Hi Richard,
Thanks for your comments. The next time I do anything like this I will make
the thing more real world. Should have done the power supply thing. I have
built enough of those. As I said, this was a rainy afternoon thought. I
sprinked in some commenys below.
Tam
"Richard Clark" wrote in message
...
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.
**********************************
I am making a not quite perfect amplifier from a perfect one. I used that so
I would not have to get into details about 80 db gain, .5V dropout voltage,
10 meg input impedance, 20 Ohm output impedance, etc. Single rail
amplifiers, including these that have a common mode range that includes
ground are quite common.
***********************************
2. You now have an op amp with 1K output impedance.
#2 in fact adds absolutely nothing but repetition.
************************************
right, but I want the reader to consider the 1K as part of the amplifier,
and I have placed it at a point where its effect is obvious
*************************************
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.
***************************************
The gain is precisely 1 at the point that I call the amplifier output. The
gain from input to the output terminal of the original amplifier is
variable, and depends on the load.
***************************************
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.
************************
Precisely. People might read this who have no idea what mu and beta are, but
they know Ohm's law.
************************
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.
***************************
I have reduced the open loop gain A of the amplifier by the ratio
Rload/(Rload + 1K), but it is still infinite. I may be using terms that are
not universally used. When feedback is used to reduce gain by say 20 db, it
is common to refer to this as 20 db of feedback (May be an audio term). I
have a unity gain amplifier, as I have defined it, and reduced the open loop
gain by infinity.
***************************8
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.
********************************
I am defining the 1K as part of the amplifier, but I placed it at a point
where you could see it. As defined, if you placed +6V on the noninverting
input, and a load of 10K on the output, the output voltage would be 6V. If
you changed the load to 2K, the output voltage would still be 6V. The
impedance is 0. A person might assume he could easily obtain 11 ma from such
a low impedance source. He can not. Not even at .01V.
**********************************
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?
***********************
I realize this sounds unclear. Consider the node that drives the 1K
resistor. It can not go higher that 10V; the short circuit current is 10 ma.
This is true both for
a) loop is closed. That is the voltage across the load ic connected to the
non inverting input.
b) loop is open with inverting input at ground and noninverting input at
+10V
************************
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:
**************************
I didn't want to do that. Instead of a 741 I would use an LM358.
**************************
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.
***************************************
I didn't want to build a good circuit, I wanted to build a bad circuit, and
show that all the feedback in the world was not going to make it a good
circuit.
***************************************
Anyone can trap another through crafted specifications.
I've got several many squirrels up a tree right now.
73's
Richard Clark, KB7QHC
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