|
Home > Archive > Electrical Engineering > August 2007 > Operational amplifier
You are viewing an archived Text-only version of the thread.
To view this thread in it's original format and/or if you want to reply to
this thread please [click here]
| Author |
Operational amplifier
|
|
| Rocky3 2007-08-11, 1:25 pm |
| Hello!
I'm getting started with the study of operational amplifier. Fist of all,
do you know any site which could explain clearly this matter? Could you
write some link?
In particular, I can't understand the reason why a saturation occurs when
the output signal is connected to the + terminal. The output is alway
connected to che other terminal, the - one.
Why? Can you help me?
Thank you anyway!
Rocky3
--
"Il sapere e la ragione parlano, l'ignoranza ed il torto urlano".
Arturo Graf / Indro Montanelli / Anonimo
| |
|
| In article <qhouphdm8h7t.oqjzakcpd5qy.dlg@40tude.net>,
prova@prova.com says...
> Hello!
> I'm getting started with the study of operational amplifier. Fist of all,
> do you know any site which could explain clearly this matter? Could you
> write some link?
Have you tried using a search engine?
> In particular, I can't understand the reason why a saturation occurs when
> the output signal is connected to the + terminal. The output is alway
> connected to che other terminal, the - one.
> Why? Can you help me?
Your statement and assumptions aren't true. A voltage-follower has
the input tied to the '+' terminal. The key thing to remember about
an (idealized) op-amp is that the differential gain is infinite.
This implies that the differential input voltage (from the '+' to
'-' terminals) is zero. Also, the input impedance is infinite and
the output impedance is zero. The rest can be easily derived from
there; solve for the currents.
> Thank you anyway!
--
Keith
| |
|
| I do not want to get into details or speculate & throw you off., but i
believe transistors in OP amplifiers are not to be considered as having
+ & - terminals.
though the choice is at hand.....
Read On - Study Hard & Google your Doubts That's more than we could do
back when........
Best Wishes,
Roy Q.T.
Urban Technician
[I don't make em, I just fix em]
| |
|
| In article <4149-46BF0553-2451@storefull-3255.bay.webtv.net>,
ROYKEY@webtv.net says...
> I do not want to get into details or speculate & throw you off., but i
> believe transistors in OP amplifiers are not to be considered as having
> + & - terminals.
The '+' and '-' terminals of an op-amp are also known as the "non-
inverting" and "inverting" inputs, respectively. The output is the
gain times the difference in these two voltages. Since the gain of
an ideal op-amp is infinite, the difference between these two inputs
must be zero. Everything you ever wanted to know about op-amp
circuits falls out from there. ;-)
> though the choice is at hand.....
>
> Read On - Study Hard & Google your Doubts That's more than we could do
> back when........
Google is more than we could do back when, but the other two options
were available. ;-)
--
Keith
| |
| daestrom 2007-08-12, 5:25 pm |
|
"krw" <krw@att.bizzzz> wrote in message
news:MPG.2127a1f9fd2365b298a8ad@news.individual.net...
> In article <qhouphdm8h7t.oqjzakcpd5qy.dlg@40tude.net>,
> prova@prova.com says...
>
> Have you tried using a search engine?
>
>
> Your statement and assumptions aren't true. A voltage-follower has
> the input tied to the '+' terminal.
He didn't say 'input', he asked about tieing the 'output' to the '+'
terminal.
Tieing the 'output' to the '+' terminal is a recipe for saturating the
amplifier.
daestrom
| |
|
| In article <46bf5166$0$8966$4c368faf@roadrunner.com>,
daestrom@NO_SPAM_HEREtwcny.rr.com says...
>
> "krw" <krw@att.bizzzz> wrote in message
> news:MPG.2127a1f9fd2365b298a8ad@news.individual.net...
Saturation occurs when the *OUTPUT* (me be bad) is tied to the '+'
input because when the output tries to go high the input forces it to
go higher (positive feedback) until it hits the power rail.
[color=darkred]
>
> He didn't say 'input', he asked about tieing the 'output' to the '+'
> terminal.
Yes, I missed that.
> Tieing the 'output' to the '+' terminal is a recipe for saturating the
> amplifier.
