EMRFD Message Archive 14758

Message Date From Subject
14758 2018-05-07 18:42:49 rcbuckiii Feedback Amplifier Equations
This is purely an academic question. I am going to write 3 programs to handle the equations given on page 2.25, Fig 2.69. This will allow me to enter different values of currents and resistors to see what the results are.

To confirm the math steps, I performed the Gain (top equation) calculation by hand on a calculator. The example I used as a comparison is the 5th row down in the 200 ohm load examples of Table 2.2. I used the following numbers:
Re = 8.1 (1.3 + 6.8)
B = 21 (14 MHz number)
Rf = 1600
Rs = 50
RL = 200

My calculation resulted in a number of 20.5189365 for the amplifier gain. Close enough for government work but doesn't match the 20.7 number given in the table.

Has anyone else used the equation and obtained a result that matches the numbers shown in the table? I guess close is good enough. I have built several of the amplifiers using the values shown. Gain is usually within 1 dB what the table shows. With component tolerances, that is more than acceptable.



14760 2018-05-08 08:38:03 kb1gmx Re: Feedback Amplifier Equations
I rarely get the same numbers but the difference is usually a fraction of a DB.

Then again From the old days the first question usually asked, "Can you even measure the difference?"
The difference between book and equation is less than .2 DB, not much of a difference at all.

FYI I use a spreadsheet I think it was off the EMRFD CD.  I'll copy it to the files area under my call.

14761 2018-05-08 08:45:33 kb1gmx Re: Feedback Amplifier Equations
I'll post it when yahoo files section starts working again.

14763 2018-05-08 10:30:26 rcbuckiii Re: Feedback Amplifier Equations

If it is on the CD, I can get it. I'll check the CD. I know there was a spreadsheet in the files section at one time. I downloaded that one a couple of years ago.

I was going to write the programs to have a little more versatility than the spreadsheet had.

14765 2018-05-08 13:33:19 kb1gmx Re: Feedback Amplifier Equations
IF you want My email is kb1gmx@arrl.net and I can reply with it.

I'm a bit annoyed as the files section had contained a large amount of useful stuff.
Its not local or yahoo in general as the other dozen groups work normal.

14766 2018-05-08 14:27:07 Mike Nothdurft Re: Feedback Amplifier Equations

Files are back  --- from somewhere.




14767 2018-05-08 14:33:12 Mike Nothdurft Re: Feedback Amplifier Equations

Whoops… wrong group.  EMRFD files not back.




14779 2018-05-11 19:19:44 rcbuckiii Re: Feedback Amplifier Equations
I have used the three equations in Fig 2.69 to write a program to compute the gain, input impedance, and output impedance based on user input. The program is written in C and is for command line use on a Linux machine. However, since it is in C, it would be easy to use the source code to adapt the program for any OS.

If anyone would like a copy of the source code and compiled program, just send me an email. My email address is rcbuckiii at yahoo dot com. I would put it in the Files section, but I guess everything there has disappeared down a black hole.


14792 2018-05-14 21:18:49 Eric J Re: Feedback Amplifier Equations

Kind of spooky. I was going through Figure 2.69 earlier today to enter the equations into Excel. I gave up. Too much like work. I'm going to use Table 2.2 and fudge the results up and down as needed with the insight from the graphs which follow.

So I can't answer your question, but I can offer an explanation. I've noticed that sometimes in EMRFD calculations are made using calculated values, then component values are rounded to the nearest standard value. In other words, the standard values reported in the table might not be the values used in the computation. I don't that as a handicap for hobbyist experiments on essentially one-off circuit designs.

I'm going to take another look at programming something to solve those equations. Excel is too easy to get lost in meaningless cell addresses. If you resolve the discrepancies, post something.

Eric KE6US