IEEE Transactions on Consumer Electronics, Vol. 51, No. 2, MAY 2005

Manuscript received March 3, 2005 0098 3063/05/$20.00 © 2005 IEEE

424

A CMOS Infrared Wireless Optical Receiver Front-end with a Variable-gain Fully-differential Transimpedance Amplifier

Roger Yubtzuan Chen, Member, IEEE, Tsung-Shuen Hung, and Chih-Yuan Hung

Abstract —A CMOS infrared wireless optical receiver front-end is presented. A stable variable-gain fully-differential transimpedance feedback amplifier is designed employing a current-mode amplifier as the feedforward gain element. For a more than triple variation of the transimpedance gain, from 0.3kΩ to 1kΩ, the variable-gain transimpedance amplifier achieves desirable gain-bandwidth independence. For an infrared wireless optical receiver front-end employing the transimpedance amplifier, the optical preamplifier achieves a transimpedance gain of ΩdB66 and a bandwidth of 114MHz with a 5pF photodiode capacitance, and a power consumption of 17.4mW.1

Index Terms — CMOS infrared wireless communications, CMOS optical preamplifiers, current amplifiers, infrared wireless optical receivers, transimpedance amplifiers.

I. INTRODUCTION

Infrared free-space optical receivers find applications in laptop computers, cellular phones, digital cameras, computer peripherals, personal digital assistants (PDAs), and many other consumer electronics equipped with a short-distance infrared communication port. In infrared wireless optical receivers, the use of sensitive current-input preamplifiers to process the signal currents from photodiodes is essential. Infrared wireless preamplifiers require a wide bandwidth, a wide dynamic range, and the ability to reject ambient light. According to Infrared Data Association, data rates of 100Mb/s and higher are now being investigated [1]. A commonly used topology is the transimpedance amplifier, whose relative low input impedance and wide bandwidth is well suited for the application. A wide dynamic range is required for infrared wireless optical receivers in order to accommodate variable link distances, 0~100cm. In the design of fixed-gain transimpedance feedback amplifiers for infrared wireless receivers, there is a direct trade-off between input noise current and the input current overload level via the value of the shunt feedback resistor employed. Therefore, in order to enlarge the dynamic range, various means have been adopted to vary the gain of the

1

This work was supported by the National Science Council of Taiwan R.O.C., Grant No. NSC 92-2218-E-224-005. R. Y. Chen and C. Y. Hung are with the Electronics Engineering Department, National Yunlin University of Science and Technology, Doulio, Yunlin 64002, Taiwan (email: chenry@yuntech.edu.tw). T. S. Hung was with the Electronics Engineering Department, National Yunlin University of Science and Technology. He is now with the Opto-Electronics and Systems Laboratory, ITRI, Hsinchu 310, Taiwan.

transimpedance amplifiers in response to the input signal levels. Variable-gain transimpedance feedback amplifiers, however, are prone to instability. Specifically, as the transimpedance gain is varied, it is difficult to maintain a fixed relative position of the unity-gain frequency with respect to the non-dominant pole of the loop gain of the feedback amplifier. Recently, several optical preamplifiers featuring adaptive transimpedance have been proposed, with different degrees of success, to overcome this problem [2]-[4].

Fig. 1 The block diagram of the infrared wireless optical receiver front-end, in which a fully-differential active feedback network is applied around the transimpedance amplifier to reject the ambient light.

Single-ended [5] and fully-differential [6], [7] active

feedback networks, as shown in Fig. 1, are applied around the transimpedance feedbabk amplifier to reject the low-frequency ambient photocurrent, and thus overcome the drawbacks of directly placing a passive RC high-pass filter at the input of the preamplifier. Alternatively, a feedforward offset-extractor is incorporated into the optical receiver to cancel the low-frequency offset voltages without the negative feedback network [8].

Out of the four amplifier types, namely voltage, transimpedance, transconductance, and current amplifiers, it is advisable to adopt the current-mode amplifier as the feedforward gain element of the transimpedance feedback

R. Y. Chen et al.: A CMOS Infrared Wireless Optical Receiver Front-end with a Variable-gain Fully-differential Transimpedance Amplifier 425

amplifier [9]. The loop gain of such a feedback configuration is equal to the current gain of the current-mode amplifier, which is independent of the closed-loop transimpedance gain of the system, i.e., the feedback resistors. In addition, due to the lower input impedance enhanced through the combination of a current-mode amplifier and the shunt feedback, the bandwidth of the transimpedance feedback amplifier displays lower sensitivity to source and parasitic input capacitances.

