1. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
INTERNATIONAL JOURNAL OF ELECTRONICS AND
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
COMMUNICATION ENGINEERING & TECHNOLOGY (IJECET)
ISSN 0976 – 6464(Print)
ISSN 0976 – 6472(Online)
Volume 4, Issue 1, January- February (2013), pp. 256-263
IJECET
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IMPLEMENTATION OF CMOS 3.8 GHZ NARROW BAND PASS (HIGH
Q) SWITCHED CAPACITOR FILTER IN 180 NM TECHNOLOGY
prashant s. patel1, mehul l. patel2
1
E&C Engg Department, L.C.Institute of Technology, Mehsana, Gujarat, India,
2
E&C Engg.Department, L.C.Institute of Technology, Mehsana, Gujarat, India,
ABSTRACT
In the recent era of nano technology, a surging demand for high-quality monolithic MOSFET
active filters in the fields of voice/data communications and instrumentations stimulated
tremendous research and development (R&D) efforts of switched-capacitor filters (SCF). The
most applications in high-frequency communication systems require narrow-bandpass filters
(Q ≈ 20), with a rather tight tolerance in the center frequency accuracy along with operational
amplifier (opamp). In this paper a SCF with the bandpass of 3.8 GHz is reported with the
simulation result obtained in Taiwan Semiconductor Manufacturing Company (TSMC)
180nm Technology using Mentor Graphics Eldo Simulation tools.
KEYWORDS: Bandpass Filter (BPF), CMOS Operational Amplifier, High Quality Factor
Q, Switched Capacitor Filter (SCF)
1 INTRODUCTION
In the VLSI system design, implementation of passive elements such as resistors,
inductors, etc on layout platform creates significant problems for the designers. Further it
requires detail knowledge of the layout process with large layout area. To overcome these
problems, Switched Capacitors (SC) techniques is significantly used instead of resistor. A
resistor can be replaced by a combination of capacitor and two switches operated on toggle
switch condition. The need to have monolithic analog filters motivated circuit designers in
the late 1970s to investigate alternatives to conventional active-RC filters. With the current
through the switched capacitor resistor proportional to the voltage across it, the equivalent
“switched capacitor resistance (Req)” is given by [1],
ܴ௘௤ ൌ 1ൗ‫ܥ ܨ‬ (1)
௦
Where Fs are the sampling frequency of the filter and C is the capacitor of the circuit.
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2. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
2. REALIZATION OF A SC FILTER
Realizing a SC bandpass filter such as cascading simple biquadratic filters, ladder band pass
filter or N-path techniques or two operational amplifiers can be implemented by various
methods. All of them, mainly, any high-order transfer function can be realized by using
cascading biquadratic filters and first-order section, generally, the resulting circuit is often
difficult to fabricate and very sensitive to finite op amp gain effects, stray resistance,
capacitance and element-value variations. For filters that have to realize higher Q-value,
ladder filter structure is employed. High capacitance spread ratio and requirement of the same
number of op amps as the filter order for the implementation are the main difficulties in
ladder filter. For achieving even higher Q-values, filter designs based on the concept of N-
path filter may be used. Several difficulties arise since the most high frequency applications
require very narrow band filters. This lead to sensitivity problems because of the rapidly
increased sensitivity of high Q filters for both to the ratio of the capacitors in the filter as well
as the gain and the settling behavior of the operational amplifier used in N path filter.
3. A HIGH Q BANDPASS SC FILTER USING TWO OPERATIONAL AMPLIFIER
In a high Q band pass filter using two operational amplifiers, the quality factor Q of
the circuit is controllable through a single resistance. In general form the transfer function of
a band pass filter is given by [2]
௔ ௦
ܶ ሺ‫ݏ‬ሻ ൌ ௔ ା௕ బ ௦మ
௦ା௕
(2)
భ భ మ
Where ao, a1, b1 and b2 are constants
The quality factor Q of the band pass filter is governed by the term b1. It has infinite Q if b1
approaches zero value but practically it is not possible although some high value up to Q≈20
is easily possible.
3.1 Two Stage Cmos Operational Amplifier
Operational amplifiers are key elements in analog processing systems. Operational
amplifiers are an important part of many analog mixed signal systems. As the demand for the
compact integrated circuits increases largely, the design of analog circuits such as operational
amplifiers in CMOS technology becomes more critical. Operational amplifiers (op-amps)
with moderate DC gains, high output swings and reasonable open loop gain band width
product (GBW) are usually implemented with two-stage structures. The op-amp which has
been designed is a two stage CMOS operational amplifier. Design has been carried out in
Mentor graphics tool. Simulation results have been verified using Eldo Simulation. The
simulation results in a TSMC 0.18um CMOS process from a 2.3V voltage supply
demonstrate the designed has a gain 59.98dB.
