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.