chapter 5 active cmos gm-c band pass...
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101
CHAPTER 5
ACTIVE CMOS Gm-C BAND PASS FILTERS
5.1 INTRODUCTION
Band Pass Filters (BPFs) are used in the transmitter and receiver
circuits of many wireless systems for channel selection and filtering in the
Radio Frequency (RF) and Intermediate Frequency (IF) ranges. The BPFs are
designed either as external filters or on-chip filters. External filters provide
very high Quality factor (Q) but require buffers to drive the off-chip
components. These buffers consume more power and in order to reduce the
power consumption, on-chip filters are preferred in wireless systems. On-chip
filters are designed with active circuits and offer very low power consumption
and good efficiency. Most of the on-chip filters are designed with Operational
Transconductance Amplifier (OTA) and capacitors and are generally called as
Gm-C filters. The Gm-C filter offers many advantages in terms of low-power
and works well at high frequencies. The Gm-C circuits represent a popular
technique of integrated realization of high frequency continuous time filters.
Gm-C filters can operate in a wide range of frequencies from several hundred
of KHz to more than 100MHz. The power consumption of the various Gm-C
filters is in the range of few milliwatts (Muhammad Qureshi and Philip Allen
2005). The Q of Gm-C filters can be adjusted by controlling the output
impedance even at lower frequencies.
In this chapter, the design, implementation and performance of the
OTAs used in the BPFs and their performance of the BPFs using these OTAs
102
with CMOS technology are discussed. Second order active Gm-C CMOS
BPFs operating at IF range with center frequencies 45MHz and 70MHz for
wireless transmitter and receiver systems respectively are proposed. The
center frequency is selected as 45MHz for the BPFs in transmitter and 70MHz
in the receiver. This is because the transmitter in GSM systems, transmitter
and receiver of Bluetooth systems use IF BPFs with center frequency of
45MHz (Stephen Kratzet 2001, ADS Design Guide and Maurice Schiff and
Stephen Kratzet 1998). The receiver in GSM system and transmitter and
receiver of WLAN use IF BPFs with center frequency of 70MHz (Maurice
Schiff and Stephen Kratzet 1998, Application note 1380-2 of Agilent
technologies and ADS Design Guide). If the system is to support more
number of carriers then there is need for bandwidth of 3MHz. For example
GSM systems can support upto 13 different carriers simultaneously. If all the
13 carriers are to be used simultaneously then a bandwidth of around 3MHz is
required. The filters that will support this bandwidth have to be designed.
Hence BPFs operating at center frequency 45MHz and 70MHz with
bandwidth of 3MHz are proposed. The filters in the transmitter and receiver
are implemented to operate with bandwidth of 3MHz.The Operational
Tansconductance Amplifier (OTA) is used as basic unit in the design of
second order BPF structures. The second order filter structure using a simple
biquad with two OTAs proposed by Jacob Baker et al (1998) and BPF
structure with three OTAs proposed by Geiger
and Sanchez-Sinencio (1985) are considered for design.
The second order BPFs are designed with CMOS technology with a
supply voltage of 1.8V using Cadence tool. The performance of the filters
designed are analyzed using various parameters like center frequency, gain,
bandwidth, Q, S-parameters, stability, 1-dB compression, third order Intercept
Point (IP3), Harmonic distortion, input noise and output noise.
103
5.2 LITERATURE SURVEY
There are several literature available on the design of BPFs. The
BPF proposed by Choi and Luong (2001) is 70MHz Gm-C filter with
bandwidth of 200KHz. These filters can be used for single carrier wireless
systems. For a wireless system where more than one signal is to be combined
using beamforming this bandwidth will not be sufficient. The BPF proposed
by Muhammad Qureshi and Philip Allen (2005) is 70MHz Gm-C filter with
bandwidth of 2MHz. The BPFs for 45MHz are made of crystal oscillators
and having crystal structures inside IC’s is difficult and hence there is need
for on-chip filters.
