Weitere ähnliche Inhalte Ähnlich wie FINALREPORTOPAMP.docx(1) (20) FINALREPORTOPAMP.docx(1)4. Introduction
The μA741 is constructed on a silicon chip and is a high performance monolithic op amp.
It is perhaps one of the most versatile integrated circuits available in the market.
Operational Amplifiers are a fundamental component of modern electronics. They have
various applications in many different circuits, and see primary usage in signal conditioning
circuits. Configurations such as the inverting, noninverting, summing and differential amplifiers
enable the realization of simple conditioning circuits, and vastly reduce the complexity of
otherwise complex tasks. The instrumentation amplifier is another useful implementation of
operational amplifiers, commonly used for measurement and test. By including buffered inputs,
and end user does not have to worry about impedance matching a unit under test, which leads to
more simplified interface circuitry.
At it's very core, the operational amplifier is a macro model for a much more complex
system. By masking the internal complexity of the system and conforming to known standards
for this type of device, end users can utilize operational amplifiers as a near dropin solution. The
ideal operational amplifier exhibits zero output impedance, infinite input impedance, infinite
voltage gain, infinite bandwidth, and zero input offset voltage. Although exact realization of
these specifications is not possible, many modern devices push the boundary of these
specifications, creating devices which behave in a very predictable and stable manner.
5.
Features of the OpAmp
● Offset Voltage null Capability
● Short Circuit and Overload Protection
● Low Power Consumption
● Large Common Mode and Differential Voltage Ranges
● No LatchUp
● No Frequency Compensation required
Specifications/Design Requirements
Low Frequency gain
Response of the amplifier to signals at the input, in terms of the ratio of output voltage to input
voltage. In our design we need to have a minimum of 100dB open loop voltage gain.
Gain Bandwidth Product
The Gain Bandwidth product (abbreviated as GBP) of an amplifier is the product of amplifier
bandwidth and unity gain. An ideal Op amp is assumed to have an infinite bandwidth, as it has
an infinite frequency response which can amplify a signal from DC to highest AC frequencies.
With real op amps, bandwidth is limited by the gain bandwidth product.
GBP = gain * bandwidth
GBP= 1M Hz (requirement)
Output Resistance
Output resistance is defined as the equivalent resistance that is measured between the output
terminal and ground. According to the required specifications we should have less than 100
ohms as an output resistance.
Slew rate:
Slew rate is defined as the maximum rate of change of output voltage per unit time, and is
expressed in volts per microseconds. It is one of the most important parameters for selecting op
amps for high frequencies. As our requirement for slew rate is very small, it is not suitable for
high frequency applications, such as oscillators, comparators, or filters.
9.
Basic Outputs of Operation Amplifier
Two Stage Op Amp
If pure telescopic, it will suffer from low output swing and medium gain. If folded, it will have
medium gain and medium output swing. For gain boost, medium output swing and better gain
than folded. Multi stage has high gain as well as high high output swing. So, Multi Stage
Topology seems more suitable for our design. In designing an opamp, numerous electrical
characteristics, e.g., DC gain, gainband width, slew rate, commonmode range, output swing,
offset, all have to be taken into consideration [1].
In the two stage op amp, first stage provides the gain and the consecutive second stage provides
the large swings. The first stage incorporates various amplifier topologies, but the second stage is
typically configured as a simple common source stage to allow maximum output swings.
Furthermore,since opamps are designed to be operated with negativefeedback connection,
frequency compensation is necessary for closedloop stability. The simplest frequency
compensation technique employs the Miller effect by connecting a compensation capacitor
across the highgain stage [2].
10. This opamp architecture has many advantages: high openloop voltage gain, high output swing,
large commonmode input range, only one frequency compensation capacitor, and a small
number of transistors. This opamp is a widely used general purpose opamp; it finds
applications for example in switched capacitor filters, analog to digital converters, and sensing
circuits [3]. But the disadvantage of multi stage topology is the stability if it has more than two
op amps but we are designing for Two stage.
Proposed Solution and Design
As seen in the table below [4], the various operational amplifier topologies have their
strengths and weaknesses.
Based on the specifications given in our project assignment, we have chosen to go with a
two stage topology, as both the gain and output swing requirements are attainable. Considering
the medium power consumption and low noise performance, the chosen topology makes a
suitable fit for our desired goals. Many attempts were made at addressing proper gain
12.