In this work, a stable CMOS variable-gain fully-differential transimpedance feedback amplifier is presented [10]. A feedback analysis is carried out for the design of a stable variable-gain transimpedance feedback amplifier in section II. Section III describes a current-mode amplifier with a low input resistance and a high output resistance, which is then employed as the feedforward gain element of our variable-gain fully-differential transimpedance feedback amplifier. The other building blocks of the infrared wireless receiver front-end are also described in section III. Results and summary are given in section IV and section V, respectively.

Ri ii Ai(s)ii Ro

Y11Y12V2Y21VoutY22

+

Vout

_

iin

+

V2__

Fig. 2 A feedback block diagram of a transimpedance feedback amplifier employing a current-mode amplifier as the feedforward gain element.

II. FEEDBACK ANALYSIS For a variable-gain transimpedance feedback amplifier

employing a current amplifier as the feedforward gain element, the feedback block diagram is shown in Fig. 2. Ai(s), Ri, and Ro represent the current gain, input resistance, and output resistance of the feedforward gain element, respectively. The Y model represents the feedback network [11], [12]. The reverse transmission through the feedback circuit is assumed negligible, compared with the feedforward transmission of the current amplifier. Loading from the feedback resistor upon the feedforward current amplifier is properly taken into account. Note that the photodiode capacitance is not included in the feedback block diagram because common-gate input buffer stages are employed to isolate the photodiode capacitance from the transimpedance amplifier.

The closed-loop gain of the transimpedance feedback amplifier is given by

)s(A1)s(AR

)R//R)(s(ARR

11

)R//R)(s(ARR

R

)s(i)s(v

i

if

foiif

foiif

f

in

out

+−≈

++

+−

=

(1)

if the condition Ri << Rf << Ro is satisfied, where Rf is the feedback resistor. The loaded feedforward gain is -RfAi(s) and the loop gain is

1p

0ii s1

A)s(Aω+

= (2)

Ai0 and p1 are the low-frequency current gain and the dominant pole of the current amplifier, respectively. The unity-gain frequency of the loop gain is

1p0it A ω=ω (3)

which is independent of Rf, or equivalently the closed-loop transimpedance gain. The equivalent non-dominant pole is usually not significantly affected by the feedback resistance. Because the unity-gain frequency of the loop gain is independent of the closed-loop variable-gain, -Rf, a safe and fixed separation between t and the equivalent non-dominant pole, can be achieved by proper design. Consequently, the stability of the variable-gain transimpedance feedback amplifier is secured.

Vb1

Vb4

Vout

Vb5

Vb3

iin

M 7

Vb2

Vb6

Vb7

M 4 M 3

M 10

M 6

M 1

M 2

M 5

M 8

M 9

Cc

Rc

A

B

VDD

Fig. 3 The half-circuit diagram of the current-mode amplifier.

On the other hand, if (2) is substituted into (1), we obtain

)A1(s1

R)s(i)s(v

0i1p

f

in

out

+ω+

−≈ (4)

IEEE Transactions on Consumer Electronics, Vol. 51, No. 2, MAY 2005 426

That is, the 3dB-bandwidth is also independent of the variable-gain of the transimpedance feedback amplifier. Thus the variable-gain transimpedance amplifier is able to exhibit a constant bandwidth, or gain-bandwidth independence, as long as the condition Ri << Rf << Ro is satisfied.

III. CIRCUIT DESCRIPTION Fig. 3 shows the half-circuit diagram of our current-mode

amplifier. A local feedback circuit is applied to increase the equivalent transconductance of the input common-gate transistor M1 and thus further reduce the input resistance of the current amplifier. In order to lower the minimum voltage level at the input node, i.e., to free the minimum value of the input node voltage from the constraint imposed by the gain boosting transistor, and consequently make the current amplifier and optical preamplifier more suitable for low-voltage application, a folded-cascode stage using an input PMOS transistor M3 is adopted (improved RGC) [11], instead of a common-source NMOS transistor.

A cascode transistor M5 is employed to increase the output

resistance of the common-source stage M2, as shown in Fig. 3. A series RC frequency-compensation network is connected from node B to node A to acquire adequate phase margin. Based upon the single-ended circuit shown in Fig. 3, the resulting fully-differential current-mode amplifier is implemented and shown in Fig. 4.

Vb1 Vb1 Vb1

iin+

Vb1

vout- Vb2

Vb2

Vb3 Vb3

Vb2

iin-

Vct

VDD V

DD

vout+

Fig. 4 The circuit diagram of the fully-differential current-mode amplifier.