OTAs are used for large gain and large bandwidth performances in
filters. Simple OTA also called as telescopic OTA proposed by Houda Daoud
et al (2006) cascades both the differential transistors and current mirror to
increase the load resistance and achieves high gain. Simple OTA also works
with low power consumption and introduces very low noise in the circuit.
Wilson Current Mirror OTA (WCM-OTA) proposed by Houda Daoud et al
(2006) has a differential stage consisting of PMOS transistors to charge
Wilson mirror. Another two MOSFETs are used to provide the DC bias
voltages. But the WCM-OTA has low output voltage swing. Cascode Current
Mirror OTA (CCM-OTA) proposed by Houda Daoud et al (2006) overcomes
the draw back of WCM-OTA and has large output voltage swing.
5.3 OPERATION OF OTAs
The OTA is used as a basic building block in many switched
capacitor filters. OTA is basically an op-amp without an output buffer and can
only drive capacitive loads. An OTA is an amplifier where all nodes have low
impedance except the input and output nodes. A useful feature of OTA is that
104
its transconductance can be adjusted by the bias current. The symbol of OTA
is shown in Figure 5.1.
+
-V-
V+
I bias
I out
Figure 5.1 Symbol of OTA
Filters made using the OTA can be tuned by changing the bias
current Ibias. Two practical concerns when designing an OTA for filter
applications are, the input signal amplitude and the parasitic input/output
capacitances. The external capacitance should be large compared to the input
or output parasitic capacitance of the OTA. Large signals cause the OTA gain
to become non-linear. This limits the maximum frequency of a filter built
with an OTA and causes amplitude or phase errors. These errors can be
minimized with proper selection of Ibias. The transconductance of the OTA is
measured using (5.1)
out
m
ig
v v (5.1)
where, iout is the output current of the OTA and v+ and v
- denote the
differential input voltage to the OTA. The voltage gain of the OTA is
measured from the ac analysis using (5.2)
105
out
v
vA
v v (5.2)
where, vout is the output voltage of the OTA. The peak to peak input signal
amplitude is in the range of 0.2V in the GSM and Bluetooth systems. Three
different OTAs, namely, Simple OTA described by Jacob Baker et al (1998),
Wilson Current Mirror OTA (WCM-OTA) and Cascode Current Mirror OTA
(CCM-OTA) proposed by Houda Daoud et al (2006) are used in the second
order filter BPF structures. The OTAs used in designing the band pass filter
are shown in Figure 5.2.
The current flowing in the input side of the simple OTA can be
varied by the control voltage. The control voltage is adjusted such that the
OTA gives better performance. The performance of simple OTA is limited by
its input and output voltage swing. To overcome the limits of simple OTA and
improve the performance, WCM-OTA and CCM-OTA are used. The WCM-
OTA has a low output voltage swing and the performance can be further
improved by using a Cascode current mirror. BPF with a Biquad structure
consisting of two OTAs denoted as BPF1 and BPF with three OTAs denoted
as BPF2 are considered. The W/L ratios of the transistors used are selected
with length L=0.18µm for both NMOS and PMOS transistors and the width
W for PMOS is greater than the width of NMOS transistors for working of
CMOS circuits. The circuit parameters for OTAs used for BPF with 45MHz
center frequency are given in Table 5.1 and Table 5.2.
