A question about slewing in an opamp

Status
Not open for further replies.
N

naalald

Full Member level 4
Joined
Dec 2, 2006
Messages
216
Helped
30
Reputation
54
Reaction score
5
Trophy points
1,298
Activity points
2,627
To my knowledge, an opamp slews if its differential input is large enough to turn off one of the two input transistor pairs. In this case the output current is limited and the output capacitor is charged with a constant current so that the output voltage will be
vo(t) = SR×t
So if the differential input of an opamp is so small that none of the two input transistors turns off, can we say that the opamp does not slew? How can we recognize if an opamp slews?
 

an opamp doesnt slew only when the diff input is large enough to turn off one of the two input transistor pairs.... an opamp slews when there is a change in the input which needs the opamp to change its output faster than it can.... whenever the opamp slews the ouptut will be a straight line with a constant slope equal to the slew rate....
 

    N

    naalald

    Points: 2
    Helpful Answer Positive Rating

Mmm...I have doubts about this. If the signal is not large, it is a high frequency small signal and first stage is fully operational. Won't the response be dictated by the linear exponential charateristic ? If both the input transistors are on, there is constant feedback about what is happening at the output which will make the response exponential. AFAIK slewing is a large signal phenomenon and it requires the feedback path to be shut off.
Or is my beleif wrong?
 


when i said change of input at a fast rate i meant high frequency only... i didnt talk about large signal... afaik i know feedback has nothing to do with slew rate... feedback doesnt have a curative effect on slew rate... say your input stage is staying on but there is a finite gap between the input and the feedback from the output... give a square wave (high frequency of small amplitude) input to your opamp which has considerably high gain you'll be able to see the slew in the output....

the higher the gain the larger will be the chances of slew... because a small input change means a large output change... correct me if there is a fault in what i'm sayin...
 


Thanks for your reply. That's right, but the change in the output rate is also dependent on the level of the input, in addition to it's rate. Is this right?
 

 


Feedback (or the lack of it) is responsible for slewing in opamps. For exponential characteristic, the rate at which the final value is reached depends on the instantaneous difference between the desired output and the instantaneous output. As long as there is a feedback link, the rise/fall can only be exponential. A major disadvantage of Slewing is that it can introduce a lot of input differential voltage which might be catastrophic (the protection diodes add lot of noise) and this is at unity gain feedback.

Have a look at this very popular paper by J.E.Solomon
**broken link removed**
 

 

When you apply a step, and if the amp is linear everywhere, you expect exponential chracteristic.
I just tried to explain the basic nature of the exponential response (dy/dt=ky)
 

saro_k_82 said:
When you apply a step, and if the amp is linear everywhere, you expect exponential chracteristic.
I just tried to explain the basic nature of the exponential response (dy/dt=ky)
Click to expand...

Yes, that´s the small signal response of an universal compensated opamp (which behaves like a first order system). But the question was related to a large signal response and the corresponding slew rate effect. OK ?
 

LvW said:
Yes, that´s the small signal response of an universal compensated opamp (which behaves like a first order system). But the question was related to a large signal response and the corresponding slew rate effect. OK ?
Click to expand...

This is exactly what I was trying to point out to Anand. Unless the loop is broken, the response cannot be anything other than exponential. He was claiming of small-signal slew response., and I tried to point this out. Cool
 

saro_k_82 said:
This is exactly what I was trying to point out to Anand. Unless the loop is broken, the response cannot be anything other than exponential. He was claiming of small-signal slew response., and I tried to point this out. Cool
Click to expand...

I think a reason for the slight misunderstanding between us is connected with the meaning of "slewing". For my understanding, there is no "small-signal slew response" since the term "slewing" is connected with a kind of non-linearity within the circuit caused by a large signal excitation (with saturation of the input stage for a short period).
Regards
LvW
 

Status
Not open for further replies.

Similar threads

Menu
Log in
Register
Cookies are required to use this site. You must accept them to continue using the site. Learn more…