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3. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
Fig: 1 a general structure of two stage operational amplifier [3]
The general structure of two-stage op-amp is shown in Figure 1.The circuit consists of an
input differential trans-conductance stage which forms the input of the op-amp followed by
common-source second stage. The common source second stage increases the DC gain by an
order of magnitude and maximizes the output signal swing for a given voltage supply. This is
important for reducing the power consumption in two stage operational amplifier. Bias circuit
is provided to establish the operating point for each transistor in its quiescent stage.
Compensation is required to achieve stable closed loop performance. High voltage gain, large
common-mode input range and a small number of transistors required for implementation are
the main advantages of this operational amplifier architecture. This op-amp is a widely used
general purpose op-amp which finds applications in switched capacitor filters, analog to
digital converters and sensing circuits.
3.2 Implementation of Cmos Two Stage Operational Amplifier Using Tsmc 180nm
Technology in Mentor Graphics Tool
Fig: 2 Schematic of two stage CMOS operational amplifier [3]
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4. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
3.3 Simulation Results of Two Stage CMOS Operational Amplifier in TSMC 180nm
Technology using Mentor Graphics Tool.
Fig: 3 Simulation result for AC analysis for two stage operational amplifier in 180nm
technology using Mentor Graphics tool.
Fig: 4 Simulation result of transient analysis of two stage operational amplifier in 180nm
technology using Mentor Graphics tool.
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5. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
3.4 Implementation of Actual Circuit for High Q Band Pass Filter using two stage
operational amplifiers [2].
Fig: 5 Schematic of high Q SC bandpass filters using two stage operational amplifier [2].
Simulation result (frequency response) of high Q SC bandpass filter using IC 3140 which is
an operational amplifier gives the bandpass frequency of 11-40 MHz with the Q value of 1.8
[2].
4. Implementation of CMOS 3.8 GHz Bandpass SC Filter Using Different Voltage
Sources in TSMC 180nm Technology using Mentor Graphics Tools.
Fig: 6 Schematic of CMOS 3.8 GHz bandpass SCF using various voltage sources [4]
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6. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
Fig:7 Simulation result of bandpass filter at 3.8 GHz in TSMC 180nm Technology using
Mentor Graphics Tools.
Here due to some mismatch in transistors parameters and CMOS operational amplifier
operating condition, there is some fluctuation in output result shown in figure 7.
Table:1 Comparison of high Q bandpass SC filter using two operational amplifier [2],
CMOS 2.3 GHz Bandpass SCF using different voltage sources [4] and CMOS 3.8
GHz Bandpass SCF using different voltage sources (my work).
CMOS 2.3 GHz
High Q Bandpass
Bandpass SCF Using My Work
SCF Using Two
Characteristics Different Voltage (180nm,
Stage Operational
Sources(350nm, TSMC)
Amplifier[2]
TSMC)[4]
Supply Voltage (V) 12 2.3 1.8
Power Dissipation (mW) 58.356 10.356 8.835
Bandpass Frequency 11- 40 MHz 2.2- 2.4 GHz 1.8 - 3.8 GHz
Slew rate N.A. 1.8721MEG 1.5955MEG
Possible Q value ≈1.8 ≈26 ≈28
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7. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
5. CONCLUSION
In most high frequency applications which require very narrow band filters which
lead to sensitivity problems because of the rapidly increased sensitivity of high Q filters.
Though the sufficient level of sensitivity is achieved and the bandpass of 3.8 GHz with high
Q≈28 value is achieved. This structure can further be explore using 90nm, 65nm, etc. and the
still high value of Q (≥28) can be achieved. From both the implementation method discussed
in this paper, implementation of CMOS 3.8 GHz bandpass filter using different voltage
sources is more preferable due to its low power dissipation and high Q value. The most
desired application of narrow bandpass filter includes fast data/voice communication and
instrumentation.
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8. International Journal of Electronics and Communication Engineering & Technology (IJECET), ISSN
0976 – 6464(Print), ISSN 0976 – 6472(Online) Volume 4, Issue 1, January- February (2013), © IAEME
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