Dominik Przyborowski said:
No, for the same bias condition and transistors area we can obtain better matching for NMOS input diff. pair and pmos load
... Can you supply some backup data / paper reference.
You can check for example a Patrick Drennan paper about matching from SSC 2001 or check his Ph.D. Also Bastos, Steyart, Tuinhot etc (many guys from esat-micas in Leuven) reached similar conclusions.
In brief:
1. The most important mismatch in mosfets is threshold voltage mismatch (current gain factor mismatch is in a range of ~1% so it's negligible)
The A_vt parameter for nmos is two times smaller than for pmos. Is in a range of 5-9 mV/um for nmos and 8-15 for pmos in technologies like 0.09-0.35.
2. The current gain factor is proportional to carrier mobility - electrons are 3 times faster than holes so K_n is also ~3 times higher than K_p.
We know that current mismatch between two identical mosfets is given by formula (assuming ideally flat output characteristic and A_K=0):
\[\sigma(I_d) \approx \frac{1}{\sqrt{WL}}\frac{2 A_{V_{th}}}{V_{od}}\]
Due to 3 times difference in current gain factor between pmos and nmos, a pmos transistor has ~3 times higher overdrive voltage (V_od) than nmos with the same dimensions and drain current. So current sources maded by pmos transistors has better matching than maded on nmos.
On the other hand threshold voltage mismatch coefficient is still two times better for nmos than pmos, so input diff. pair maded with nmos has smaller vgs variation than pmos diff pair with the same dimensions.
Additional factor for offset voltage in differential amplifier is bounded with input transistor tranconductance and overdrive voltage (input pair should be in moderate inversion). A few years ago I lost a few minutes to derive a formula for offset and i obtained following formula:
\[V_{offset} \approx \sqrt{\sigma^2_{V_{th_{in}}} + \frac{I^2 \cdot \sigma^2_{I_{load}}}{gm_{in}^2} + \frac{V^2_{od_{in}} \cdot \sigma^2_{\beta_{in}}}{4}}\]
Dominik Przyborowski said:
Slew rate depends only to tail current and compensation capacitor values.
... See "Basic Opamp Design and Compensation" by Johns & Martin, pg. 231.
I have both edition of this book, in second one it's on the page 252 ;-)
It's still the same formula: SR=I_{tail}/C_c
The consideration in this paragraf are maded with a few assumptions like the same dimensions of input transistors for p- and nmos version, etc. But still slew rate is the ratio between tail current and compensation capacitor.
Dominik Przyborowski said:
Only PSRR+ is better when the output is made on nmos, PSRR- is as bad as PSRR+ for pmos output.
In two stage opamp PSRR depends to compensation technique.
With an ndiff input, high frequency noise will couple from the supply through the Cgs of the driving PFET and then
the compensation cap to the output.
With a pdiff input, high frequency noise will couple from the substrate through the Cgs of the driving NFET and then
the compensation cap to the output.
You will typically use a quite / isolated substrate for sensitive analog blocks such as opamps. Therefore, would you not agree that noise
from the supply is typically worse than from the substrate. I have not seen the opposite but that is not to say it cannot occur. Have
you experienced such issues?
We could use a triple-well nfets for better isolation, but still the most important contribution to psrr is caused by compensation technique, because the C_c is the highest capacitor in two stage opamp (with capacitance almost two orders of magnitude higher than internal mosfets capacitances) and for higher frequencies (higher then dominant pole) becomes biasing output transistor as diode so psrr could fall to 0dB (or in worst case goes below) until frequency reach to zero in psrr bounded with input transkonductance and C_c.
For example in cascode compensation technique we have a current buffer between C_c and 2nd stage input so we get much better psrr.
Concluding, for the same two stage opamp with classic miller compensation, psrr depends only from the locations of poles/zeros in transfer function, input/output transconducatnce, compensation capacitar values, etc. but do not depends from type of transistors (for the same parameters of opamp).