106
VDD
Vcontrol
IOUT
R
V+ V-
M9 M8
M1 M2
M51
M41M31M3 M4
M5
M6M7
a) Simple OTA Circuit
V+ V-M9 M10
VDD
M1 M2
M3 M4
M5 M6
M7 M8
I' bias
I bias
IOUT
M11
M12
b) WCM-OTA Circuit
V+ V-M9 M10
VDD
M1 M2
M3 M4
M5 M6
M7 M8
I' bias
I bias
IOUT
M11
M12
c) CCM-OTA Circuit
Figure 5.2 Circuit of different OTAs used
107
Table 5.1 Circuit Parameters of the Simple OTA for Transmitter BPF
ParametersSimple OTA for
BPF1
Simple OTA for
BPF2
PMOS W/L
(M3, M4, M8, M9,
M41 and M31)
22µm/0.18µm 22µm/0.18µm
NMOS W/L
(M1,M2, M51, M5,
M6, M7 and M10)
2µm/0.18µm 2µm/0.18µm
R (K ) 100 100
Ibias (µA) 25 25
Vcontrol (V) 0.8 0.8
Table 5.2 Circuit Parameters of the WCM-OTA and CCM-OTA for
Transmitter BPF
WCM-OTA CCM-OTAParameters
BPF1 BPF2 BPF1 BPF2
PMOS W(µm )/L(µm)
(M1, M2, M3 and M4)9/0.18 9/0.18 6/0.18 9/0.18
NMOS W(µm )/L(µm)
(M5, M6, M7,
M8, M11 and M12)
3.08/0.18 3.08/0.18 3.08/0.18 3.08/0.18
PMOS
W(µm )/L(µm)
(M9 and M10)
10/0.18 10/0.18 8/0.18 10/0.18
Ibias (µA) 60 60 80 60
The transient analysis is performed for all the three OTAs to find
their transconductance and the simulation results show that the
transconductance is 160µS, 52.44µS and 58.68µS for Simple OTA, WCM-
OTA and CCM-OTA respectively for BPF1 and 82.76µS, 59.6µS and
108
56.11µS for Simple OTA, WCM-OTA and CCM-OTA respectively for BPF2
in the transmitter. The circuit parameters for OTAs used in the BPFs with
70MHz center frequency are given in Table 5.3 and Table 5.4.
Table 5.3 Circuit Parameters of the Simple OTA for Receiver BPF
ParametersSimple OTA for
BPF1
Simple OTA for
BPF2
PMOS W(µm)/L(µm)
(M3, M4, M8, M9,
M41 and M31)
22/0.18 22/0.18
NMOS W(µm)/L(µm)
(M1,M2, M51, M5,
M6, M7 and M10)
2/0.18 2/0.18
R (K ) 100 100
Ibias (µA) 50 29
Vcontrol (V) 0.8 0.8
Table 5.4 Circuit Parameters of the WCM-OTA and CCM-OTA for
Receiver BPF
WCM-OTA CCM-OTAParameters
BPF1 BPF2 BPF1 BPF2
PMOS
W(µm)/L(µm)
(M1, M2, M3
and M4)
6/0.18 6/0.18 4/0.18 6/0.18
NMOS
W(µm)/L(µm)
(M5, M6, M7,
M8, M11 and
M12)
3.08/0.18 3.08/0.18 3.08/0.18 3.08/0.18
PMOS
W(µm)/L(µm)
(M9 and M10)
9/0.18 12/0.18 7/0.18 12/0.18
Ibias (µA) 75 85 85 85
109
The transient analysis is performed for all the three OTAs to find
their transconductance. The simulation results show that the transconductance
is 267.5µS, 457.8µS and 450.9µS for Simple OTA, WCM-OTA and CCM-
OTA respectively for BPF1 and 234µS, 460.6µS and 460.2µS for Simple
OTA, WCM-OTA and CCM-OTA respectively for BPF2 in the receiver.
5.4 PROPOSED BAND PASS FILTERS
Two 2nd
order BPF structures working in the intermediate
frequency range of 45MHz and 70MHz are proposed. Designing one of the
filters is based on a simple Biquad structure and other with three OTAs. They
are useful for both receiver and transmitter. The BPFs are designed with OTA
as basic building block along with capacitors. The circuit of BPF with two
OTAs (Biquad structure) represented as BPF1 is shown in Figure 5.3.
