gain of 2X
greater than that will ever be needed, and then add a volume control (or use
AGC).

 

[Eeyore]

Without doing an accurate analysis, how would one know whether this is
true or not? How does your "simple" analysis enable a person to determine
the sensitivity of gain and input impedance to transistor parameters?

If you want to crunch the numbers it does that just fine.

After doing a number of accurate analyses, a new designer of circuits
will eventually gain the experience to know when approximations are good
enough.

Accurate analysis has its place.

I reckon I can produce an answer as accurare as you like. Expereience shows it's not
vital to do this. Relying on transistor parameters is kinda odd anyway !

He says he measured an input impedance of 1.08 Kohm, which is 8% higher
than 1 Kohm, not 0.9%.

I thought he said 1.088 kOhm? That's close enough to 9% for me.

As for accuracy, k is lower case and most units including Ohms start with a capital
letter !

So you agree with my statement?

Which one ?

Graham

 

[Eeyore]

I *did* analyse it, so how is it that I failed?

Perhaps you could elaborate on your analysis ?

The Vishay data sheet only gives tolerances for hie (3 times variation)
and HFE (2 times variation) for gain group A.

Given those tolerances, what does your "simple" analysis method say the
variation in input impedance and gain would be?

Minute. That's off the top of my head.

The circuit without feedback will have a more variable input Z with hfe but gain will
be almost unaffected.

Graham

 

[The Phantom]

Perhaps you could elaborate on your analysis ?

As I said in an earlier post, and you apparently didn't notice:

"I've posted the analysis over on alt.binaries.schematics.electronics with
the subject line "Andersen circuit analysis". Unzip it into a temp folder
and double click the index file. It should open in your browser."

Minute. That's off the top of my head.

The circuit without feedback will have a more variable input Z with hfe but gain will
be almost unaffected.

How about sensitivity to the other 3 h parameters? How about some
numbers rather than just saying "more variable" and "almost unaffected"?

 

[The Phantom]

If you want to crunch the numbers it does that just fine.

Show us how to use your simple method and get the change in input
impedance and gain as hfe changes from 100 to 110.

I reckon I can produce an answer as accurare as you like. Expereience shows it's not
vital to do this.

But how would you get the experience without first having done some
accurate analyses?

Relying on transistor parameters is kinda odd anyway !

How else would you get an accurate analysis? I guess you're saying that
you rely on approximations.

I thought he said 1.088 kOhm? That's close enough to 9% for me.

But *you* said: "He measured 0.9% in fact.". You did *not* say 9% until
just now.

As for accuracy, k is lower case and most units including Ohms start with a capital
letter !



Which one ?

The one right above yours, where I said "Without feedback, R6 is not so
dominant.".

 

[Eeyore]

How about sensitivity to the other 3 h parameters?

What parameters. There are none of interest.

How about some
numbers rather than just saying "more variable" and "almost unaffected"?

Hiow about you learn to analyse real circuits ?

Graham

 

[Eeyore]

Show us how to use your simple method and get the change in input
impedance and gain as hfe changes from 100 to 110.

So minute as to be negligible.

Sure I can work it out. Who'd want to in reality ?

How else would you get an accurate analysis? I guess you're saying that
you rely on approximations

Transistor parameters of interest here vary by at least +- 50%.

There is therefore no such thing as an accurate analysis hence it's stupid to even bother
with one.

The trick is to design parameter sensitivity out as far as possible.

Graham

 

[The Phantom]

So minute as to be negligible.

Sure I can work it out.

So you keep saying, but why are you reluctant to share with us how your
simple method would solve this problem?

Who'd want to in reality ?

People who want to increase their knowledge base would want to. I'm sure
some of the other readers of this thread as well as myself are eager to
learn a simpler method to calculate sensitivities to variations in h
parameters.

Transistor parameters of interest here vary by at least +- 50%.

There is therefore no such thing as an accurate analysis hence it's stupid to even bother
with one.

The trick is to design parameter sensitivity out as far as possible.

A worthy goal. To know when the goal has been reached, you must have a
method of determining what the parameter sensitivities are. Will you share
your method of doing this?

 

[John Fields]

As for accuracy, k is lower case and most units including Ohms start with a capital
letter !

 

[John Larkin]

Absolutely everything !



Where did you get these 'standard equations' ???? They're nonsense.

Incidentally the actual numbers are fairly free of the influence of h
parameters. For this simple kind of circuit you can assume almost any small
signal transistor to be 'the same' within reason.

The impedance looking directly into the base is proportional to beta,
pretty close to

Zin = 26 * B / Ie Ie = emitter current in mA.

which ain't very free, since beta could range from 20 to 1000 for
various transistors.

Knowing that, you can calculate the open-loop voltage gain of the
transistor. And knowing that, you can calculate the effective
impedance looking into the feedback resistor. Now put all the relevant
impedances in parallel, add the 1K series, and you're done.

John