12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
A novel Single-Input-Single-Output multifunctional filter based on
Current-Starved Pseudo-Floating Gate
MEHDI AZADMEHR
Institute for Microsystems Technology
Vestfold University College
Raveien 197, 3184 Horten
NORWAY
YNGVAR BERG
Institute for Microsystems Technology
Vestfold University College
Raveien 197, 3184 Horten
NORWAY
Abstract: In this paper a Multifunctional single-input-single-output (SISO) voltage-mode filter, based on CurrentStarved Pseudo-Floating Gate (CSP F G) inverters is presented. Using 3 bias voltages we are able to change the
circuits functionality to perform various types of filter operations such as band pass, band reject, low and high pass.
These filters have the advantage of being tunable, i.e. the frequency band can be tuned using bias voltages. Typical
applications are detection of high frequency components in sensor signals, i.e. airbag sensors. AC simulation of
the inverter is presented to show that the circuit is suited for high performance filter design.
Key–Words: Analog, CMOS, Floating gate, Inverter, Multifunctional filter.
1 Introduction
2 CSPFG inverter
As the demands regarding power consumption and
compactness increases in analog VLSI and ULSI circuits design, multifunctional circuits has gained a lot
of attention. One of areas where multifunctionality
has become very popular is in filter design. There
has been proposed many circuits that can realize various filters such as low pass, high pass, band pass,
band stop outputs. These filters can be categorized
based on the number of inputs and outputs. Four main
categories are single-input-multiple-output (SIM O)
[1, 2], multiple-input-single-output (M ISO) [3, 4],
multiple-input-multiple-output (M IM O) [5, 6] and
single-input-single-output (SISO) [7, 8]. In this paper we propose new type of SISO multifunctional
filter based on Current-Starved Pseudo Floating-Gate
(CSP F G) inverters [9,10]. These filters enjoy tuning
ability, low power consumption, large gain and compactness. The first frequency selective circuit based
on CSPFG was a band pass amplifier presented by Y.
Berg et al [9].
This paper is organized as follow: Section 2 presents
an analog CSPFG inverter and describe the inverters
operation principle. In section 3 the CSPFG SISO
multifunctional filter is presented where the different
filters are presented in subsections. Section 4 is the
conclusion of this work. The simulations presented in
this paper are being done in the cadence environment
with 90 nm CM OS transistor models from the ST M
with a V DD equal 1.2 volts and threshold voltage of
0.25 volts.
ISBN: 978-960-6766-82-4
Figure 1: Schematic of the CSPFG inverter
The analog CSPFG inverter is shown in figure
1 (schematic) and 2 (symbol). This circuit is an
current-starved inverter with a weak positive feedback. The positive feedback circuitry is basically a
current starved inverter where the P M OS transistors
are connected between the output and GN D and the
N M OS transistors between output and V DD, the
opposite of the inverter. This positive feedback has
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12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
Realization of different filters using the SISO
multifunctional filter, shown in figure 3, is done
through the bias voltages V hf 1, V hf 2 and V hf 3.
Each bias voltage controls the highest cut-off frequency of it’s respective inverter. The bias values for
each mode is listed in table 1.
Low pass
High Pass
Band Pass
Band Stop
All pass
All Stop
Figure 2: Symbol of the analog CSPFG inverter
2 important functions in the circuit, it sets the operational point (DC level) of the circuit [11, 12] and it decides the lower cutoff frequency. Bias voltages on Vhf
and Vlf limit the current flowing through the inverter
and the feedback circuitry respectively. In the symbol
the biases V lf ′ and V hf ′ are excluded since they are
biased V DD − V lf and V DD − V hf respectively in
all circuits presented in this paper. Input capacitance
Cin, blocks DC signals from the other circuitry connected to the input and make the circuit floating [13].
Capacitors connected in parallel with the input capacitor can be used for weighting different input signals
compared to each other simply by choosing the right
values [14]. The closed loop gain of the circuit can be
adjusted by changing the value of the feedback capacitor, Cf compared to Cin, and is given by:
Cin
A≈−
(1)
Cf
Vhf1
GND
F
F
F
GND
VDD
Vhf2
GND
VDD
F
VDD
VDD
GND
Vhf3
F
GND
GND
F
VDD
GND
Table 1: The V hf values for each bias for each operation mode. If the value is F , the bias is used to tune
the filter
To understand the filters operation we have divided it into 3 parts marked as A, B, C and separated by a dashed square. The various filter operations
are achieved by activation and/or deactivation of these
blocks.
