frequency characteristics of transistor

[geozog86]

Hello!

I am a bit confused about the unity frequency of a MOS transistor.

I see in one book that wunity=gm/Cgd, which makes sense when the transistor is ideally driven (by an ideal V source)

I see in Gray/Hurst book that wunity=1/[Rsource*(Cgs+Gain*Cgd) which makes sense if there is non ideal driving of the transistor so we see a miller capacitor in the place of Cgd.

BUT i see in another book a wt=gm/Cgs. It says that wt is where Ig=Id, and not always the same like wunity, but where does this formula come from? Since in most of the cases wt is usually wunity (or if we take it for granted)

Thx

 

[FvM]

Transistor configuration? Chapter?

 

[geozog86]

From the Gray Hurst Lewis booK? My mistake it's the w3dB. I have a translated version, so if they follow the same ordering, chapter 7(frequency response) for the common emitter configuration, the pole 1. But this gives me no answer as to where the wunity formula comes from

 

[FvM]

I assume, that you mean unity frequency as a synonym of transition frequency. fT.

In Gray/Hurst/Lewis/Meyer (I'm referring to the 4th edition), fT is introduced in paragraph 1.6.8 MOS Transistor Frequency Response.

In chapter 7, fT is referred as a reference value in discussion of the CS, CD and CG circuit frequency responses. The value isn't directly observed in any of this circuits, but several pole and zero frequencies have a similar order of magnitude, so a known fT value can be used as a rough estimation.

Generally, you need to read thoroughly. In Gray/Hurst ωT=gm/Cgs is a simplified expression, neglecting Cgd and Cds. gm/Cgs is also an exact expression for the zero in CD circuit (source follower) frequency response in paragraph 7.2.3.2 Frequency Response of the Source Follower.

 

[geozog86]

Cool, thx, i'll try to pay a bit more attention to understand what's going on, and i'll be back with more Q if there will be any.