a wideband cmos current-mode operational amplifier and its use for band-pass filter realization...
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A Wideband CMOS Current-Mode Operational Amplifier and Its Use for Band-Pass
Filter Realization
Mustafa Altun* , Hakan Kuntman*
*Istanbul Technical University, Faculty of Electrical and Electronics Engineering, Department of Electronics and Communication Engineering,
34469 Maslak, Istanbul, Turkey.
Introduction
Current-mode operational amplifiers (COAs) have several applications in closed-loop analogue signal processing, as conventional amplifiers (VOAs) perform the same function in the voltage domain [1].
Generally, COAs have better closed-loop bandwidth and enable high-speed operations with lower voltage supplies [2], [3].
COA ideally exhibits zero input resistance and infinite output resistance and current gain.
Fig. 1 (a) Circuit symbol (b) Equivalent circuit
Methods to Decrease Input Resistance Some complicated negative feedback configurations can
be applied [4], [5] to reduce input resistance. However, it considerably worsens frequency response of the amplifier.
The bigger β values we select, the better input resistance values we can obtain.
inI inV
A
AA
I
Vr
in
inin
1
Fig. 2 Negative feedback configuration
Methods to Decrease Input Resistance Positive feedback is another solution for getting better
input resistance [6]. If we can choose the value of Aβ a bit smaller than 1,
very small input resistance values can be achieved. For stability: Aβ < 1, rin > 0
inIinV
A
A
A
I
Vr
in
inin
1
Fig. 3 Positive feedback configuration
Proposed COA
The COA is formed by class A input and output stages. In the input stage M5, M4 and M3 compose positive feedback loop to reduce input resistance.
The output stage of the amplifier is folded-cascode structure.
V b 1
I o +
V b 2
0
I o -
V b 4
M 1 3
M 8
M 4
M 9
M 1 7
M 1 0
M 1 9
M 1 6
M 2
M 1 4 M 1 5
M 1
M 5 M 1 8
M 1 2
M 3
M 7
V b 1
V S S
V b 5
M 1 1
M 2 0 M 2 2
V b 2
0
M 2 1
C cI in
V b 3
M 6
V D D
V S S
0
Fig. 4 Schematic of the proposed COA
Proposed COA
Shown in the equation of rin below, second term mainly affects input resistance value.
If we select its value close to zero, rin also goes near zero. Moreover, its value must bigger than zero to overcome the stability problem.
53
4
334552
532
334
42552
2255
1
mm
m
dsmdsdsmds
mmm
dsmds
mmdsmds
dsmdsmin
gg
g
gggggg
gggA
ggg
ggggg
ggggr
Proposed COA
Transistor M11 works like a resistance and only improves frequency response of the COA.
To get better frequency response, bias currents and differential pairs are implemented with PMOS transistors.
Output resistance, DC current gain and gain-bandwidth product equations are given respectively.
Cc
gf
g
gg
g
gggA
g
gg
g
gggr
mGBW
m
dsds
m
dsdsmi
m
dsds
m
dsdsdsout
22
1
2
)0(
13,12
1
10
610
9
8913,12
1
21,19
16,1521,19
22,20
18,1713,1222,20
Filter Realization with the COA
As seen in Fig. 5, a well known band-pass filter topology is used and additionally COA is selected as an active element instead of VOA.
Fig. 5 Multiple feedback band-pass filter topology
31221
111
RRRCCwo
312
21
221 11
RRCC
RCCQ
Filter Realization with the COA
Because COA has better bandwidth compared to conventional op-amp, this band-pass filter can operate properly up to the value of frequency ≈ 250MHz.
Another advantage of using COA is being able to get two output signals.
