Re: Q: feedback and input impedance in a emitter follower

On Sat, 21 Oct 2006 23:23:54 +0200, "Peter Andersen"
<peterandersen@xxxxxxxxxxx> wrote:

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

Go to this site and get the data*** for the BC547; get the one from
Vishay because it has h parameters:
http://www.datasheetcatalog.com/datasheets_pdf/B/C/5/4/BC547A.shtml

The DC collector current in your circuit is around 1.15 mA. The
data*** 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.



.

Quantcast