A common-mode feedback (CMFB) circuit [13], [14], which is shown in Fig. 5, monitors the common-mode of the output voltages and feedbacks a control voltage Vct to adjust the output bias currents. The error amplifier, employed in the ambient photocurrent rejection circuit shown in Fig. 1, is implemented by a fully-differential two-stage operational transconductance amplifier, as shown in Fig. 6.

IV. RESULTS AND DISCUSSION

The transimpedance feedback amplifier is designed using a 0.35um CMOS technology. The frequency response of the open-loop current amplifier is shown in Fig. 7. A low input resistance of roughly 0.04kΩ is achieved and is attributed to the improved RGC circuits employed at the input. The simulated output resistance of the current amplifier is approximately equal to 30kΩ.

Vb1 Vb1

Vct

Vcm

VDD

vout- vout+

Fig. 5 The circuit diagram of the common-mode feedback (CMFB) circuit.

VDD

Vct

VipVim

Vbp

Vbp

CC Vo2Vo1

Vbp

Fig. 6 The circuit diagram of the error amplifier.

The frequency response of the closed-loop transimpedance feedback amplifier employing the current amplifier as the feedforward gain element is shown in Fig. 8, in which the feedback resistor Rf is varied from 0.3kΩ to 1kΩ. For more than three times variation of the magnitude of the feedback resistor, from 0.3kΩ to 1kΩ, the 3dB bandwidths of the transimpedance amplifier remain relatively constant, as shown in Fig. 8. The frequency response of the error amplifier are given in Fig. 9.

The frequency response of the infrared wireless optical

receiver front-end, as shown in Fig. 1, employing our transimpedance feedback amplifier, is shown in Fig. 10. The DC photocurrent rejection circuit, consisted of the error amplifier and the differential-pair transconductance amplifier,

R. Y. Chen et al.: A CMOS Infrared Wireless Optical Receiver Front-end with a Variable-gain Fully-differential Transimpedance Amplifier 427

serves as a voltage-current feedback network of the optical preamplifier to reject the low-frequency current, as shown in Fig. 10. A transient pulse response of the output of the optical preamplifier is shown in Fig. 11. Fig. 12 shows the simulated output eye diagram of the optical preamplifier. The layout of the infrared wireless optical receiver front-end is given in Fig. 13. All the results shown are post-layout simulation (performed with back-annotated netlists). Table 1 shows the performance summary of the transimpedance amplifier and the optical preamplifier.

Fig. 7 The frequency response of the current-mode amplifier.

Fig. 8 The frequency responses of the transimpedance amplifier.

V. SUMMARY A transimpedance feedback amplifier, employing a current-

mode amplifier as the feedforward gain element, is described. The stable variable-gain fully-differential transimpedance amplifier also exhibits a relative constant bandwidth. The variable-gain fully-differential transimpedance feedback amplifier finds applications, among others, in infrared wireless data communications. For an infrared wireless optical receiver front-end employing the transimpedance amplifier, the optical preamplifier achieves a transimpedance gain of ΩdB66 and a bandwidth of 114MHz with a 5pF photodiode capacitance, and

a power consumption of 17.4mW.

Fig. 9 The frequency responses of the error amplifier.

Fig. 10 The frequency response of the infrared wireless optical receiver front-end.

Fig. 11 The transient pulse response of the output of the infrared wireless optical receiver front-end.

Frequency (Hz)

Cur

rent

Gai

n (d

B)

Frequency (Hz)

Rf=0.3kΩΩΩΩ

Rf=1kΩΩΩΩ

Tran

sim

peda

nce

Gai

n (d

B)

Frequency (Hz)

Rf=1kΩΩΩΩ

Rf=0.3kΩΩΩΩ

Gai

n (d

B)

IEEE Transactions on Consumer Electronics, Vol. 51, No. 2, MAY 2005 428

Fig.12 The output eye diagram of the infrared wireless optical receiver front-end.

TABLE 1 THE PERFORMANCE SUMMARY OF THE TRANSIMPEDANCE

AMPLIFIER AND THE OPTICAL PREAMPLIFIER. Technology 0.35um CMOS 3.3V

Power dissipation 11.9 mW

Bandwidth 304MHz for Rf=0.3kΩ 256MHz for Rf=1kΩ

Tran

sim

peda

nce

ampl

ifier

Gain range 55 dB ~ 66 dB

Photodiode capacitance 5pF Active area 720um × 560um

Power dissipation 17.4 mW

Dynamic Range (dB) 48 dB Opt

ical

pr

eam

plifi

er

Bandwidth 94MHz for Rf=0.3kΩ 114MHz for Rf=1kΩ

Fig. 13 The layout of the infrared wireless optical receiver front-end.

