Why we care unity gain frequency of amplifiers?

[pansoo]

unity gain care

Is it right that the usuable operation frequency of amplifiers are ~3db frequency?

But when we design an amplifier, why care unity gain frequency?

Is there any reason or just see because 3db freq =unity gain freq/gain?

And also why we care PM at unity gain frequency? The region is not usuable frequency.. isn't it?

 

[naalald]

Hi,
The time domain behavior is dependent to UGF. PM Settling ...

 

[iamxo]

I think, because we use amp in feedback loop normally. under this condition, the BW of closed-loop amp is approximately the UGF of the amp in open-loop.

so, we consider the UGF of the amplifier.

 

[btbass]

The unity gain frequency of the op-amp is the maximum bandwidth of the op-amp.
This falls with rising gain.
A rule of thumb for bandwidth of the amp is:
unity gain frequency/ gain
So, if the unity gain frequency = 4MHz and the circuit gain is 100, the amp is good up to 40KHz.
.

 

[pansoo]

Thank you very much!! iamxo

It's my wanted answer. I simulated and checked it is similar.

But I have more question.

1. In feedback loop, -3db point, y/x=A/1+A=0.707=1/sqrt2 ==> A(open loop gain)=2.41=7.64dB

Why OL 0dB point is CL -3dB point?

2. When we design an opamp, we compensate PM to load capacitor using capacitor or resistor. In commercial opamp, how compensate the PM, how to know the load capacitor? um.. If I use big compensate capacitor, then is it always ok?

 

[A.Anand Srinivasan]

I think, because we use amp in feedback loop normally. under this condition, the BW of closed-loop amp is approximately the UGF of the amp in open-loop.

so, we consider the UGF of the amplifier.

in closed loop operation the bandwidth is not effectively equivalent to the UGF,,,, the closed loop system obeys the gain bandwidth product if it is a one pole system....

the unity gain frequency is the frequency at which the oscillatory characteristics of a closed loop system might begin...
if u observe carefully for amplifiers we usually talk in terms of 3dB cut off frequency and for oscillatory characteristics or oscillators we talk about UGB,,,,

 

[naalald]

Hi pansoo,
For single pole opamps if you write the equations you can see that the OL 0dB is the CL -3dB.
The compensation in single stage opamps is done with the load capacitor and you should know its value. But in two stage and more opamps the load capacitor does not have the role of the compensation.

 

[lijianheng]

It should be concerned when the loop is closed and there is negaive feedback.

 

[jnuhope]

Hi pansoo,
For single pole opamps if you write the equations you can see that the OL 0dB is the CL -3dB.
.....

I think the above statement only valid when β=1. Pls correct me if I am wrong.

 

[naalald]

Hi,
Thanks jnuhope. For single pole if you write the equations you will have:
OL 0dB = ω1×A
CL -3dB = ω1×β×A
(A is the open loop DC gain).
And you can equal these two values for β=1.