Self-Cascode Current Controlled CCII based-Tunable Band Pass Filter Issa ELDBIB, Vladislav MUSIL Department of Microelectronics, Brno University of Technology, Udolni 53, 602 00 Brno, Czech Republic E-Mail:
[email protected],
[email protected] Abstract. In this paper we present new construction of CMOS current conveyor based self cascode current mirrors used in tunable band pass filter. Thus the first step in our design was to improve the CCII performance characteristic. The low voltage self cascode technique in CMOS technology was first studied. A tunable currentmode band pass filter using active elements unity gain current conveyor based self cascode current mirror is presented. The Q-factor and the central frequency can be electronically controlled by a DC bias current. To verify this , a Pspice simulations were carried out using 0.7um technology CMOS process from AMIS.
Keywords Self cascode current mirror, Self cascode CCII, CMOS CCII-based Band pass filter.
2. Self cascode current mirrors Current-mode structures performances depend strongly on performance of employed current mirrors, Usually, high current transfer accuracy, high output impedance, low input voltage (VIN) and low minimum output voltage are expected from a current mirror; The simple current mirror Figure 1a is usually far from achieving high accuracy and high output impedance, mainly due to channel length modulation effect. Both accuracy and output impedance of a conventional doublecascode current mirror Figure 1b, are much higher with respect to a simple current mirror, however, input and output voltage swings are restricted. [2],[3].. To improve output swings, a simple self-cascode current mirror has been reported [5], which utilizes the self-cascode structure show in Figure 1c. Iin
1. Introduction Second generation current conveyors are one of them most well known current mode analog blocks. The problem of designing a high performance integrated CCII circuit seems to be still open, especially in CMOS technology [1] where the high frequency response of current mode circuits is greatly improved.. In this paper, we deal with optimizing secondgeneration current conveyors. In order to reduce its parasitic resistance (RX) at port X, we consider an improved topology of this configuration. The proposed configuration was considered for optimal sizing of the transistors. The paper is organized as follows: in Section 3,we present the CCII based self cascode current mirror and improved CCII based self cascode current mirror configurations. In section 4, a tunable current mode second order band-pass filter with a very high Q-factor and central frequency is presented.
978-1-4244-2088-9/08/$25.00 ©2008 IEEE
Iin
M1
Iout
M3
M2
a)
M1
b)
Iout
M4
Iin
Iout
M3
M2
M1
M4
M2
c)
Fig. 1 (a) simple CM. (b) cascode CM. (c) self cascode CM.
The small signal input resistance and output resistance of Figure 1c can be found as:
rin =
1 g m (effective )
(1)
gm effective bigger than gm of equivalent conventional transistor, then input resistance rin of composite transistor is smaller.
rout = ( g m 4 r3 − 1)r4
(2)
It is obvious from equation 2 output resistance of composite transistor is bigger than of conventional.
3. Controlled CCII based-self cascode current mirrors
80
70
simple CM composite CM
60 1.0Hz
1.0KHz
1.0MHz
1.0GHz
Frequency
DB(r:2)
Fig. 2input impedance for composite and conventional CM.
A proposed implementation of the second-generation current conveyors based self cascode current mirrors with a positive current transfer from X to Z (CCII+) is shown in Figure 6. The considered circuit between points Y and X allows the function of voltage follower, by means of M1 to M4. The transistors M9-M12 allow the mixed loop to be dc biased. The output NMOS and PMOS current mirrors duplicate the current flowing through port X at port Z.
From last Figure 2 we realize that no much difference in the values of input impedance between simple current mirror and self cascode current mirror.
M10a
M9a
M5a
M6a
-0 M9
-10
M5
R1 M1
Simple CM
-15
Z
X M2
M4
R2
1.0GHz
1.0KHz 1.0MHz IDB(Z) Frequency
M3
Y
0
-20 1.0Hz
M6
M10
Composite CM
-5
M12
M11
M7
M8
Fig. 3
Current mirror AC simulation.
This composite transistor has a transconductance-tooutput conductance ratio as high as that of a long-channel transistor but a shorter “physical channel length”, Figure 3 presenting higher current transfer ratio for self cascode current mirror. 60uA simple CM
40uA
composite CM
20uA
M12a M11a
M7a
M8a
Fig.6 LV CCII based self cascode CM.
The rx impedance simulation plotted 2.9 k ohm which means high value resistance as depicted in Figure 7 and this is draw back in many applications when x impedance loaded with low load and advantage in some other applications. 4.0K
0A 0V
0.4V
0.8V
1.2V
ID(M3) V_V1
3.0K
Fig. 4 Input voltage vs input current characteristics 500u
2.0K 1.0Hz 100KHz V(VX) / I(Ix) Frequency
250u
10GHz
Fig. 7 rx for conventional self cascode CCII CM. 0A 0A I(out)
200u
400u
500u
I_Iin
Fig. 5 DC simulation for Iin and Iout self cascode CM Current
While x node impedance value is affected by the biasing condition of the transistors that form output stage. Starting from this consideration the designer can adjust the value of x node impedance by controlling the biasing of output stages.
