Download Low power CMOS operational amplifier

Done By: Raza Hanif (3336928)
Raheel Choudhary
Fawad Usman Rathore
Contents of Presentation
o Background and literature review
o Different applications of CMOS OpAmp
o Choice of complex electronic system
o proposed circuit (cmos opamp) to be
implemented in the system -> In the Book
o analysis of proposed circuit
o Improved, and more complex circuit
o Analysis
o System Analysis
o Conclusion
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.
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.
In Context with Textbook
OP741
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
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
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.
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.
Operational Transconductance Amplifier (OTA)
Pg. 885 in
textbook
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
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.
One possible design for a Low power CMOS
Operational Amplifier
V OS
+
V+
V DD
Vo
OUT
-
V-
V in
V SS
CL
2p
AC Analysis of the amplifier
W
W
( ) 2  ( )6
k p  2k n
g m 2  g m6
vo
L
L



vi ( g ds 2  g ds 4 )  ( g ds6  g ds7 ) ( 2   4)  ( 6   7)
I D5  I D7
V OS
V DD
+
V+
V in
Vo
Power Dissipation of the amplifier
-
V-
OUT
CL
2p
V SS
(ID1 + ID2 + ID3)*(VDD-VSS)
The circuit dissipates only about 180 uW of power, which is quiet low compared
to the circuit analyzed before
Result on the Oscilloscope
Transient Response of the Circuit
Time domain response of the circuit when it is excited with a sinusoidal signal.
OUT
RF2
V in
V+
+
-
V-
Closed Loop Circuit with Rf & Rs in
inverting configuration
V DD
1M
V SS
RF1
10M
V out
CL
2p
Result on the Oscilloscope
Many other design possibilities
VDD
5V
Q1
JFET_P_VIRTUAL
Q3
Q9
JFET_P_VIRTUAL
JFET_P_VIRTUAL
Q4
Q10
Q13
Q17
JFET_P_VIRTUAL
JFET_P_VIRTUAL
Ibias
Q23
100uA
JFET_P_VIRTUAL
Q2
JFET_P_VIRTUAL
Q14
Q18
JFET_P_VIRTUAL
JFET_P_VIRTUAL
JFET_P_VIRTUAL
C1
JFET_P_VIRTUAL
Q19
Q21
JFET_P_VIRTUAL
15pF
Q5
Q6
Q11
Q15
JFET_N_VIRTUAL
JFET_N_VIRTUAL
JFET_N_VIRTUAL JFET_N_VIRTUAL
JFET_N_VIRTUAL
Q22
V1
Q20
Q8
Q7
JFET_N_VIRTUAL
JFET_N_VIRTUAL
1V
1kHz
0Deg
Q12
JFET_N_VIRTUAL
Q16
JFET_N_VIRTUAL
JFET_N_VIRTUAL JFET_N_VIRTUAL
C2
VSS
-5V
15pF
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.
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.