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-3db frequency of the op-amp open loop gain

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Junus2012

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Dear friends,

What is the importance of the -3 dB freuquency in the open loop charasterics of the operational amplifier. We mostly are design to satisfy the GBW value then f-3dB just comes as a consequence of that

Thank you very much
 

It's a parameter of a simplified ideal OP model with first order characteristic together with the DC gain. I think it's not so important for real OP with more complex transfer function.
 
FvM said:
It's a parameter of a simplified ideal OP model with first order characteristic together with the DC gain. I think it's not so important for real OP with more complex transfer function.
Click to expand...

Dear FvM

Thank you for your reply

It means that the dominant pole (which represent the -3 dB frequency in the open loop) has no effect for circuit performance for example speed and settling time
 

What is the importance of the -3 dB freuquency in the open loop charasterics of the operational amplifier....t
Click to expand...

Usually it is 3db power or gain point; -3db is approximate equal to 50% of the original gain (sometimes power) and 3db is similarly equal to double (twice) the gain.

3db=0.3 bel=10^(0.3)=1.995

-3db=-0.3 bel=10^(-0.3)=0.5012

the 3db point (and its multiples) are very handy points for a quick overview.
 
It means that the dominant pole (which represent the -3 dB frequency in the open loop) has no effect for circuit performance for example speed and settling time
Click to expand...
Not with sufficiently high open loop DC gain.
 
We do design for unity GBW and DC gain and in the case of a -20db/dec roll-off the -3db frequency in the open loop transfer comes as a consequence. If you imagine your opamp as an ideal integrator, then the -3db frequency is nowhere to find but you still care about the unity GBW.
 
Junus2012 said:
Dear friends,

What is the importance of the -3 dB freuquency in the open loop charasterics of the operational amplifier. We mostly are design to satisfy the GBW value then f-3dB just comes as a consequence of that

Thank you very much
Click to expand...

If you want to use your op-amp as a crude open loop comparator, your -3dB can play some role in the initial part of the output before slewing effects take over.
 
vivekroy said:
If you want to use your op-amp as a crude open loop comparator, your -3dB can play some role in the initial part of the output before slewing effects take over.
Click to expand...

Dear Vivekroy

So for closed loop, GBW is the important parameter which also has an impact on the settling time of closed loop feedback, which beside the slew rate affect the speed of the op-amp

in open loop comparator then the -3 dB is becoming a paramter to consider, right ? how it will affetc the operation ?
 

It looks to me like a purely theoretical consideration. Comparator is in saturation most of the time, how should open loop bandwidth affect the operation. Recovery time and slew rate do, but they are not directly related to o.l. bandwidth.
 
Slewing is the most important characteristic I think as well.

Assume that there is a two stage amplifier with the dominant pole at the end of the first stage. Assume that the input difference is not enough to cause slewing in the first stage but high enough to cause slewing in the second stage. Then for the output stage to start slewing, the first stage output needs to go high (or low) enough and the speed at which it does is going to be given by the open loop bandwidth.

This is an exaggerated example of -3dB pole affecting the initial part before slewing takes over.
 
Comparators are more simply characterized by propagation delay and maximum switching frequency rather than -3db bandwidth.
 
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