Figure 5.3 Circuit of BPF1 with Biquad structure
The input condition for the BPF1 with Biquad to operate as band
pass filter is V2=Vin, V1 and V3 are grounded as described by Jacob Baker et al
(1998), where V1, V2, V3 are the inputs to the Biquad as shown in the
110
Figure 5.3. The capacitors C1 and C2 are tuned to obtain the required center
frequency and Q for the BPF1 with the three different OTAs for transmitter
and receiver. Assuming that the transconductances of each stage are equal, the
expressions for center frequency ( o ) and the Q of the filter are given as
mo
1 2
g
C C (5.3)
2
1
CQ
C (5.4)
The value of the capacitors C1 and C2 for the BPF1 with the three
different OTAs for transmitter and receiver are given in Table 5.5 and Table
5.6 respectively.
Table 5.5 Value of capacitors in BPF1 for Transmitter
Capacitor
values
BPF1 with
Simple OTA
BPF1 with
WCM-OTA
BPF1 with
CCM-OTA
C1(fF) 500 140 140
C2 (pF) 3.6 1.6 1.6
Table 5.6 Value of capacitors in BPF1 for Receiver
Capacitor
values
BPF1 with
Simple OTA
BPF1 with
WCM-OTA
BPF1 with
CCM-OTA
C1(fF) 500 160 160
C2 (pF) 3.8 6.1 6.1
111
The transfer function of the second order BPF1 is given as
1 m
2 2
1 2 1 2 m
sC gH (s)
s C C sC C g (5.5)
The second type of band pass filter BPF2 with three OTAs is a
modified structure where the circuit can be tuned for bandwidth and Q
separately. The structure of the BPF2 is shown in Figure 5.4.
The filter has three OTAs with transconductance gm1, gm2 and gm3
and two capacitors C1 and C2. The input condition for the BPF2 with Biquad
to operate as band pass filter is V2=Vin, V1 and V3 are grounded. V1, V2, V3
are the inputs to the filter as shown in the Figure 5.4. The transfer function of
the BPF2 is given as
1 m2
21 2 m3 1 m1 m2
sC gH(s)
s C C sg C g g (5.6)
Figure 5.4 Circuit of BPF2 with three OTAs
The center frequency can be adjusted linearly with gm1 = gm2 = gm
and gm3=constant, which is called as bandwidth movement. The Q can be
varied by simultaneously adjusting the transconductance of all the three
OTAs. The expressions for center frequency ( o ) and Q are given as
112
m1 m2o
1 2
g g
C C (5.7)
m1 m22
1 m3
g gCQ
C g (5.8)
The capacitors C1 and C2 are tuned to obtain the required center
frequency and Q for the BPF2 with the three different OTAs for transmitter
and receiver. The values of the capacitors C1 and C2 after tuning are given in
Table 5.7 and Table 5.8 respectively.
Table 5.7 Value of capacitors in BPF2 for Transmitter
Capacitor
values
BPF2 with
Simple OTA
BPF2 with
WCM-OTA
BPF2with
CCM-OTA
C1(fF) 400 450 500
C2 (pF) 5.6 5.8 5.2
Table 5.8 Value of capacitors in BPF2 for Receiver
Capacitor
values
BPF2 with
Simple OTA
BPF2 with
WCM-OTA
BPF2with
CCM-OTA
C1(fF) 170 170 160
C2 (pF) 5 5.6 5
5.5 SIMULATION RESULTS
The simulation results are shown for the BPF1 with Simple OTA
designed for the receiver to work with center frequency 70MHz and the
results for other filter structures are tabulated. The simulation is performed
113
with the three OTAs individually for the BPF1 and BPF2, to find the gain and
bandwidth of the filter from the AC response. The AC response of the BPF1
with Simple OTA is shown in Figure 4.5. M0 (69.98MHz, 18.46dB) denotes
the marker at the center frequency with corresponding gain. M2 (71.55MHz,
15.47dB) and M3 (68.23MHz, 15.45dB) denote the half power points for
finding the bandwidth.
