A simple differential pair gain and phase response question

[CAMALEAO]

Simple differential pair gain and phase response question

Hi everyone,

When plotting the phase response of the first stage in a two-stage amplifier I see the following behavior:



Does anyone knows why there is a zero in the differential input stage? Where does it come from?

Regards.

 

[LvW]

Re: Simple differential pair gain and phase response question

Unfortunately, it is not easy to read and interpret the graph. Furthermore, I cannot see any scaling of axes.

 

[FvM]

Re: Simple differential pair gain and phase response question

Seeing a schematic of the actual two-stage topology would also help.

 

[CAMALEAO]

Re: Simple differential pair gain and phase response question

You are right. Forgot to mention which topology.

The two-stage amplifier that I am talking about is the typical two-stage amplifier, where the input stage is a PMOS differential to single-end amplifier with mille compensation. It is the typical 2-stage amplifier that we see for example in Allen's book.

 

[CAMALEAO]

Re: Simple differential pair gain and phase response question

The circuit that I am referring to is the following:



I have measured the differential gain between the output and the input and I got the above plot.

The thing that I am not understanding is why are we seeing a zero? Does anyone tried to do this and saw the same effect? Where is this zero coming from?

Regards.

 

[frankrose]

Re: Simple differential pair gain and phase response question

1, could you post a real graph with scaling as LvW mentioned?
2, could you show how is your testbenck looks like and how you calculate the plotted curves?
3, could you post an other graph where you just calculate this: dB20(VF("/Vout"))?

 

[LvW]

Re: Simple differential pair gain and phase response question

The thing that I am not understanding is why are we seeing a zero? .

...."we are seeing a zero"? Who is "we"? I cannot see any zero.
Perhaps it is helpful to explain your understanding of a "zero".
* A zero in the p-n diagram, or
* Magnitude with 0 dB?
* Magnitude with an absolute zwero (impossible)
* Zero phase shift?