Depends on the phase shift. ;-)
--
Keith
| |
|
|
True. But a LITTLE positive feedback can be useful in applications where
the output is TRUE/FALSE (e.g.: thermostat) and you want a "dead zone"
where the output will not swith.
[color=darkred]
>
> Depends on the phase shift. ;-)
>
> --
> Keith
| |
| Carlton Stedman 2007-08-13, 9:25 am |
| On Aug 12, 5:04 pm, <nni/gil...@nni.com> wrote:
>
> True. But a LITTLE positive feedback can be useful in applications where
> the output is TRUE/FALSE (e.g.: thermostat) and you want a "dead zone"
> where the output will not swith.
>
>
>
>
Aka, a "schmitt trigger", being a comparator with positive feedback to
set a hysteresis window ("dead zone" as the above called it). One
common application being using the hysteresis to debounce on something
that switches.
With positive feedback, the output will saturate to one or the other
rails with a very small difference in the inputs. As real op-amps are
not ideal, their open loop gain is *not* infinite, so, one
approximation for output voltage is simply:
Vo=Aol(V+ - V-)
This shows that as Aol increases, the differential voltage needed to
saturate the op-amp (since, the output voltage should really be very
high or low, but gets clamped to the rails) decreases.
You can bias up an op-amp by providing a very small differential
voltage across the inputs such that the output is some value between
the rails. This becomes more difficult with a higher open loop gain,
since the higher the gain, the smaller the differential voltage
required to cause the op-amp to saturate.
[color=darkred]
>From a transistor level on the simplest of op-amps, whats happening is
that the input stage to the op-amp is a differential pair of
transistors with large current mirrors set as an active resistance,
providing much more current than just using simple resistors and
adapting over temp and such. Basically, the current gets divided
between the + and - legs of the diff pair which in turn appropriately
biases the output stage of an op-amp, like a common source amplifier.
Depending on the amount of current, it changes how well this amplifier
can pull up to the positive rail or down to the bottom rail. In real
op-amps, there are additional stages, which provide more gain (meaning
the output transistors can pull close to the rails with less current
from the differential pair).
Positive feedback is not well-suited for amplification purposes.
| |
| dave y. 2007-08-14, 3:25 am |
|
>Positive feedback is not well-suited for amplification purposes.
Funny, my very first shortwave radio used positive feedback, and
it worked sorta ok.
| |
|
| In article <ei62c3520ajcnniv2r6v8kgvafaqvc778f@4ax.com>,
nospam@myhouse.com says...
>
>
> Funny, my very first shortwave radio used positive feedback, and
> it worked sorta ok.
Telephones have worked for a hundred years or so too.
--
Keith
| |
| VWWall 2007-08-15, 3:25 am |
| krw wrote:
> In article <ei62c3520ajcnniv2r6v8kgvafaqvc778f@4ax.com>,
> nospam@myhouse.com says...
>
> Telephones have worked for a hundred years or so too.
>
One of the early open wire carrier amplifiers was conditionally stable.
When turned on in the normal way, it would oscillate. The procedure
was to remove one particular tube, turn on the power, and after the
remaining tubes had warmed up, insert the missing tube. This avoided
the gain level at which the amplifier was unstable.
These were in service for many years!
--
Virg Wall, P.E.
| |
| Salmon Egg 2007-08-15, 9:25 am |
| On 8/14/07 6:22 PM, in article
MPG.212b850622095c5f98a8d0@news.individual.net, "krw" <krw@att.bizzzz>
wrote:
> In article <ei62c3520ajcnniv2r6v8kgvafaqvc778f@4ax.com>,
> nospam@myhouse.com says...
The regenerative receiver was invented by Edwin Armstrong before he was a
full fledged adult. To me, the performance from a single tube device was
amazing. I still marvel at it. Moreover, it worked when it was unstable. The
oscillating detector enabled listening to cw signals.[color=darkred]
>
> Telephones have worked for a hundred years or so too.
In the old days when the microphone and hand-held receivers, it was great
fun to place the receiver over the mouthpiece to get howling from the
feedback. I just tried it again. It still howls B I had to use two
telephones in order get receiver to mouthpiece proximity.
Bill
--
Iraq: About three Virginia Techs a month
|
|
|
|
|