Figure 5.5 AC response of the BPF1 with simple OTA for receiver
The ac response of the BPF1 and BPF2 with different OTAs for the
transmitter is given in Table 5.9. The ac responses of the 2nd order BPF1 with
different OTAs shows that the filter with WCM-OTA provides high Q. The
filter with CCM-OTA provides relatively large bandwidth and high gain. The
ac responses of the 2nd order BPF2 with different OTAs shows that the filter
with CCM-OTA provides high Q.
114
Table 5.9 AC response of the 2nd
order BPF1 and BPF2 for transmitter
BPF1 BPF2Filter with
type of
OTA
Designed
values Simple
OTA
WCM
-OTA
CCM-
OTA
Simple
OTA
WCM-
OTA
CCM-
OTA
Center
Frequency
(MHz)
45 45.71 45.71 45.71 45.71 45.71 45.71
Gain (dB) > 0dB 15.14 14.54 16.51 8.05 0.9 0.4
Bandwidth
(MHz)3 3.05 3.01 3.35 3.29 3.2 3.14
Q 15 14.98 15.18 13.64 13.89 14.28 14.55
The filter with Simple-OTA gives relatively large bandwidth and
high gain. The ac response of the BPF1 and BPF2 with different OTAs for the
receiver is given in Table 5.10.
Table 5.10 AC response of the 2nd
order BPF1 and BPF2 for Receiver
BPF1 BPF2Filter with
type of
OTA
Designed
values Simple
OTA
WCM
-OTA
CCM-
OTA
Simple
OTA
WCM-
OTA
CCM-
OTA
Center
Frequency
(MHz)
70 69.98 69.28 69.18 69.18 69.89 69.18
Gain (dB) > 0dB 18.46 18.92 17.26 11.61 3.594 6.596
Bandwidth
(MHz)3 3.32 2.67 2.72 2.73 3.32 3.5
Q 23 21.09 25.94 25.43 25.34 21.05 19.76
115
The ac responses of the 2nd order BPF1 with different OTAs shows
that the filter with WCM-OTA gives more gain and high Q. The filter with
simple OTA provides relatively large bandwidth than the BPF with WCM-
OTA and CCM-OTA. The filter with CCM-OTA gives a Q near to filter with
WCM-OTA and bandwidth greater than filter with WCM-OTA.
The ac responses of the 2nd order BPF2 with different OTAs show
that the filter with Simple-OTA provides more gain and Q than the BPFs with
other two OTAs. The filter with CCM-OTA gives low value of Q and more
bandwidth than BPFs with other two OTAs.
The S-parameter simulation is performed for the BPFs for
impedance matching of 50 ohms on input and output of the filter. The S-
parameters are found by considering the filter structures as two port network.
The various S-parameters found are input return loss in dB (S11), reverse
voltage gain in dB (S12), forward voltage gain in dB (S21), output return loss
in dB (S22), Power gain (GP), transducer gain (GT) and available gain (GA) of
the circuit. Transducer power gain, GT, is defined as the ratio between the
power delivered to the load and the power available from the source and is
given as
2
21GT S (5.9)
Operating power gain, GP, is defined as the ratio between the
power delivered to the load and the power input to the network and is given as
2
212
11
1GP S
1 S (5.10)
Available power gain, GA, is defined as the ratio between the
power available from the network and the power available from the source
and is given as
116
2
21 2
22
1GA S
1 S (5.11)
The circuit will have good input impedance matching when GP and
GT are closer and good output impedance matching when GA and GT are
closer. The circuit is checked for unconditional stability using the K- test.
