On Mon, 1 Sep 2003 17:52:43 -0400, "Tarmo Tammaru"
wrote:
Hi Richard,

Everything, except the output dynamic range. Please go through my numbers.
The equation I gave is for the point at which the amplifier goes non linear,
and there actually is no feedback. There is a large error voltage at the
input, but the amp can't do anything about it because it is already in
saturation. For a 5V input/output

What exactly does this mean? That you expect 5V in will track with 5V
out? Say so. Shortcuts in specification lead to disaster. If this
is not what you mean (and you did not specify an input in the
original) your ambiguity allows for any interpretation.

and a 1K load the output terminal of the
op amp is already at VCC. A more practical example might have been a 13.8
volt power supply regulator running off 16 volts, with a current sensing
resistor in series with the output. Anyway, it was a rainy afternoon kind
of thing.

Tam/WB2TT

Hi Tam,

Let's review as you asked:

Consider the following. An ideal DC op amp operating from a 10V supply with
0 output impedance, and infinite gain.

1. Put a 1K resistor in series with the output.

Hence the "ideal" Op Amp is crippled from the start, any expectations
of superlative performance have already been abandoned. Further, the
"ideal" Op Amp is not rail limited, nor is it convention to discuss Op
Amp designs with single ended supplies. These are more short cuts
that could have been expressed with very little greater length to
conventional usage.


2. You now have an op amp with 1K output impedance.

#2 in fact adds absolutely nothing but repetition.


3. Connect the inverting input of the op amp to the other end of the 1K
resistor. Call that the output terminal. and connect a load resistor Rl from
there to ground. ( The gain from the non inverting input to Rl =1 in the
linear range)

That being the new output of the amplifier, R1 as you call it, now
loads that amp to ground. (By the way, if you are going to have two
resistors, convention would label them R1 R2; not R and R1.)

The gain is NOT 1 by sheer, obvious placement of the resistors you
describe. You elsewhere supply a gain that does not agree with this
#3. What you imply are the mu and beta gains, but you do not really
go into that distinction, nor do you perform the math that bears on
their usage.


4. Because of the infinite feedback,

Infinite feedback? Poor specification where I have to presume you are
in error and meant infinite gain (for the previously "ideal" amplifier
- which it is not now). If the output were strapped back to the
inverting input, that "might" qualify as infinite, but your load is
the gain determinant of the amplifier. If you meant that the Op Amp
output is impressed upon the inverting input, that goes without saying
for all linear applications doesn't it? If by this your statement of
a gain of 1 above was along the same lines, it suffers by similar
degree.

the output impedance is now 0 again,

No, it is not, you have ascribed (by description) a 1000 Ohm output
impedance (resistance). The amplifier is not the Op Amp component, it
is the assembly of components presented to the load and is modified by
the gain that you incorrectly ascribe above.

but all of the load current still flows through the 1K series resistor.

Which confirms my statement and directly follows from the addition you
originally offer.

It
will not, for instance, deliver 5V into a 910 Ohm resistor because the
amplifier will have saturated before that point. For a given RL, the maximum
voltage you can get out is (10 x Rl)/(1000 + Rl) with or without feedback.


With OR without feedback? Which is it? Do you have feedback or don't
you? The additional baggage of your statement both adds nothing, and
is self conflicting. Do I now have the choice to express it has no
feed back?

Dynamic range is not the same thing as rail limited and rail limiting
is certainly not within the canon of "ideal" amplifiers. Dynamic
range is dimensionless and generally described in dB and is a function
of noise. Feed back has a direct correlation to the amount of noise
added by the amplifier and thus impacts Dynamic Range directly.

It would have taken a whole lot less to simply use the conventional
741 Op Amp as an example, warts and all, to express the same issue
which merely points out that a poor design works poorly. You even
anticipate this poor aspect through the modifications after the fact:
A more practical example might have been a 13.8
volt power supply regulator running off 16 volts, with a current sensing
resistor in series with the output.
which illuminates how a design engineer builds from known limitations
toward known loads. In other words, if the engineer faces a 10V rail
limitation, he could have as easily added a DC-DC up converter to
solve it. Anyone can trap another through crafted specifications.
I've got several many squirrels up a tree right now.

73's
Richard Clark, KB7QHC