Using equations found in both [4] and [5], and using the low voltage op amp paper as a basis for
redesign, a suitable solution has been realized. Following the presented equations from [5]:
We can observe that the transconductance of the input PMOS transistor, as well as the
resistance of the cascode load dominate the gain of the first stage. The load resistance in the
cascode is proportional to the transconductance and the drainsource resistance of the transistors.
Further, the gain of the inverting amplifier stage is directly influenced by the output resistance of
the second stage, as well as the transconductance of the driving transistor. During the design
process it was noted that the transistor dimension and bias currents would directly affect the
transconductance values, thus influencing our gain. This relation is given by:
As the bias current increases we can theoretically boost our gain, at the cost of power
consumption and potential stability. The low power operational amplifier presents an equation
for the gain bandwidth product in relation to the stability capacitor and the transconductance, as
well as an equation to calculate the slew rate. During the design process these equations were
kept in mind, and are given as follows:
It can be seen by the gain bandwidth product equation that by decreasing the size of the
coupling capacitor we achieve a larger gain bandwidth product. This translates to a larger 3dB
rolloff frequency, and thus the minimal value for capacitance was used. Using the gpdk180
CMOS library in cadence, we’ve implemented this capacitor as a mimcap model with a
capacitance of 17.6 pF. Additionally, it can be seen that the slew rate is influenced by the
coupling capacitor and the bias current. Using these two equations we can predict the influence
of altering the coupling capacitors size.
Additional design equations have been presented in [5], but are omitted for brevity. The
final design was based on the initial transistor sizing found in the low power, large swing
21. Comaparison Results
Specifications/Parameter (Open
Loop)
Actual Design UA 741 Op Amp
Low Frequency Gain 76.68436 dB > 100 dB
3dB Frequency 506.5934 kHz
Gain Bandwidth Product (GBP) 233.9106 MHz 1 MHz
Output Resistance 12.4 kΩ < 100 Ω
Slew Rate 53.7 V/us >0.5 V/us
Output Voltage Swing 1.6886 V > 800 mW
Supply Voltage 1.8 V
CMRR
simulation issue > 70 dB, typical: 90 dB
Load Capacitor 20pF
Frequency 100KHz 100 KHz
Amplitude 100mV 100 mV
Power Dissipation 1.933 mW <85 mW, typical: 50 mW
Minimum Output Voltage 1.765781 V
Maximum Output Voltage 76.44189 mV
Parameters (Closed Loop: Unity
Gain Buffer Connection)
Minimum Output Voltage 900 mV
Maximum Output Voltage 1.0999 V
Output Impedance (low
frequency)
796.7883 mΩ
HD2 in dB 6.75246 dB
HD3 in dB 13.5049 dB
22. Summary
A design for a low power 1.8V CMOS operational amplifier is presented. Various design
topologies are attempted, finalizing with a two stage amplifier implementing a cascode on the
input. Several design decisions are explained, and trade offs are discusses. The pre and post
layout simulation results are presented and compared in tabulated format with a modern 741
operational amplifier, the specifications presented in the project prompt, and specifications from
a research paper. Despite not meeting the given gain requirements, performance post simulation
is acceptable given the project prompt.
References
[1] P.R. Gray, P.J. Hurst, S.H. Lewis and R.G. Meyer, “Analysis and Design of Analog
Integrated Circuits”, Forth Edition. John Wiley &Sons, Inc., 2001.
[2] B. Ahuja, “An improved frequency compensation technique for CMOS operational
amplifiers”, IEEE J. SolidState Circuits, vol. SC18, pp. 629633, Dec, 1983.
[3] Anshu Gupta, D.K. Mishra and R. Khatri, “A Two Stage and Three Stage CMOS OPAMP
with Fast Settling, High DC Gain and Low Power Designed in 180nmTechnology” International
Conference on Computer Information Systems and Industrial Management Applications
(CISIM) pp 448453, 2010.
[4] Razavi, Behzad, “Design of Analog CMOS Integrated Circuits”, First Edition. McGrawHill,
2000.
[5] Ehsan Kargaran, et. al. “A 1.5v High Swing UltraLow Power Two Stage CMOS OPAMP in
0.18 um Technology”. 2nd International Conference on Mechanical and Electronics Engineering.
Volume 1. pp 6871, 2010