CMOS Transimpedance Amplifiers for Optical Wireless Communications
CMOS transimpedance amplifiers (TIAs) suitable for optical wireless communications are presented. The shunt-feedback TIAs are designed using a feedforward current amplifier (CA). In the presence of a large input photodiode capacitance, the CA-based TIA po
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Abstract CMOS transimpedance amplifiers (TIAs) suitable for optical wireless communications are presented. The shunt-feedback TIAs are designed using a feedforward current amplifier (CA). In the presence of a large input photodiode capacitance, the CA-based TIA posses a wide dynamic range with a relatively constant bandwidth. The optical wireless preamplifiers are validated by both simulation and measurement. Keywords CMOS • Feedback amplifier • Optical wireless communications • Optical receiver • Transimpedance amplifiers • TIA
1 Introduction Transimpedance amplifiers (TIAs) are adopted in optical receivers to sense the input current generated by the photodiode and generate an amplified output voltage. The commonly used TIA configuration is to apply a shunt-shunt feedback to close the loop around a voltage amplifier to allow absorbing the photodiode current and yielding better drive capability [1–8]. Because the impact of the usually nonnegligible and even dominant photodiode capacitance, e.g., in optical transmissions with a large diameter photodiode to relax alignment requirement or optical wireless involving eye-safety regulations, can be suppressed by the lower input impedance, implementing a feedback TIA using a feedforward current amplifier (CA) is an attractive alternative [9]. In this chapter a CMOS feedback transimpedance amplifier consisting of a feedforward current amplifier (CA-TIA) is studied. In addition to low cost, CMOS
R.Y. Chen (*) • Z.-Y. Yang Department of Electronics Engineering, National Yunlin University of Science and Technology, Doulio, Yunlin 64002, Taiwan e-mail: [email protected] J. Juang and Y.-C. Huang (eds.), Intelligent Technologies and Engineering Systems, Lecture Notes in Electrical Engineering 234, DOI 10.1007/978-1-4614-6747-2_66, # Springer Science+Business Media New York 2013
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facilitates integration with back-end systems. The CA-TIAs are designed using a low-cost 0.35 μm CMOS and adopted in an optical receiver capable of rejecting lowfrequency ambient noise in Sect. 2. Post-layout simulations are shown in Sect. 3. In Sect. 4 the experimental verification of the optical receiver front end are presented.
2 Circuit Description The block diagram of the CA-based TIA is shown in Fig. 1. The photodiode is represented by a current source in parallel with its junction capacitance. A schematic diagram of the feedforward current amplifier is given in Fig. 2. A regulated cascade (RGC) [10] is added to increase the equivalent transconductance of the RF
ACA
Vout
Zin iin
CPD
Zout
Fig. 1 Block diagram of a CA-TIA
VDD VB1
VB1
CMFB
MP2
MP1 Q MPB
MP3 Cc Rc
Q1
MN1
VB2 MP4
VB2 VB4 MN2 VB3
MNA MPA MN3
Input RGC
I0
is
I0
Fig. 2 Schematic diagram of a current amplifier
VB2
CMOS Transimpedance Amplifiers for Optical Wireless Communications
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CA-TIA
Photodiode Bias Input Stage (PBIS)
RF
CPD
IDC+is
RGC
is
+
_
_
+ RF
+ _
+
RS
Error Amplifier
MB
_
MB
RS
2I0 Ambient Photocurrent Rejection Circuit (APRC)
Fig. 3 The
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