Q: feedback and input impedance in a emitter follower

It is not an emitter follower ... that should help you do your homework!

Arhhh. your right. Sorry, that should of cause be common emitter!

BTW: nope, not homework..

Peter

The loading effect of R5 is set by the current through it ... which in this case is larger due to the inverted signal at the collector of the transistor (in other words, the effective value of R5 is much smaller than 10 k in the ac equivalent circuit). That's why the input impedance drops with the collector feedback connected. The approximations for this circuit are simple and can be found in all the standard textbooks.

The standard textbooks tells me that:

Zin'=Zin/M Zin' = inputimpedance with feedback Zin = inputimpedance without feedback M = Feedback factor M = Ao/A' Ao= openloopgain A' = gain with feedback.

So in this case Zin is 4K, M is 11,86 and the Zin' should then be 337ohm - which is not correct.

What is wrong here? - and what kind of standard textbooks could you recommend me? (give me some titles). AoE doesn't cover this subject.

Peter

I don't see a C6 in your circuit; do you mean C4?

Do you have a spec sheet for the BC547A transistor? Is one available on the net? And, what are the h parameters at the operating point (I assume the circuit is operating at low, audio, freqencies; if not, what is the operating freqency)?

Do you want to include the impedance of the capacitors in the calculations, or do you want to assume that the caps can be taken as AC short circuits?

Yes, of cause! sorry.

Yes the amp is LF. I don't have any specifications on the BC547A. It is a simulation from Electronic Workbench 9 where hfe is held at 100.

I take them as AC shorts.

Peter

Are you simulating this whole circuit in Electronic Workbench 9? Are you asking for a procedure to determine the input impedance and gain in Workbench? Or do you want a method to calculate these just with algebra?

Are you perhaps evaluating the gain taking the input signal as that which appears at the right end of R6 but evaluating the input impedance from the left end of R6?

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.).

Peter

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

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.

Peter

Peter

Go to this site and get the datasheet for the BC547; get the one from Vishay because it has h parameters:

The DC collector current in your circuit is around 1.15 mA. The datasheet gives the h parameters (at 1 KHz) on pages 2 and 3 for a collector current of 2 mA. This transistor is available in 3 hfe groups. The lowest hfe for which the h parameters are given is 220. I extrapolated the parameters down to hfe = 100. Then using the curves in Figure 8, I adjusted the parameters for a collector current of 1 mA. My final numbers are:

hie = 1800 hfe = 100 hre = .00015 hoe = 6 * 10^-6

Another person might get slightly different numbers, but these are good enough for a reasonable analysis. The analysis I performed is a nodal analysis. It doesn't make any approximations for feedback, but it can be a bit complicated. However, with modern mathematical software, the math is done for you; the hardest part is getting it set up correctly. In fact, once you have it set up, you can do the matrix math on a calculator. 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.

With these values and with feedback, I get an input impedance of 1054.2 ohms, and a gain of -9.12

Without feedback, I get an input impedance of 2596 ohms, and a gain of

-160.5. If I change hie to 3750 ohms, and hfe to 111, I get an input impedance without feedback of 4111 ohms and a gain of -105.3, closer to your measured (I think) values.

The results without feedback are sensitive to the actual h parameters of the transistor, of course.

And you reckon you need these for a simple analysis ?

Sheesh !

Graham

That's a horrid circuit btw. I hoped I'd seen the last of that kind of thing in the late 70s. Why on earth are you using such a thing ?

Graham

What input imedance and gain do you actually want ? Do you actually want it to be inverting ? If not, I can improve the noise figure too while I'm at it.

Graham

You're worrying too much about formulas.

OTOH if you had a firm grasp of practical ( as opposed to theoretical ) circuit theory you wouldn't need to worry about them !

Frankly this is more of 'back of a cigarette packet' job.

Graham

Correction. It would still be invertsing but using series feedback instead of shunt so the input Z goes up and the noise goes down.

Graham

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.

Graham

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.

Graham

The first sensible advice I've seen so far in this thread !

Graham