operational amplifiers

S

Simon Ward

Guest
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.

Cheers
 
Simon Ward wrote:
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.
Click to expand...
What have you done to determine the answer to date?
 
"Simon Ward" <simon.ward@pgen.net> wrote in message
news:5e9319db.0307281555.2ffd4b2f@posting.google.com...
| I have been given the following problem for a college assignment. I
| was hoping that someone with more knowledge than I could help me out;
|
| Operational amplifier (op-amps), which have negative feedback applied,
| can have their overall gain fixed using a simple resistor network in
| the feedback path. Explain why this is so.

Theoretically, an op amp with no feedback would have infinite gain.

FrediFizzx
 
Simon Ward wrote:
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.

Cheers
Click to expand...
Take a look at application notes for Op-Amps. Here are a couple links
to get you started.

http://merchant.hibbertco.com/servlet/mtrlext.MtrlExtServlet?tp=archsearch
http://www.national.com/
http://www.onsemi.com/home



--


Michael A. Terrell
Central Florida
 
Simon Ward wrote:
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.
Click to expand...
have you heard of the parable of
the ass and the carrot and the stick?


--
Achilles: I wish my wish would not be granted.
< an undescribable event occurs >
Achilles: What happened? Where's my Genie?
Tortoise: Our context got restored incorrectly.
Achilles: What does that cryptic comment mean?
Tortoise: The system crashed.


To email me send to :

rb <my last name> AT ieee DOT org
 
On Mon, 28 Jul 2003 18:21:22 -0700, "FrediFizzx"
<fredifizzx@hotmail.com> Gave us:

"Simon Ward" <simon.ward@pgen.net> wrote in message
news:5e9319db.0307281555.2ffd4b2f@posting.google.com...
| I have been given the following problem for a college assignment. I
| was hoping that someone with more knowledge than I could help me out;
|
| Operational amplifier (op-amps), which have negative feedback applied,
| can have their overall gain fixed using a simple resistor network in
| the feedback path. Explain why this is so.

Theoretically, an op amp with no feedback would have infinite gain.

FrediFizzx
Click to expand...

And no control. And "infinite" has starvation points elsewhere in
the chip and circuit. It would be a single, always on device. Hell,
a transistor works in that instance. Feedback loops are critical to
control circuitry. They don't just guess. They have to be
responsive, active control elements, and that requires data from what
they are controlling. We call it feedback. Why does that word scare
some people? One monitors a point, and utilizes the data to control
the system that controls the point being monitored.

Feed forward should even be more scary, then. ;]

If one lives in the basement of a house or building in a wet region
of the world, are they considered to be "critically damped"?
 
On Mon, 28 Jul 2003 23:55:10, simon.ward@pgen.net (Simon Ward) rolled
up his sleeves and typed:

I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.
Click to expand...
What's your college attendance record?

--
Jim Backus OS/2 user
bona fide replies to jimb-thecirclethingy-jita-dp-demon-dp-co-dp-uk
or remove "NOT" from address
remove dashes and make the obvious substitutions for valid email
address
 
simon.ward@pgen.net (Simon Ward) wrote in message news:<5e9319db.0307281555.2ffd4b2f@posting.google.com>...
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.

Cheers
Click to expand...
Douglas Eagleson
eagleson@yahoo.com




You are asking the question called why does the op-amp's internal
circuit implement the defined external ampilfier. And it is
designed that way using transistors.

And when the instructor claims infinite causality of gain by the
added feedback short, it is an incorrect claim in physics. The
gain is always limited by the transistor's hfe.

So the defined op-amp is an abstract theory of its specific
circuit.
 
"Simon Ward" <simon.ward@pgen.net> wrote in message
news:5e9319db.0307281555.2ffd4b2f@posting.google.com...
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.
Click to expand...
Simon, you do understand that this group has little use for students using
them in lieu of doing their "homework"? Having said that, picture an
amplifier with two inputs. One of these inverts the input signal and the
other does not. If the output signal, or some part of it, is fed back to
the inverting input, this is called negative feedback. If all of it is fed
back, the gain is 1. In the history of electronics, negative feedback looms
as one of big, big ideas.

Anyway, if the amplifier has lots of gain and if the entire signal is fed
back to the inverting input, the result is an amplifier that follows the
input with no gain. Most times, only a part of the output is fed back to
the input.

The classic model used to understand and analyze op-amps is that the
difference between the two inputs is zero when the amplifier is dealing with
normal signals. So, if you connect a voltage divider between the output and
the inverting input, you get an easy to analyze circuit that predicts the
gain to be equal to ratio of two resistors. If the feedback resistor is
10,000 ohms and the input resistor is 1,000 ohms the gain is Rf/Ri or 10 in
this case.

Hope that helps.
 
The amp on it's own has huge gain so in normal use the two inputs are at
more or less the same voltage. In the classic circuit the + input is
connected to 0V so the circuit behaves as if the -ve input was also at 0V.

Now think about current flows in the circuit....

The current going through the input resistor (Rin) is Vin/Rin

The current flowing in the feedback resistor (Rfb) is Vout/Rfb

Since the input impedance of the opamp is very high these two currents are
equal (it has nowhere else to go..

Vin/Rin = Vout/Rfb

Rearrange this to give

Vout/Vin = Rfb/Rin

eg Gain = Rfb/Rin




"Simon Ward" <simon.ward@pgen.net> wrote in message
news:5e9319db.0307281555.2ffd4b2f@posting.google.com...
I have been given the following problem for a college assignment. I
was hoping that someone with more knowledge than I could help me out;

Operational amplifier (op-amps), which have negative feedback applied,
can have their overall gain fixed using a simple resistor network in
the feedback path. Explain why this is so.

Cheers
Click to expand...
 
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