3.1 Low pass
Different values for vhf=vlf
10
5
0
Assuming the inverter is ideal with infinite gain
and Vhf larger than vdd/2.
20dB
−5
3 Multifunctional CSPFG filter
0.25
0.3
0.35
0.4
0.45
0.5
0.55
0.6
−10
−15
−20
−25
−30
10
2
10
4
10
6
10
8
10
10
Hz
Figure 4: The AC response of the analog CSPFG inverter for different values of V hf , assuming V lf =
V hf
Figure 4 Shows the AC response of the CSPFG
inverter when the bias voltages, V hf and V lf are varied from 0.25 to 0.6 volts simultaneous. We can see
that the forwarding inverter suppresses high frequencies depending on the value of V hf , but it can operate
Figure 3: Multifunctional CSPFG filter
ISBN: 978-960-6766-82-4
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12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
on frequencies as high as 800 MHz. The positive feedback suppresses low frequencies based on the voltage
on V lf . The lowest frequencies that can pass are as
low as 5 KHz. This shows that the circuit behaves
as wide band pass filter where the lowest and highest
cut-off frequencies are adjustable.
If we ignore the lowest cut-off frequencies of the
inverter and consider only the highest cut-off frequencies the single inverter becomes a low pass filter. The
bias voltages V lf 1, V lf 2, V lf 3 are set to 0.35 volts
to achieve a stable, weak DC value at the input of
the inverter. This is valid for all circuits presented in
this paper. The highest cut-off frequency of the inverter can be obtained using simple transistor models
and assuming that the starving transistor operates in
the linear region:
fmax ≈
βW b Vhfef f ective
2Cl V DD
Figure 5: CSPFG high pass filter
result is attenuation of the signal at output for frequencies within the pass band of the inverter and a signal
equal to the input signal for frequencies higher than
the pass band of the inverter. The behavior of the filter
is presented in figure 6.
AC response for differnt values of vhf
(2)
5
0
where Vhfef f ective is Vhf − V t and Wb is the
starving transistor, see fig 1. If Cin and Cf are equal,
the transfer function of the inverter can be obtained
by:
Vout
1
SCl
−5
−10
Gain 20dB
−15
Cl
=> Sτ Vout = −Vin − Vout
gm
Vout
−1
H(S)lp =
=
Vin
1 + τS
−25
−35
−40
−45
−50
6
10
7
10
8
10
Hz
9
10
10
10
11
10
(3)
Figure 6: The AC response of the analog CSPFG high
pass filter for different values of Vhf
Equation 3 shows that the inverter has a 1 order
low pass filter behavior. In the multifunctional filter
shown in figure 3 there are 2 low pass filters, these are
marked as block B and C.
The multifunctional circuit will become low pass
filter if we turn off block A and B by biasing vhf 1 and
vhf 2 to GN D. The only active component will then
be the inverter in block C. To achieve maximum gain
the inverter has no feedback capacitor . The cut-off
frequency of the filter in the low pass mode is adjusted
using bias V hf 3.
The transfer function of the high pass filter is
given by:
−1
+1
1 + τS
Vout (S)
τS
=
=
Vin (S)
1 + τS
H(S)hp = H(S)lp + H(S)Cf 2 =
H(S)hp
(4)
Block A in the SISO multifunctional filter
shown in figure 3 performs the high pass operation.
Block B is biased V DD so it becomes a all pass filter
with a large Gain. Block C is turned of by biasing it
to GN D.
3.2 High pass filter
A CSPFG high pass filter is shown i figure 5. The high
pass behavior is achieved by adding the input signal to
the output of the low pass filter through the capacitor
Cf 2. All the frequencies within the passband of the
inverter are being summed at the output with the input
signal. Due to the opposite sign of these signals, the
ISBN: 978-960-6766-82-4
−20
−30
= −gm(Vin + Vout )
SCl Vout
= −Vin − Vout
gm
τ=
0.25
0.35
0.45
0.55
0.65
3.3 Band pass filter
Cascading a high pass filter and a low pass filter in
series as shown in figure 7 results in a band pass filter
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12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
AC Response of the band pass filter
5
0
Gain 20dB
−5
−10
0.25
0. 3
0.35
0. 4
0.45
0. 5
0.55
0. 6
−15
−20
Figure 7: The CSPFG band pass filter
−25
−30 6
10
given that the low pass filter is biased higher that the
high pass filter. These circuits will combine additively
in bode plot and multiplicatively in the S domain. In
the multifunctional filter presented in figure 3, block
A and B will form a band pass filter.
