Transcript A Wideband CMOS Current-Mode Operational Amplifier and Its

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.
I in


A
Vin


Fig. 2 Negative feedback configuration
Vin
A
rin 

I in 1  A
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
Vin


A


Fig. 3 Positive feedback configuration
I in
Vin 1  A
rin 

I in
A
Proposed COA
VDD


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 foldedcascode structure.
M7
M8
M14
M15
Vb1
M16
Vb1
M1
M9
M19
Vb2
M21
Vb2
M12
M13
Io+
Io-
Iin
Cc
0
M11
M2
M3
0
M20
M10
M22
VSS
Vb4
0
Vb3
M5
M4
M6
M17
M18
Vb5
VSS
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.


g m2 g m4
rin 
g ds 2  g m5  g ds 5  

g m5  g ds 5 g m 2  g ds 2  
g ds 4  g m3  g ds 3 
g m 2 g m3 g m5
A
g ds 2  g m5  g ds 5 g ds 4  g m3  g ds 3 
1
g m4

g m3 g m5
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.
rout
 g ds 20, 22 g ds12 ,13  g ds17 ,18  g ds19 , 21 g ds15,16 



g
g
m 20 , 22
m19 , 21


g
g g
g g 
Ai (0)  m12 ,13  ds 9 ds 8  ds10 ds 6 
2  g m9
g m10 
1 g m12 ,13
f GBW 
2 2Cc
1
1
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.
wo 
Q
Fig. 5 Multiple feedback band-pass filter topology
1
C1C2 R2
1
1 
 

 R1 R3 
C1C2 R2  1
1 



2 
C1  C2   R1 R3 
Filter Realization with the COA
Vout

Vin


1
s
R1C1
s2  s
C1  C2 
C1C2 R2

1
C1C2 R2
 1
1 
 

R

 1 R3 
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.
Simulation Results
Transistors
W(μm)/L(μm)
Parameter
Value
M1, M9
30/1.4
VDD – VSS
±1.5 V
M2
10/0.7
Vb1, Vb2
0.5V, 0.2V
M3
9.2/0.7
Vb3, Vb4,Vb5
0.3V, -0.2V, -0.7V
M4, M5, M6
5/0.7
ID1,2
15uA
M7, M8
11/1.4
ID12,13
100uA
M10
5/0.7
ID17,18
200uA
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
Table. 1 Transistor dimensions
Table. 2 DC values of the COA
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
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
Simulation Results
According to the Figure 6,


Unity gain bandwidth is nearly 200MHz
Open-Loop gain is close to 100dB
Phase margin exceeds 45˚
100.00
200.00
50.00
100.00
0.00
0.00
-50.00
-100.00
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
Fig. 6 Open-loop frequency response of the COA
Phase (Degree)

Current Gain (dB)

Simulation Results
6.00
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
Output Current (uA)
4.00
2.00
0.00
-2.00
-4.00
-6.00
0.00
40.00
80.00
120.00
160.00
200.00
Time (ns)
Fig. 7 Response of the COA in unitygain feedback to a ±5 μA input step

Actually, except slew rate
performance, other
performance values are
satisfactorily nice.
Table. 3 Performance parameters of the COA
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.
Ideal
Simulated
0.00
-10.00
Voltage Gain (dB)

10.00
-20.00
-30.00
-40.00
-50.00
1.0E+5
1.0E+6
1.0E+7
1.0E+8
1.0E+9
Frequency (Hz)
Fig. 8 Simulated and ideal filter responses
Simulation Results
7.0
Up to 0.8 V peak to peak
input signal value, THD is
small enough to allow
band-pass filter work
properly.
6.0
5.0
4.0
THD (%)

3.0
2.0
1.0
0.0
0.0
0.2
0.4
0.6
0.8
1.0
Peak to Peak Input Voltage (V)
Fig. 9 Total Harmonic Distortion (THD) values
of the filter versus input peak to peak voltage
at 10 MHz frequency
Conclusion
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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 COAbased filter.
References
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[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
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
[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.