Author: John PopelishDate: 10:30 09-11-06
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archie wrote:
> why do we prefer to supply voltages at the inverting terminal,or
> conventionally known as the negative terminal of an op-amp???
I don't really understand your question. I often provide a
voltage source connection to the + (non inverting) input and
current to the - (inverting) input. But there are few hard
rules, considering the different uses for opamps.
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Author: archieDate: 10:38 09-11-06
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why do we prefer to supply voltages at the inverting terminal,or
conventionally known as the negative terminal of an op-amp???
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Author: Bob MyersDate: 14:39 09-11-06
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"archie" <haughtynarcissist@gmail.com> wrote in message
news:1163086722.766134.62130@m73g2000cwd.googlegroups.com...
> why do we prefer to supply voltages at the inverting terminal,or
> conventionally known as the negative terminal of an op-amp???
I'm assuming that when you say "supply voltages," you are
asking why the input signal to an op-amp circuit is very often
connected to the inverting input - is that the case?
If my assumption is correct, then the answer is as follows:
Many circuits which are constructed from op-amps make
use of a technique known as "negative feedback" to
stabilize the performance of the circuit and set such
parameters as the overall gain, etc.. In this method, a
portion of the output signal is basically being subtracted
from the input signal (or a portion of it). Since in most
cases it is not important to preserve the absolute "polarity"
or phase of the output relative to the input, it is simpler to
just use the inverting input of the op-amp to achieve this,
as opposed to having completely separate "gain" and
"invert and add" blocks.
Most such circuits rely on the fact that the gain of the
op-amp is extremely high (what's called the "open-loop
gain," i.e., the gain in the absence of such external
feedback), such that when all is said and done the
behavior of the complete circuit is actually determined by
the components external to the op-amp. If you Google
for "inverting amplifier op-amp" you will no doubt find
any number of analyses with circuit diagrams which will
explain why this is so. The analysis of such circuits will
typically assume that the op-amp in fact has "infinite"
gain, as well as "infinite" input impedance (meaning that
its inputs do not represent a load on the input signal or
on the portion of the output which is being "fed back").
Bob M.
>
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Author: ChrisDate: 18:49 09-11-06
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archie wrote:
> why do we prefer to supply voltages at the inverting terminal,or
> conventionally known as the negative terminal of an op-amp???
Hi, Archie. Is this what you're talking about?
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| ___
| .----|___|---.
| | Rf |
| | V+ |
| Vin ___ | |\| |
| o----|___|-o----|-\ |
| Ri | >----o----o
| .--|+/ Vout
| | |/|
| | V-
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
There are many different op amp configurations which do different
things. This one is convenient because of its simplicity. This
amplifier is inverting (a negative output voltage if the input is
positive) and the gain Av = -(Rf/Ri), with input impedance is Ri.
A basic non-inverting amplifier has input voltage supplied at the
non-inverting input (you might call it the + terminal):
|
| V+
| Vin |\|
| o-----|+\ Vout
| | >---o---o
| .--|-/ |
| | |/| .-.
| | V- Rf| |
| | | |
| | '-'
| | |
| '---------o
| |
| .-.
| Ri| |
| | |
| '-'
| |
| ===
| GND
(created by AACircuit v1.28.6 beta 04/19/05 www.tech-chat.de)
This configuration is non-inverting (the output is a positive voltage
if the input is +), and the gain Av = 1 + (Rf/Ri). Input impedance is
the input impedance of the op amp.
Hope this has been of help. If this is a homework question, I'll bet
there's a lot of good information in the textbook.
Cheers
Chris
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Author: defaultDate: 19:08 09-11-06
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On 9 Nov 2006 07:38:42 -0800, "archie" <haughtynarcissist@gmail.com>
wrote:
>why do we prefer to supply voltages at the inverting terminal,or
>conventionally known as the negative terminal of an op-amp???
"We" don't.
The gain on an inverting amp is the ratio of input and feedback
resistor - in the non-inverting amp it is the ratio plus one.
So you can never have less than unity gain and you can never have
unity gain, except when used as a voltage follower - no resistors.
Maybe that's why it is more common to see inverting amps?
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Author: redbellyDate: 22:25 09-11-06
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archie wrote:
> why do we prefer to supply voltages at the inverting terminal,or
> conventionally known as the negative terminal of an op-amp???
If you're talking about the input signal voltage, then actually it can
go to either the + or - input. There are reasons for doing both,
depending on the situation.
If you're talking about the output's feedback path, it's because only a
negative feedback will produce a stable output somewhere between the
supply rails. A positive feedback would result in the output going to
one of the rails.
Mark
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Author: BanDate: 03:50 10-11-06
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archie wrote:
> why do we prefer to supply voltages at the inverting terminal,or
> conventionally known as the negative terminal of an op-amp???
We don't.
Supply voltages are usually something like +/-5V or +/-15V. They are
connected to the positive respective negative terminal of the opamp, which
are not the inputs. If you want to use the opamp lineary in an inverting
configuration, then either you ground the non-inverting input, or apply a
reference voltage there for an offset. The inverting input is always
connected to the output (through a divider or resistor) and to the input
voltage through another resistor, the input current is compensated by the
feedback current from the output, the opamp tries to keep the inverting
input at the reference voltage.
The output will then produce a voltage referenced to the above reference
voltage.
I hope this helps. :-)
--
ciao Ban
Apricale, Italy
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On 9 Nov 2006 07:38:42 -0800, "archie" <haughtynarcissist@gmail.com>
wrote:
>why do we prefer to supply voltages at the inverting terminal,or
>conventionally known as the negative terminal of an op-amp???
assuming you mean the inverting input (-)
we don't. A circuit is designed as a series of trade-offs:suitability
for the job versus cost & size. others might be how useful it could be
for feature-add.
if the subsequent design calls for one or the other, it doesn't
indicate a pet liking for that terminal
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