Hi, I was looking for a CMOS OpAmp circuit which doesnt need need any special Common Mode Feedback circuit for stabilizing the bias point. Anybody has come across a circuit like that ?
Please let me know regards Jean
How to do it is right there in your textbook ;-)
...Jim Thompson
-- | James E.Thompson, P.E. | mens | | Analog Innovations, Inc. | et | | Analog/Mixed-Signal ASIC's and Discrete Systems | manus | | Phoenix, Arizona 85048 Skype: Contacts Only | | | Voice:(480)460-2350 Fax: Available upon request | Brass Rat | | E-mail Icon at
Take a look at some of the CMOS op-amps that RCA made back in the stone age. They had a very low transistor count.
Do you mean fully differential CMOS opamp?
Yes, I was looking for fully differential design.
Jim, which is the textbook you are talking about ? GHLM ?
regards Jean
Unless you have something really lame such as accurate resistor loads and a diff pair driven by an accurate bias current, I don't see any way to do such a design without common mode feedback. Basically, to do such a circuit without common mode feedback, you need to be able to look at the circuit and predict the bias conditions, specifically the voltage at the high impedance nodes (gain nodes).
You can do common mode with switch cap, but that doesn't mean you don't have common mode feedback.
As "miso" points out, I don't think a full differential OpAmp can be done without a common-mode loop; though I think (without putting pencil to paper) that you _may_ be able to do it if you have a ground pin. (But I think that can be argued as a variation of a common-mode loop.)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
=A0 =A0 ...Jim Thompson
=A0 | =A0 =A0mens =A0 =A0 |
=A0 | =A0 =A0 et =A0 =A0 =A0|
=A0|
=A0 =A0 =A0 |
Even with a ground pin, I don't see a way to establish a common mode at the outputs of the op amp without some sort of feedback, with the one exception I stated in my previous post, i.e. well known current and resistor loads. High impedance nodes tend to go to a rail without feedback.
I suspect the original poster isn't phrasing the question properly.
[snip]
On further thought, full differential _has_to_have_ a common-mode loop.
CMOS-specific textbooks I own...
Razavi Laker Sansen (presented a seminar with Sansen in Melbourne in '86) Allen Holberg Gregorian Temes Johns Martin IEEE paper collections: Gray Hodges Brodersen Gray Wooley Brodersen
plus numerous general analog design texts.
I collect almost anything published... one good idea obtained is worth the price ;-)
...Jim Thompson
--
| James E.Thompson, P.E. | mens |
| Analog Innovations, Inc. | et |
| Analog/Mixed-Signal ASIC\'s and Discrete Systems | manus |
| Phoenix, Arizona 85048 Skype: Contacts Only | |
| Voice:(480)460-2350 Fax: Available upon request | Brass Rat |
| E-mail Icon at http://www.analog-innovations.com | 1962 |
I love to cook with wine Sometimes I even put it in the food
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