Hi,
In SLV662, it says in the conclusion:
For OTA, both R1 and R4 enter into AC analysis, and note that only the ratio of the feedback resistors is important.
Yes, the ratio of the R1 and R4 will determine the output voltage. But I can't understand why only their ratio is important. It seems their impedance in parallel will affect the fp2 too. May someone clarify this for me?
Thanks.
Hello,
I have pasted the fig for reference below.
Regarding a GM error amplifier, the mid band gain is set by the resistor divider ratio (Kfb) times the Gm times R2. The parallel combo of R1 and R4 is not considered here as its different from a standard op amp.
the transfer function for the OTA is
where
Even if you were to add an Cff, a cap in parallel with R1 Kfb would be:
It is not Cff//R1//R4.
The above is true assuming C1>>C3 & Rout (Gm amp)>>R2
hope this helps.
David.
Hello,
sorry correction for above, I got my ref designators mixed up.
Type 2
The Kfb term changes to the formula below.
R4 being the bottom FB resistor.
Type 3 with an RC in parallel with the tope FB resistor
Hope this helps.
David.
Hello,
Equation 50 is indeed correct. COUT is a short circuit as its assumed to be much greater than Cff.
therefore, with COUT being a short circuit the R1 is indeed in parallel with R4.
You can choose to follow either approach, seeing as the app note is complete suggest following this as it is correct.
Interesting that both approaches produce exactly the same result. refer to Equation (41) and (42) as it relates to H1(s).
Vcomp/Vout
Sorry for the confusion.
David.
