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24th March 2019, 15:52 #1
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Miller OpAmp  PMOS input differential vs NMOS input differential
So, I've encountered two stage miller with PMOS differential pair as input while others have NMOS differential pair input. Are there any advantages/disadvantages of using one over the other? When is it preferable to use a PMOS differential pair as input and when is it preferable to use NMOS differential pair as input?
If you can recommend or know references or materials that discuss this deeply, it would be greatly appreciated. Thanks!

24th March 2019, 16:04 #2
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
The choice is often dictated by the minimum input of the signal. Assume you have a simple OTA with a NMOS input pair, then the minimum input voltage is given by the sum of Vgs across your input pair plus the overdrive of the tail current source. Assume you have an overdrive of 0.2V across the tail current course and an overdrive of 0.2V for the input pair transistors and a Vth of 0.5V. Then the lowest voltage you can apply is 0.9V. On the other hand if your input signal has a large upper voltage, PMOS will be a problem.
The other reasons can be matching. PMOS and NMOS have a different standard deviation of Vth mismatch.
Noise can be different as well which may be another consideration.

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25th March 2019, 04:52 #3
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
1. To clarify, for large input signal, PMOS will be a problem whereas for small input signal NMOS will be a problem. But why would this be a problem for PMOS or NMOS? Sorry, still kind of confused.
2. Is Vth mismatch a casetocase basis for NMOS and PMOS? Or like, is one always better than the other?
3. Similar question with previous, regarding noise performance.
If you can recommend references or books with this discussion, it would also be greatly appreciated. I'm still not that knowledgeable with this. Sorry if the questions seem kind of stupid.

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25th March 2019, 08:58 #4
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
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25th March 2019, 12:04 #5
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
Ah, I think I now understand what you mean regarding the input. Sorry if it sounds unclear. I'm just trying to find out when and why a pmos or nmos input can be used. Regarding the casetocase, what I want to know is, for example, is it always true that using PMOS as input have better standard deviation and noise performance than NMOS as input (or the other way around)? Or will this depend?

25th March 2019, 16:23 #6
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
Regarding to noise, PMOS have smaller flicker noise coefficient, so with same area they produce smaller noise power at lower frequencies than NMOS input stages, always, this is coming from their process.
However the gm of PMOS input stages also smaller (PMOS mobility  µP  always smaller than NMOS's), so over a corner frequency the thermal noise contribution usually bigger for PMOS.
So low bandwidth direct conversion receivers prefer PMOS active mixers for example, but heterodyne or even lowIF mixers have better performance sometimes with NMOS input stages, same true for amplifiers.
Mismatch probably also process dependent, but unfortunately I haven't experienced big differences between the types, I cannot recall a case."Try SCE to AUX." /John Aaron/
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26th March 2019, 19:35 #7
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
Noises and mismatch are process dependent.
In most of the CMOS PSub NWell processes, pfets are placed in nwell, which has to be created in psub. It result with one step more with doping the silicon, so it increase variability of doping in pmosfets and thus it worse mismatch coefficient in these processes. It is not true in complementary processes (not used for years NSub with Pwells) or in FDSOI, where mismatch for the same transistor flavor is the same. There is an additional story in decananometer CMOS related to halo implantation making mismatch coefficient dimensions and bias dependent.
Flicker noise also depends to the process. Considering CMOS PSub NWell process, pmosfet is created in the nwell, and (omitting physical details) thus it channel is formed deeper than in nmosfet. So, thicker interface layer between channel and oxide results with different mechanism of flicker noise. In such processes, flicker noise in pmosfet is mostly related to fluctuation of carrier mobility and thus is lower and current density dependent (proportional to transconductance  almost constant in weak inversion and prop to square root of current in strong inversion). The channel in nmosfets is formed close to oxide (channeloxide interface layer is thinner) and flicker noises are generated by interface traps, fluctuating carriers number and thus flicker noises are constant vs operating point and usually much higher. And again, above does not longer true for FDSOI or CMOS with NSub and PWell.
In general, transconductance of both transistor types is the same for the same operating point (with accuracy to slope factor), the difference between p and nfets caused by differences in mobility of electrons and holes is in capacitance and thus in transit frequency.
In general, the complementary Miller OpAmps, designed in the same process, with transistors biased in the same operating point will have almost the same performance  gain, stability, bandwidth, offset, noises and PSRR.
There is a lot of related topics on this forum showing a comparison of complementary opamps also.
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27th March 2019, 03:12 #8
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential

28th March 2019, 12:49 #9
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
Take a look at chapter 1.1.1 here: https://payhip.com/b/5Srt (CLICK PREVIEW AT THE TOP RIGHT CORNER). Especially, Fig. 1.3 and 1.4. Maybe that could help?
Regarding other resources Razavi and Baker are always good.
Generally, whether it's NMOS or PMOS input stage depends on the specification. If you want to pass 01V input signal you choose PMOS input stage. If you have voltage follower for 1V input (e.g voltage reference / bandgap) you may choose NMOS or PMOS. This question is specific. When you have concrete cases it's easier and very often clear. :)
Regarding matching, take a look into foundry documentation. There are plots for matching.

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28th March 2019, 21:34 #10
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Re: Miller OpAmp  PMOS input differential vs NMOS input differential
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