31221221
212
11
111
1
RRRCCRCC
CCss
CRs
V
V
in
out
Simulation Results
SPICE is used for simulation with the process parameters of a 0.35 μm CMOS technology
Threshold voltages are nearly 0.5 V for NMOS and -0.7 V for PMOS
Table. 1 Transistor dimensions
Table. 2 DC values of the COA
Transistors W(μm)/L(μm)
M1, M9 30/1.4
M2 10/0.7
M3 9.2/0.7
M4, M5, M6 5/0.7
M7, M8 11/1.4
M10 5/0.7
M11 9/1
M12, M13 80/1
M14 140/1.4
M15, M16 70/1.4
M17, M18 41/1
M19, M21 120/1.4
M20, M22 30/1
Parameter Value
VDD – VSS ±1.5 V
Vb1, Vb2 0.5V, 0.2V
Vb3, Vb4,Vb5 0.3V, -0.2V, -0.7V
ID1,2 15uA
ID12,13 100uA
ID17,18 200uA
Simulation Results
1.0E+0 1.0E+1 1.0E+2 1.0E+3 1.0E+4 1.0E+5 1.0E+6 1.0E+7 1.0E+8 1.0E+9 1.0E+10
-50.00
0.00
50.00
100.00
Cur
rent
Gai
n (d
B)
-100.00
0.00
100.00
200.00
Pha
se (
Deg
ree)
According to the Figure 6, Unity gain bandwidth is nearly 200MHz Open-Loop gain is close to 100dB Phase margin exceeds 45˚
Fig. 6 Open-loop frequency response of the COA
Simulation Results
0.00 40.00 80.00 120.00 160.00 200.00T im e (ns)
-6.00
-4.00
-2.00
0.00
2.00
4.00
6.00
Out
put C
urre
nt (
uA)
Parameter Value
Power Dissipation 1.3 mW
Open-Loop Gain 95 dB
GBW 202 MHz
Phase Margin (Cc=0.3p) 65˚
Output Voltage Range ±1 V
Slew Rate 20uA/ns
Input Resistance 8Ω
Output Resistance 11.2 MΩ
Input Voltage Offset ≈ 8mV
Table. 3 Performance parameters of the COA
Fig. 7 Response of the COA in unity-gain feedback to a ±5 μA input step
Actually, except slew rate performance, other performance values are satisfactorily nice.
Simulation Results
These following values are selected to realize a COA based multiple feedback band-pass filter. Quality factor (Q) is 1,
center frequency is 10 MHz
Element values in the circuit are chosen as
R1 = R3 = R2/2 = 3.18 kΩ, C1 = C2= 5 pF.
1.0E+5 1.0E+6 1.0E+7 1.0E+8 1.0E+9Frequency (H z)
-50.00
-40.00
-30.00
-20.00
-10.00
0.00
10.00
Vol
tage
Gai
n (d
B)
S im ulated
Ideal
Fig. 8 Simulated and ideal filter responses
Simulation Results
Up to 0.8 V peak to peak input signal value, THD is small enough to allow band-pass filter work properly.
0.0 0.2 0.4 0.6 0.8 1.0Peak to Peak Input Vo ltage (V )
0.0
1.0
2.0
3.0
4.0
5.0
6.0
7.0
TH
D (
%)
Fig. 9 Total Harmonic Distortion (THD) values of the filter versus input peak to peak voltage
at 10 MHz frequency
Conclusion
In this work, a high performance COA is proposed Higher than 200 MHz GBW is achieved with using very
simple COA structure. It also offer very low input resistance ≈ 8Ω and ±1V
output voltage swing. In filter realization part, it can be easily seen that using
COA instead of VOA apparently improves frequency range of the filter.
While VOA-based multiple feedback band-pass filter works usually in some kHz center frequencies, 10 MHz is selected as a center frequency by using the COA-based filter.
References
[1] G. Palmisano, G. Palumbo, S. Pennisi, CMOS Current Amplifiers, Boston (MA), Kluwer Academic Publishers, pp. 1-9, 1999.
[2] T. Kaulberg, “A CMOS Current-Mode Operational Amplifier,” IEEE J. Solid-State Circuits, Vol.28, No.7, pp. 849-852, July 1993.
[3] E. Abou-Allam, E. El-Masry, “A 200 MHz Steered Current Operational Amplifier in 1.2-μm CMOS Technology,” IEEE J. Solid-State Circuits, Vol.32, No.2, pp. 245-249, Feb. 1997.
[4] W. Surakampontorn, V. Riewruja , K. Kumwachara and K. Dejhan, “Accurate CMOS-Based Current Conveyors,” IEEE Trans. Instrum. Meas., vol. 40, pp. 699–702, Aug. 1991
References
[5] G. Palmisano and G. Palumbo, “A Simple CMOS CCII+,” International Journal of Circuit Theory and Applications 23(6),. pp. 599-603, November 1995
[6] W. Wang, “Wideband class AB (push-pull). current amplifier in CMOS technology,” Electronics. Letters, 26, No. 8, pp 543-545, April 1990.
[7] W. Jung, Op Amp Applications Handbook, USA, Analog Devices, pp. 374-392, 2005.
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