For the circuit to be unconditionally stable the Rollett’s stability factor K > 1
and < 1. The value of K and are calculated from the S-parameters as
2 2 2
11 22
11 21
1 S SK 1
2 S .S (5.12)
and
11 22 12 21S .S S .S 1 (5.13)
The S-parameter simulation results are given in Figure 5.6.
a) S21 and S11
Figure 5.6 S-parameters of the BPF1 with simple OTA for receiver
117
b) S12 and S22
c) GT,GA and GP
Figure 5.6 (Continued)
118
d) Kf and B1f
Figure 5.6 (Continued)
The S-parameter simulation results of the BPFs at transmitter with
different OTAs are given in the Table 5.11.
The S-parameter simulation of the BPF1 and BPF2 shows that the
voltage gains (S21 and S12) are at maximum at the center frequency of the
filters and the losses (S11 and S22) are at minimum at the center frequency of
the filters. The S-parameter simulations for the gains show that the input
impedance matching is good as GP is closer to GT for all the filters. Output
impedance matching is good for the BPF1 with different OTAs as GA and GT
are closer for these filters than for BPF2 with different OTAs. The stability
factor Kf greater than one and B1f ( ) less than one shows that the BPF1 and
BPF2 with all the three OTAs is unconditionally stable.
119
Table 5.11 S-parameters of the BPF1 and BPF2 for transmitter
BPF1 BPF2
S-ParametersSimple
OTA
WCM-
OTA
CCM-
OTA
Simple
OTA
WCM-
OTA
CCM-
OTA
S21 (dB) -3.184 -1.36 -1.15 -6.523 -6.615 -6.386
S11(dB) -16.39 -14.39 -19.98 -12.7 -15.37 -16.77
S12(dB) -3.607 -1.36 -1.118 -6.94 -6.615 -6.386
S22(dB) -7.777 -16.11 -12.62 -3.271 -2.655 -2.718
Power Gain GP
(dB)-3.084 -1.201 -1.167 -6.285 -6.504 -6.295
Transducer Gain
GT (dB)-3.184 -1.36 -1.118 -6.523 -6.615 -6.386
Available Gain
GA (dB)-2.935 -1.253 -0.878 -3.761 -3.224 -3.065
Rollet stability
factor (Kf)1.059 1.002 1.001 1.132 1.005 1.002
Stability
measure (B1f)/
)
0.6993 0.6485 0.5918 0.5792 0.4776 0.4703
The S-parameter simulation results of the BPFs at receiver with
different OTAs are given in Table 5.12.
Table 5.12 S-parameters of the BPF1 and BPF2 for receiver
BPF1 BPF2
S-ParametersSimple
OTA
WCM-
OTA
CCM-
OTA
Simple
OTA
WCM-
OTA
CCM-
OTA
S21 (dB) -3.17 -3.64 -3.303 -6.43 -6.387 -5.871
S11(dB) -14.99 -23.42 -18.67 -11.43 -12.38 -13.41
S12(dB) -3.354 -3.638 -3.301 -6.789 -6.393 -5.871
S22(dB) -8.713 -4.561 -4.728 -3.512 -3.11 -3.318
Power Gain GP
(dB)-6.301 -3.532 -3.249 -6.098 -6.157 -5.655
Transducer
Gain GT (dB)-6.65 -3.763 -3.345 -6.43 -6.394 -5.854
Available Gain
GA (dB)-3.964 -1.774 -1.516 -3.876 -3.473 -3.146
Rollet stability
factor (Kf)1.059 1.002 1.001 1.116 1.005 1.001
B1f ( ) 0.7282 0.4331 0.3876 0.621 0.5612 0.5513
120
The S-parameter simulation of the BPF1 and BPF2 shows that the
voltage gains (S21 and S12) are at maximum at the center frequency of the
filter and the losses (S11 and S22) are at minimum at the center frequency of
the filters. The S-parameter simulations for the gains show that the input
impedance matching is good as GP is closer to GT for all the filters and
output impedance matching is good for the BPF1 with WCM-OTA and CCM-
OTA and for BPF2 with CCM-OTA as GA and GT are closer for these filters.
