why open loop configuration in not used in op amps ?

[desperado1]

open loop configuration

why open loop configuration in not used in op amps ?

 

[Externet]

open loop comparator

Because its gain is controlled by implementing the feedback. Open loop is like a car at full throttle all the time and with no brakes. Hardly useable.

 

[Audioguru]

open loop configuratio of op amp

An opamp has an open loop gain of about 200,000 from DC to about 5Hz then less gain at higher frequencies.

The input offset voltage could be 5mV which will cause the output to be saturated high or low when amplified 200,000 times.

 

[jerins]

op-amp as comparator in open loop configuration

because opamp is a high gain device .even a noise voltage can be amplified to large extent . if u r not using a feedback(normaly negative) the out put of open loop configuration is always saturated we get only a constant saturated voltage
such types of configuration is useless in real application thats way open loop configuration is not using ........becoz of this we control the gain of opamp usig a negative feedback

 

[anasense]

op not used

Open Loop Opamp basically = comparator.

 

[xulfee]

open loop op-amp for

it is basically a difference amplifier,means it actually amplifies the difference of voltages applied to both inputs,its gain is 10^6,so if i'm applying 1 v n 2v respectively to both pins my output will be 1V*10^6, which will bring ur opamp into saturation region,so to control its gain according to our desire we use -ve feedback

 

[Kral]

comparator open loop gain

desperado1,
The main advantage of an op amp is the ability to design a circuit whose transfer function depends only on external components, not the characteristics of the op amp itself. Of course, this is achieved exactly only with an ideal op amp, the gain of which is infinity. Such a device would not have many useful applications, except, as anasense pointed out, in a comparator application.
Regards,
Kral

 

[desperado1]

op amp configurations

thanks a lot mates for your reply i have another question for you all
we know that a LED is made up of Gallium Arsenide. Plus we also know that it is a diode having p and n junctions.
Normally a silicon is doped with born to make p type and with phosphorus to make n type.

we also know that a LED has p and n type junctions. then WHAT IS THIS JUNCTIONS MADE OFF ? IS IT THE SAME GALLIUM ARSENIDE OR OTHERS ?

thanks in advance

 

[Audioguru]

lm139 negative comparator

There are many newer LEDs that are made with different chemicals.
Gallium Arsenide was used in dim old red LEDs.

 

[rosarioantony]

openloopconfiguration

for an ideal opamp open loop gain is infinity . comparators dont have feed back.

 

[Audioguru]

op amp open loop configuration

comparators dont have feed back.

Oh yeah?
I said the same thing until somebody showed me the datasheet of the LM139 quad comparator by ST Micro:

 

[LvW]

open loop op amp configuration

comparators dont have feed back.

Oh yeah?
I said the same thing until somebody showed me the datasheet of the LM139 quad comparator by ST Micro:

In the above application the LM139 is used as a linear amplifier !! Therefore, negative feedback is used.
However, comparators very often have POSITIVE feedback because of a safe and clean switching function.

 

[desperado1]

op-amp in open-loop configuration

MR. LVW, How can one distinguish between a negative and positive feedback by simply visualizing an op-amp ?

 

[LvW]

=;p;lo=op[[[[=ii-op=o]p-[os

MR. LVW, How can one distinguish between a negative and positive feedback by simply visualizing an op-amp ?

I am not sure if I understood your question, however, the circuit as shown by audioguru incorporates negative feedback. Don´t you agree ?

 

[Audioguru]

what is open loop configuration in amplifiers

Negative feedback feeds some of the output to the (-) input.
Positive feedback feeds some of the output to the (+) input.

 

[LvW]

op amp openloop transfer function

Negative feedback feeds some of the output to the (-) input.
Positive feedback feeds some of the output to the (+) input.

Quite right - with some restrictions: From a system point of view it is interesting to realize that, for real systems, each negative feedback turns into positive feedback above a certain frequency limit; hopefully with a level which leads not to instability !

 

[desperado1]

op amp open loop comparator

MR. LvW
for real systems, each negative feedback turns into positive feedback above a certain frequency limit; can u give me a specific example ?

 

[Audioguru]

comparator open loop

MR. LvW
for real systems, each negative feedback turns into positive feedback above a certain frequency limit; can u give me a specific example ?

Opamps have an internal frequency compensation capacitor that reduces its gain above about 10Hz so that at the frequency where it has 180 degrees phase shift the gain is reduced to less than 1 so it does not oscillate.

 

[LvW]

gallium arsenide op amp

MR. LvW
for real systems, each negative feedback turns into positive feedback above a certain frequency limit; can u give me a specific example ?

Yes, no problem.
It is a simplified sight to define neg. feedback as the case when a signal is fed back to the neg. terminal of the opamp. Consider, for example, a case where you have an inverter in the feedback path (it´s not academic, it´s the principle of active phase compensation). Then, the feedback must be connected to the pos. terminal.
Therefore, a more general definition is needed:
We have pos. feedback if a signal at a specific frequency arriving at the opamp input is in phase with the incoming signal. This happens for each opamp which has a feedback path to the neg. terminal at a frequency (and above it) where the phase shift is equal to resp. above 180 deg. And to prevent oscillations, the loop gain at this point must be below unity. This case is called stable pos. feedback.
(By the way: This definition is not new. It was introduced by BODE more than 50 years ago).

Quote AUDIOGURU:
Opamps have an internal frequency compensation capacitor that reduces its gain above about 10Hz so that at the frequency where it has 180 degrees phase shift the gain is reduced to less than 1 so it does not oscillate.

Yes, that´s obvious (if the opamp is universal-compensated !).
But, nevertheless, as explained above : We have pos. feedback at this frequency (and above).

 

[desperado1]

negative feedback configuration

Mr. Lvw ii am not clear about this point

"It is a simplified sight to define neg. feedback as the case when a signal is fed back to the neg. terminal of the opamp. Consider, for example, a case where you have an inverter in the feedback path (it´s not academic, it´s the principle of active phase compensation). Then, the feedback must be connected to the pos. terminal. "