40K
Io Y
vy
Io=1uA
iz
CCII+ Z
20K Io=140uA
X
ix
0 1.0Hz
Fig. 8 current controlled basic block.
100KHz
10GHz I(Ix)
V(VX) Frequency
⎡ iY ⎢v ⎢ X ⎢⎣ i Z
M10a
M10
⎤ ⎡0 0 0⎤ ⎡v Y ⎤ ⎥ = ⎢1 r 0⎥⎥ ⋅ ⎢⎢i X ⎥⎥ X ⎥ ⎢ ⎥⎦ ⎢⎣0 ±1 0⎥⎦ ⎢⎣v Z ⎥⎦
M9a
M5a
M9
M5
M1 Io
Fig. 10 Io effect on rx of CCII based self cascode current mirror.
4. Self cascode CCCII-based band pass filter
(3)
The structure is used as a basic building block of a tunable current mode Band-pass filter is depicted in Figure 11. When the CCII1, CCII2 and C1 implement the non ideal inductance, C2 is the shunt capacitor. The controlled negative resistance is used to cancel the effects of the parasitic shunt resistors (rx1 and rx2). Finally the out put current signal is obtained through CCII4.
M6a
M6
M3
Y
Io
Z
X
0
M12
M11
M4
M7
Y 4 Z CCII X
M2
Io
Y 3 CCII Z X
X
Io
C1
Y 2 CCII Z 0
0
Y 1 CCII Z X
Io
C2
M8 0
output
Fig. 11 Second order filter block M12a
M11a
M7a
M8a
Fig. 9 modified CCII based self cascode current mirror
Figure 8 shows CCCII block, the impedance seen at x node is not more a parasitic element, but becomes a part of the block specifications, as presented in matrix form reported above [4]. In order to control the biasing of the output stage, the circuit presented in Figure 6 may be modified as shown in Figure 9 to get current controlled current conveyer (CCCII), The current Io controlling the biasing of the output stage, so modifying the parasitic resistance rx, Simulation results plotted in Figure 10 show that this resistance can be controlled between 22.5 kΩ and 1.4 kΩ by varying the current control in the range [1uA-140uA].
The transfer function of the filter shown in Figure 11 is expressed by:
H (s ) =
i out S = r r i in rX 4C 2S 2 + X 4 S + X 4 rg L
1 1 1 1 1 = + − + rg rX 1 rX 2 rX 3 rX 4
(4)
(5)
Its resonance frequency and its quality factor are respectively given by the following expressions:
ω0 =
Q=
1
rX 1rX 2C 1C 2 rg rX 1rX 2
C2 C1
(6)
(7)
•
solution comes by varying values of the rx1 and rx2 (Io1 or Io2).
SIMULATION RESULTS
The Q-factor and the central frequency can be electronically controlled by mean of DC bias current. To validate this result, a Pspice simulation results plotted in Figure 12, Figure 13 are showing very interesting frequency and Q factor performances, a tunable central frequency in the range of 100MHz to 120MHz and a high tunable Q-factor. 30 Io3=120u
5. Conclusion Finally a new architecture of self cascode CCII is presented. The CCII operates at low supply voltage and consumes low power. Simulations using Pspice confirm the good performance of such conveyor. The current mode high Q-factor and frequency band pass filter is constructed. This architecture is electronically tuned by varying the DC bias current of the conveyor.
20
References [1] 80u 60u 40u 30u
0
100MHz IDB(R3)
237MHz IDB(R3)
Frequency Fig. 12 tuning of Q factor of band pass filter.
100MHz
120MHz
20 80uA Io2=30uA 10
100uA
40uA
135uA
50uA 0
56uA
-10 75MHz
[3]
[4]
-20 43MHz
30
[2]
100MHz IDB(R3)
148MHz
IDB(R3) Frequency Fig. 13 resonance frequency tuning of band pass filter.
the central frequency can be adjusted independently of the Q-factor with C1*C2 if the ratio C2/C1 is left constant. And the Q-factor can be tuned from rx3 or rx4 (Io3or Io4) without changing the central frequency. It’s desirable to adjust the central frequency after integration but not allowed to change the capacitor’s values, the
[5]
Cheng and Thoumazou, “3 V MOS CC cell for VLSI tec.” Electron. Lett., vol. 29, no. 3, 1993. C. C. Enz, “High precision CMOS micropower amplifiers,” Ph.D. thesis no. 802, EPF-Lausanne, Switzerland, 1989. H. Wallinga and K. Bult, “Design and analysis of CMOS analog signal processing circuits by means of a graphical MOST model,” IEEE J. Solid-State Circuits, vol. 24, no. 3, pp. 672-680, June 1989. G. Ferri, P. De Laurentiis, G. Stochino. “Current Conveyors II”, Electronic World ; April 2001 ; Galup-Montoro, C., Schneider, M. C. and Loss, I. J. B. “Series-Parallel association of FET’s for high gain and high frequency applications,” IEEE J. Solid-state Circuits, vol. 29, no. 9, 1994,