The stability factor Kf greater than one and B1f less than one, show that the
BPF1 and BPF2 with all the three OTAs is unconditionally stable.
The Periodic Steady State (PSS) analysis and Periodic Noise (PN)
analysis for large signal and nonlinear analyses are performed to find the
behavior of the BPFs around the periodic steady state point. As the input
power level increases, the circuit becomes nonlinear, the harmonics are
generated and the noise spectrum is folded. PSS analysis is performed for the
BPFs to find its 1dB compression point, 3rd order Input Intercept Point (IIP3)
and harmonic distortion. 1dB compression point is the point where the circuit
gives an output power of 1dB less than the actual output power required. 1dB
compression point is determined from the curve plotted for gain with respect
to input power. 1dB compression point is input referred 1dB compression
point as it is calculated with reference to input power.
IIP3 curve is measured using two-tone test where the two tones, 1
and 2, with the same amplitude and coming from two adjacent channels,
drive the BPF simultaneously. Harmonic distortion is characterized as the
ratio of the power of the fundamental signal divided by the sum of the power
at the harmonics. Harmonic distortion is obtained by plotting the spectrum of
any node voltage. For the BPFs harmonic distortion is obtained by plotting
the spectrum of load when the input power level is around 20dBm. The PSS
parameters are shown in Figure 5.7.
121
a) 1-dB compression
b) IIP3
Figure 5.7 PSS analysis of the BPF1 with simple OTA
122
c) Harmonic Distortion
Figure 5.7 (Continued)
The Periodic Steady State (PSS) parameters of the BPFs at
transmitter and receiver are given in Table 5.13.
Table 5.13 PSS response of the 2nd
order BPF1and BPF2
PSS parameters
Type of Filter Type of OTA1-dB
Compression
(dBm)
IIP3
(dBm)
Harmonic
Distortion
(dB)
Simple OTA -29.593 -29.94 -24.2
WCM-OTA -38.635 -29.68 -36.1BPF1
CCM-OTA -38.609 -29.69 -36.06
Simple OTA -31.84 -29.95 -27.61
WCM-OTA -31.77 -29.95 -27.69
BPF at
Transmitter
with Center
frequency of
45MHZBPF2
CCM-OTA -31.41 -29.95 -27.15
Simple OTA -28.759 -22.5 -23.61
WCM-OTA -36.3 -24.167 -29.59BPF1
CCM-OTA -35.7515 -24.1 -28.53
Simple OTA -37.7811 -29.94 -30.25
WCM-OTA -37.702 -24.10 -30.38
BPF at
Receiver
with Center
frequency of
70MHZBPF2
CCM-OTA -37.8629 -24.2 -30.38
123
The PSS response of the BPFs at receiver and transmitter show that
the input referred 1dB compression is in the range of -30dBm for all the
filters.
The BPF1 with WCM-OTA for the transmitter provides a very
good compression of -38.635dBm and the BPF2 with CCM-OTA for the
receiver provides very good compression of -37.8629dBm. The third order
input intercept point (IIP3) is at minimum for the BPF1 with simple-OTA and
BPF2 with all the three OTAs in the transmitter at -29.95dBm and -29.94dBm
for BPF2 with simple-OTA in the receiver. The harmonic distortion is at its
minimum at -36.1dB and -36.06dB for the BPF1 with WCM-OTA and CCM-
OTA respectively on the transmitter and is at its minimum at -30.38dB in
BPF2 with WCM-OTA and CCM-OTA in the receiver.
Figure 5.8 Periodic noise response of the BPF1 with simple OTA
124
The Periodic noise analysis is performed for the BPF with the three
OTAs to visualize the contribution of different noise sources in the total
noise. The Periodic noise response of the BPF is shown in Figure 5.8 and the
input and output noise of the filter with the OTAs is given in Table 5.14.
