EMRFD Message Archive 814

Message Date From Subject
814 2007-05-30 19:50:12 kanewderfish AC formulas on p2.25 vs FBA program from CD
Hi all,
I have been playing around with the two sources quoted in the
subject. The formulas for Rin and Ro on p 2.25 do not appear to
include any reference to the value 'C col-base' as the FBA program
does. This (I believe) leads to results for Rin and Ro that differ by
quite a bit in the examples I have tried.
Am I correct that if I did include the effect of the collector to
base capacitance in the Rf value in the formulas on p 2.25 the results
for Rin and Ro would match better to what is reported by FBA with a
given value for C col-base?
Assuming this is correct, am I also correct that the "adjusted" value
for Rf in the formulas would have to be calculated as a complex number
as it would now include real (resistor) and imaginary (capacitor)
I guess my real question is how to get the two methods to agree. If
anyone is interested I can forward my circuit values.

Tnx es 72

815 2007-06-01 11:49:41 Wes Hayward Re: AC formulas on p2.25 vs FBA program from CD
Hi Bob and group,

Good question. Yes, there is a big difference between the two
evaluations for the feedback amplifier. We would expect this, for
the two cases used quite different assumptions for the analysis.

The equations in Fig 2.69 assumed an ideal transistor in the circuit
of Fig 2.63. The transistor has a beta value that holds for all
frequency. The intrinsic emitter resistance is lumped in with the
external emitter resistance of Re. I plugged some numbers into the
equations of Fig 2.69 with the following parameters:
emitter current=20 mA and 10 Ohms degeneration for Re=11.3

The results were Gt=18.0 dB, Rin=61.3, and Ro=202.

The analysis in FBA uses a modified hybrid pi model. See the info
in IRFD, or many other places for details on this model. It is a
small signal model that includes F-t for the transistor. The F-t is
used to generate a shunt capacitor that appears in parallel with the
emitter base junction. I did further modifications by placing a
small resistor in series with the input, called Rb'. This is the so
called base spreading resistance. A "Miller C" of a few pF is also
tacked from collector to base. Within the program, I generate the y-
parameters for this combination based upon the external emitter
degeneration (R and inductance) and an assumed standing current. The
resulting y matrix is modified with the collector to base feedback
network, which includes series R, L, and C. The termination at the
input is set while that at the output is picked by setting an overall
load and a transformer turns ratio. If we pick RL=50 and N=2 (which
is the default built into the program) we end up with 200 Ohms as the
load at the collector. The program lets the user alter F-t, beta,
Rb', and Miller C = C col-base.

I wanted to see what we would get with this analysis if we assume
parameters that might fit with the previous analysis. I picked the
following :
Transistor: beta=100, F-t=30000, Rb'=.001, Ccb=.001
F = .01
Rs=50, RL=50, N=2
Ie=20 mA, R degeneration = 10, with .01 nH inductor
Rfb=1000 with 100000000 pF and .01 nH

The analysis results are then Gt=18.01 dB, Zin=61.4+j.008, and Zout=

Essentially, we have excellent agreement. You can now start
changing the frequency dependent parameters to see what the impact
might be.

There are a lot of things that we can do with this. You can change
the assumptions for the initial analysis and derive your own
equations to replace those of Fig 2.69. For example, what will
happen if we replace all (beta+1) terms with beta.

Another thing to do is to find some published scattering parameters
for a transistor of interest and then analyze a feedback amplifier
using them. This will take some serious arithmetic or a suitable
computer program. The old ARRL Radio Designer would do it.

Another thing to do is to use LT SPICE. The SPICE models for our
common transistors are easy to find all over the web. I'll do this
analysis and report the results here.

Bottom line: This is a game of modeling that we are playing. We
are picking models of varying complexity in order to try and predict
circuit behaviour. The transistor model used for Fig 2.69 equations
is purely scalar, so we don't encounter any complex mathematics.
This was not the case with the action that happens within FBA. If we
go deeper and become even more thorough in the mathematical models we
use for the transistors, we come closer and closer to a description
of the physical world that we would measure when we build the circuit.

I hope that this clarifies the situati
816 2007-06-01 15:23:03 Wes Hayward Re: AC formulas on p2.25 vs FBA program from CD
Hi all,

In a previous posting, I discussed two methods in EMRFD that address
single transistor feedback amplifiers. I just posted a PDF that
summarizes some of these comparisons and includes a SPICE analysis.
The same circuit is used for all three.

I hope this clarifies the situation.

73, Wes