TL971: The Unit gain bandwidth question for TL971

Mickey,

Quote : "For the figure 2 of the TL971 datasheet, when Gain is 0dB, the frequency(the blue line) is 12MHZ and the phrase (the red line) is 0."

Yes that is true. Look close at other points. For all frequencies Gain dB = Phase degrees.
Both curves are the same data which is gain in dB.

The good news it that the phase margin was put into the label. Therefore that label phase margin is credible.
I will add this to the data sheet error update database.

In figure 1, find gain 0dB, go vertical to phase line and read PM directly. No math involved.

Hi Ron,

I still do not understand that you said.

Q1: For figure 1 and figure 2, they are differences are φM and f0. But what are the φM and f0?

Q2: For figure 1 and figure 2, you mean that find gain 0dB, go vertical to phase line, then the intersection of the vertical line
and the red line is the phase margin?
If the intersection of the vertical line and the red line is not the phase margin, what is the meaning of this phase?

For the current TL971 datasheet, how to determine the phase margin from figure 1 and figure 2?

Q3: Would you suggest a common method to determine the phase margin from bode chart?

Mickey,

φM is phase margin for a unity gain amplifier and f0 is frequency where open loop gain is 0dB.

Let's' do this graphically. For figure 1, figure 2 is broken so this won't work.

Follow purple line starting at left side of graph at 0dB 

Notice that phase is +180 at lowest frequency and gain is flat, this is a closed loop response. This means Y axis is phase margin with no additional math.

If phase at lowest frequency was +90 and gain is falling, then loop is open and Y axis is phase margin with no additional math.

If phase at lowest frequency was -90 and gain is falling, then loop is open and Y axis is amplifer lag so phase margin is Y axis + 180  

Hi Ron,

Thanks for your help.

Q1: Why is the figure 2 broken?

Q2 : Regarding "If phase at lowest frequency was -90 and gain is falling, then loop is open and Y axis is amplifer lag so phase margin is Y axis + 180 ", why the phase margin is change to +180? not-90?

Q2: For the figure 1, since the phase margin is 63.7 and the unity gain frequency is 8.4MHZ.

If the customer needs the system to remains stable, should the input signal frequency be less than 8.4MHZ?

Mickey,

A1: Both red and blue curves are gain plots. There is no phase plot. The phase degree scale is actually dB for gain.

A2: see picture. -105 -90 = -195 degrees (not 75) ; -105 + 180 = +75 (matches text of graph) 

Note +180 is the same as - -180

Stability is based on negative feedback and op amp performance. Input signal is not a factor. 

Input signal can exceed loop bandwidth. This won't affect stability.