Table 5.14 Periodic noise response parameters of the 2nd
order BPFs
Periodic noise parameters
Type of Filter Type of OTA Input noise
(dB)
Output noise
(dB)
Simple OTA -149.5 -152.7
WCM-OTA -141.0 -142.6BPF1
CCM-OTA -141.5 -142.7
Simple OTA -146.7 -153.2
WCM-OTA -150.1 -156.8
BPF at
Transmitter
with Center
frequency of
45MHZBPF2
CCM-OTA -150.7 -157.2
Simple OTA -143.7 -150.3
WCM-OTA -145.8 -149.5BPF1
CCM-OTA -145.9 -149.2
Simple OTA -143.6 -150.0
WCM-OTA -146.5 -152.8
BPF at
Receiver
with Center
frequency of
70MHZBPF2
CCM-OTA -146.9 -152.8
The periodic noise analysis gives the noise performance of the
device which contributes the maximum noise. The periodic noise analysis of
the BPFs for the input and output noise shows that the BPF2 with CCM-OTA
provides a low noise level of -150.7dB and -157.2dB input and output noise
respectively in the transmitter and of -146.9dB and -152.8dB input and
output noise respectively in the receiver. Hence the BPF with CCM-OTA can
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be used in the transmitter and receiver of the wireless systems where the noise
level has to be very low. The power consumption of the BPFs designed for
the wireless transmitter and receiver are given in Table 5.15.
Table 5.15 Power consumption of the various BPFs
Type of Filter Type of OTA
Power
consumption
(µW)
Simple OTA 117.74
WCM-OTA 55.275BPF1
CCM-OTA 75.438
Simple OTA 169
WCM-OTA 173.79
BPF at Transmitter
with Center frequency of
45MHZBPF2
CCM-OTA 181.695
Simple OTA 578
WCM-OTA 839BPF1
CCM-OTA 929
Simple OTA 693
WCM-OTA 949
BPF at Receiver
with Center frequency of
70MHZBPF2
CCM-OTA 1179
The power consumption of the various BPFs designed is in the
range of µW and the filters referred in the literature have power consumption
in mW.
5.6 CONCLUSION
The design and implementation of 2nd
order CMOS band pass
filters using OTA as the basic element in two different structures have been
126
presented. It is shown from the simulation results that the filters can work at a
low supply voltage of 1.8V. The filters for transmitters are designed for center
frequency of 45MHz and bandwidth of 3MHz with a Q of 15. The filters at
receiver are designed for center frequency of 70MHz and bandwidth of 3MHz
with a Q of 23.
The simulation results of the filters on the transmitter side show
that the Q is high for BPF1 with WCM-OTA than BPFs with CCM-OTA and
simple OTA. The Gain and bandwidth are high for BPF1 with CCM-OTA.
The S-parameter analysis also shows that the BPF with WCM-OTA and
CCM-OTA perform well in terms of matching and gain at the designed
frequency and bandwidth. The PSS simulation shows that the distortion is less
in BPF1 with WCM-OTA and CCM-OTA. The periodic noise analysis shows
that the input and output noise are very low for BPF2 with CCM-OTA.
The simulation results of the filters on the receiver side show that
the Q and gain are high for BPF with WCM-OTA than the BPF with simple
OTA and CCM-OTA. The S-parameter analysis also shows that the BPF with
WCM-OTA and CCM-OTA perform well in terms of matching and gain at
the designed frequency and bandwidth. The PSS analysis shows that the
distortion is less in BPF2 with WCM-OTA and CCM-OTA. The periodic
noise analysis shows that the BPF2 with CCM-OTA provides good
performance in terms of input and output noise. The power consumed by
BPF2 with CCM-OTA designed for both receiver and transmitter is more than
the power consumed by all other filter structures.
The filters designed are tested for different ac input voltages from
10mV to 0.6V and it is found that they are highly robust and the performance
of the filters was not affected. All the filter structures can be used in IF
channel selection and filtering in wireless applications as they consume very
less power and provide good filtering characteristics.