REFERENCES [1] Infrared Data Association. IrDA-Standard-Physical Layer,

Version 1.3 (2001, March), http://www.irda.org. [2] R. G. Meyer and W. D. Mack, “A wide-band low-noise

variable-gain BiCMOS transimpedance amplifier,” IEEE J. Solid-State Circuits, vol. SC-29, no.6, pp. 701~706, June 1994.

[3] H. Khorramabadi, L. D. Tzeng, and M. J. Tarsia, “A 1.06Gb/s-31dBm to 0dBm BiCMOS optical preamplifier featuring adaptive transimpedance,” IEEE International Solid-State Circuits Conference (ISSCC) Digest of Technical Papers, pp. 54~55, Feb. 1995.

[4] K. Phang and D. A. Johns, “A CMOS optical preamplifier for wireless infrared communication,” IEEE Trans. on Circuits and Systems-II, vol. 46, no. 7, pp. 852~859, July 1999.

[5] K. Phang and D. A. Johns, “A 3-V CMOS optical preamplifier with DC photocurrent rejection,” in Proc. IEEE International Symposium on Circuits and Systems (ISCAS), pp. I-305~308, 1998.

[6] B. Zand, K. Phang, and D. A. Johns, “Transimpedance amplifier with differential photodiode current sensing,” in Proc. IEEE International Symposium on Circuits and Systems (ISCAS), pp. II-624~627, 1999.

[7] B. Zand, K. Phang, and D. A. Johns, “A transimpedance amplifier with DC-coupled differential photodiode current sensing for wireless optical communications,” in Proc. IEEE Custom Integrated Circuits Conference (CICC), pp. 21-3-1~4, 2001.

[8] C. S. Hsieh and H. Y. Huang, “A high-bandwidth wireless infrared receiver with feedforward offset extractor,” in Proc. IEEE International Symposium on Circuits and Systems (ISCAS), pp. I-73~76, 2003.

[9] B. Wilson and J. Drew, “Novel transimpedance amplifier formulation exhibiting gain-bandwidth independence,” in Proc. IEEE International Symposium on Circuits and Systems (ISCAS), pp. 169~172, 1997.

[10] R. Y. Chen, T. S. Hung, and C. Y. Hung, “A CMOS variable-gain fully-differential transimpedance amplifier for infrared wireless data communications,” in IEEE International Conference on Consumer Electronics Digest of Technical Papers, paper no. 9.2-5, Jan. 2005.

[11] B. Razavi, Design of Analog Integrated Circuits, McGraw-Hill: Boston, 2001.

[12] A. S. Sedra and K. C. Smith, Microelectronic Circuits, 4th ed., Oxford University Press: New York, 1998.

[13] D. A. Johns and K. W. Martin, Analog Integrated Circuit Design, John Wiley & Sons: New York, 1997.

[14] R. A. Whatly, “Fully differential operational amplifier with DC common-mode feedback,” U.S. patent no. 4573020, Feb. 1986.

R. Y. Chen et al.: A CMOS Infrared Wireless Optical Receiver Front-end with a Variable-gain Fully-differential Transimpedance Amplifier 429

Roger Yubtzuan Chen received the M.S. and Ph.D. degrees from the University of California, Los Angeles (UCLA), both in Electrical Engineering.

He had been with CCL/ITRI, ERSO/ITRI, Southern Taiwan University of Technology, and Fengchia University. In February 2003, he joined the Electronics Engineering Department, National Yunlin University of

Science and Technology, Taiwan, where he is now an associate professor. He has been involved in the design of CMOS analog and digital integrated circuits for communications.

Tsung-Shuen Hung received the B.S. degree in electronics engineering from United University, Miaoli, Taiwan, and the M.S. degree in electronics engineering from Yunlin University of Science and Technology, Taiwan, in 2002 and 2004, respectively.

In 2004, he joined Opto-Electronics and Systems Laboratories, ITRI, Hsinchu, Taiwan, where he is a

associate Engineer. His current research is the design of optical disk systems.

Chih-Yuan Hung was born in Taichung County, Taiwan, in 1980. He received the B.S. degree from the Electronics Engineering Department, National Yunlin University of Science and Technology, Taiwan, in 2003. He is currently working toward the M.S. degree at the same University.

His main research interests are analog IC Design and mixed signal IC design.