About bode plot (mag and phase plot)

Hi, all.

Suddenly, I am wondering how I got bode plots of common source amplifiers.
As you know, without dominant pole consideration, the gain of the common source amp is -GmRout. There IS a negative sign!!!

Since bode plots show both magnitude and phase reponse, the magnitude only indicates the absolute value of the amplitude (of course, in the bode plot, in dB) - So, there is no sign information!

Now, we typically starts with 0 degree until we hit the 1/10 of the (dominant) pole frequency. However, ISN'T -180 (or 180) corresponding to a negative sign?? With 0 degree phase shift, at DC, the voltage gain should be GmRout!!

I know I am wrong here..., but I am blind for it..Please let me open my eyes to see the truth^^

Please let me know....

Thanks,

yes , this is true , just write the gain in the frequency domain and include all ploes and all signs, signs are as u said a 180 degree shift

well i couldnt get you .the common source is an inverting amp. but in gain we take mag. irrespective of the sign. and that is why it is above 0db and if it is attenuating then it is below as my Vo/Vin is less than 1 for which log value is negative.

for phase u can plot starting at any angle. only thing is that u have t0 take care that ur closed loop phase shd not be regenerative (0 or 360) . thats it. phase margin is of no concern when it is in openloop.generally it is wise to plot the phase from 0 degree as it is pretty straightfwd

Hi,
To make my question short, at DC, phase shift is 180 degree....(assuming poles/zeros are far way from the origin), but why we don't draw the phase plot from -180.

Is the phase plot drawn relative to other poles and zeros?
In other words, if the bode plots are given , ie, one pole - some positive mag and phase plot starting from 0 degree, then you cannot tell that there is an inversion at DC??
Is that true???

the idea is that the important thing is to avoid the positive feedback which happens at a phase shift of 360 deg (zero deg),
so if u start at zero (we didnt draw the -ve sign effect) then u compare to -180 and if u start from -180(when taking the -ve sign into consideration) u compare to -360 , so in both cases we compare to the +ve feedback condition and in both cases we take the -Ve sign into our consideration, but one case draw it and the other dont.
regards

ee484 said:

Hi,
To make my question short, at DC, phase shift is 180 degree....(assuming poles/zeros are far way from the origin), but why we don't draw the phase plot from -180.

Click to expand...

What do you mean by this, I mean, can you tell which component's phase plot we don't draw from -180.

Regards