The transfer function of the band pass pass filter
is given by:
H(S)bp = H(S)hp × H(S)lp =
H(S)bp =
Vout (S)
=
Vin (S)
S2 +
−1
τS
×
1 + τS 1 + τS
− τ1lp S
(τlp +τhp )
τhp τlp S
+
1
τhp τlp
Q≈
τlp
τhp
and
ωc ≈ √
1
τhp τlp
(7)
fmax ≈
βW b Vhf 2ef f ective
2Cl V DD
(8)
(5)
βW b (Vhf 2ef f ective − Vhf 1ef f ective )
2Cl V DD
fcenter ≈
Vhf 2ef f ective +Vhf 1ef f ective
)
2
2Cl V DD
ISBN: 978-960-6766-82-4
The band stop filter is the parallel connection of the
low pass (block C) and the high pass filter (block A).
Block B is biased to V DD to operate as an all pass
filter. In order for the filter to work the high pass filter
must be biased larger then the low pass filter. In simulations presented in this paper the high pass filter is
biased 40mV higher.
The transfer function of the band stop filter can be
obtained by summing the transfer function of the low
pass filter, (block C) and the high pass filter (block A)
and is given as:
Vout (S)
= H(S)hp + H(S)lp
Vin (S)
τS
−1
H(S)bs =
+
1 + τS 1 + τS
τhp τlp S 2 − 1
H(S)bs =
(τ +τ )
S 2 + τlphp τlphp S + τhp1τlp
(11)
Figure 9 shows the AC simulation result of the
band stop filter for different values for vhf as shown
in the legend box.
The bandwidth of the band stop filter can be estimated as:
(9)
and the center frequency is:
βW b (
11
10
H(S)bs =
The frequency width is:
fwidth ≈
10
10
3.4 Band stop filter
The low pass filter can also be used as an amplifier to increase the gain of the filter without adding
any extra amplifier to the output of the filter. The bias
voltages V hf 1 and V hf 2 can be used to change the
bandwidth of the filter as they change the highest cutoff frequencies of the A and B respectively. We may
express the cutoff frequencies as:
βW b Vhf 1ef f ective
2Cl V DD
9
10
Figure 8: The AC response of the band pass filter
shown in figure 6 where vhf 2 is biased 40mV larger
than vhf 1, the results are for different voltages at
vhf 1 according to the legend box
(6)
fmin ≈
8
10
Hz
This gives:
s
7
10
(10)
fwidth ≈
111
βW b (Vhf 3ef f ective − Vhf 1ef f ective )
2Cl V DD
(12)
ISSN: 1790-5117
12th WSEAS International Conference on CIRCUITS, Heraklion, Greece, July 22-24, 2008
AC response for different values of vhf
20
15
0.15
0.25
0.35
0.45
0.55
[4]
Gain in 20dB
10
5
0
[5]
−5
−10 1
10
2
10
3
10
4
10
5
10
6
10
7
10
8
10
9
10
10
10
Hz
[6]
Figure 9: AC response of the filter shown in figure 3
for different values of vhf1 when biased as a band stop
filter, assuming vhf1 40mv larger than vhf3
[7]
4 Conclusion
[8]
The CSPFG SISO multifunctional filter presented
in this paper offers multifunctionality and tunability
without the use of switches. The circuit has low component spread, made of only capacitors and inverts.
The capacitors are used only as coupling capacitors
without effecting the frequency response of the filter
directly and can be chosen small resulting in spacesaving circuits, very well suited for use in VLSI and
ULSI circuits. The CSPFG filters are useful in application where a narrow band is to be detected or
rejected. One important area is detecting frequency
bands in resonating sensors. Other good properties of
the CSPFG filters are simplicity and versatility. The
ability to choose functionality and frequency area simply using a few bias voltages allows tuning even after
production and installation.
[9]
[10]
[11]
[12]
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