Low Power CMOS Operational Amplifier

1. Low Power CMOS Operational Amplifier Done By: Raza Hanif (3336928) Raheel Choudhary Fawad Usman Rathore

2. Contents of Presentation • Background and literature review • Different applications of CMOS OpAmp • Choice of complex electronic system • proposed circuit (cmos opamp) to be implemented in the system -> In the Book • analysis of proposed circuit • Improved, and more complex circuit • Analysis • System Analysis • Conclusion

3. BackGround Complementary Metal-Oxide-Semiconductor (CMOS) technology is circuit implementation using both pMOS and nMOS transistors on the same silicon chip. CMOS designs typically offer high gain and speed at low power consumption. CMOS scales well to smaller devices without drastic changes in performance.

4. This device is commonly used to amplify small signals, to add/subtract voltages, and in active filtering. It must have high gain, low current draw, and should function over a variety of frequencies.

5. In Context with Textbook TTL LOGIC OP741

6. Why CMOS over TTL? MOSFETS consume less power in the driver circuit. MOSFETS have greater bandwidth. MOSFETS are thermally more stable. MOSFETS are considerably ‘faster’ than BJT’s. CMOS ICs use much less power than TTL

7. Analysis of a Complex Electronic System The input consists of differential electrode inputs and the outputs are from: an amplifier, a peak detector, a trough detector, and a level detector. A trough is a noticeable decline in the signal strength

8. Implication of CMOS (OTA) amplifier in neural amplifier There is a great demand for technologies that enable neuroscientists and clinicians to observe the simultaneous activity of large numbers of neurons in the brain. While there are microelectronic devices being developed for small-scale amplification of the weak bioelectrical signals, existing circuits typically have unacceptable noise levels or consume too much power to be fully implanted in large quantities. In our electronic system, the bioamplifier must dissipate little power so that surrounding tissues are not damaged by heating. A heat flux of only 80 mW/cm2 can cause necrosis in muscle tissue. In small chronic impants, the power dissipation should not exceed a few hundred miliwatts. Necrosis: accidental death of cells and living tissue.

9. Main Objective Design and testing of a fully integrated amplifier suitable for recording biological signals from the millihertz range to 7 kHz. Design such that the amplifier offers the best power – noise tradeoff.

10. Operational Transconductance Amplifier (OTA) Pg. 885 in textbook

11. Operational Transconductance Amplifier (OTA) The input voltage controls an output current by means of the device transconductance, labeled gm. What is important and useful about the OTA’s transconductance parameter is that it is controlled by an external current, the amplifier bias current, IABC , so that one obtains From this externally controlled transconductance, the output current as a function of the applied voltage difference between the two input pins, labeled v+ and v-, is given by

12. Analysis of OTA Circuit Power dissipation in the circuit is acceptable, but it is still not close to 1 mW, which is ideal for biosignal amplifiers. Possible Solutions: I) Either, try to improve this circuit II) Come up with another circuit that can supply high enough open loop gain, with low power dissipation.

13. One possible design for a Low power CMOS Operational Amplifier

14. AC Analysis of the amplifier DC Gain of this circuit is about 145 dB

15. Power Dissipation of the amplifier (ID1 + ID2 + ID3)*(VDD-VSS) The circuit dissipates only about 180 uW of power, which is quiet low compared to the circuit analyzed before

16. Result on the Oscilloscope

17. Transient Response of the Circuit Time domain response of the circuit when it is excited with a sinusoidal signal.

18. Closed Loop Circuit with Rf & Rs in inverting configuration

19. Result on the Oscilloscope

20. Many other design possibilities

21. Conclusion The design process that was followed resulted in a CMOS operational amplifier design that at least met and, in a few cases, exceeded the design objectives by a large margin. The notable performance areas were the DC open loop gain of 145 dB, and the power consumption of 180 uW. Also, the settling time was quite low as can be seen by the transient response of the circuit, which means the circuit is relatively ‘fast’. A great deal was learned in the design process, including how to approach a design project, the tradeoffs involved in a CMOS op-amp design, patience, and how to stay up late. There could still be a lot improved in this circuit, but requires knowledge that is beyond the scope of this course, mainly in the field of VLSI.

22. References R. R. Harrison and C. Charles, "A low-power low-noise CMOS amplifier for neural recording applications," IEEE J. Solid-State Circuits, vol. 38, pp. 958-965, 2003. Yiqin Chen, Mark E. Schlarmann and Randall L. Geiger, "An Improved design Formulation for design and Optimization of Operational Amplifiers," MWSCAS’99 The 43rd Midwest Symposium on Circuits and Systems, New Mexico, USA, 8-11 August, 1999. Electrophysiology, From: [http://en.wikipedia.org/wiki/Electrophysiology]. Retrieved on November 10th, 2006.