11-04-2012, 04:24 PM
Design and Implementation of Ultralow Current-Mode Amplifier for Biosensor Applications
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INTRODUCTION
SINCE emerging in the past century, the biomedical technology
has been believed to be one of the most promising
industries in the twenty-first century, together with microelectronics
and nanoelectronics. Recently, deoxyribonucleic acid
(DNA)-molecule-based biosensors have been reported by many
technical literatures [1]–[3]. Naturally, the integrated biosensing
system (IBS), which monolithically integrates the biomedical
sensor arrays and ASICs, such as the ultralow current-mode
amplifier (ULCA), the analog-to-digital converter, and the digital
signal processor, in a single chip, is avidly expected to
greatly reduce the cost of common sensors used in hospitals
and markets.
CIRCUIT DESIGN AND DESCRIPTIONS
The proposed ULCA is shown in Fig. 2. In this circuit,
the current from DNA biosensors is input to a complementary
regulated current mirror consisting of an N-type opamp AN0,
a P-type opamp AP0, and transistors M0, M1, M3, and M4,
where it is amplified by a factor of 10 (20 dB). The opamps
AN1 and AP1 and the transistors M6−M11 compose a voltage
limiter.
CIRCUIT ANALYSIS
As shown in Fig. 2, when a positive input is applied, the
gain is provided by the regulated current mirror consisting of
M0, M1, and opamp AP0, whereas M3−M5 and opamp AP1
are serving as current sources providing the quiescent current
for the stage, which can be simplified to the circuit shown in
Fig. 4(a). A complementary discussion of the negative input
case leads to the topology shown in Fig. 4(b).
EXPERIMENTAL VERIFICATION AND DISCUSSION
The proposed ULCA is realized in the SMIC 0.18-μm
CMOS mixed-signal technology, and Fig. 5 shows the die
micrograph of the chip. The box encloses the ULCA circuitry,
which occupies about 230 × 80 μm2 of the chip area. The
performance of the ULCA is measured in terms of gain, bandwidth,
noise, offset, etc. The results are shown and discussed in
this section.
CONCLUSION
In this brief, a novel subthreshold Class AB ULCA aiming
at the application of signal preamplification in the IBS has
been demonstrated in SMIC 0.18-μm CMOS technology. Experimental
results show that the proposed ULCA completely
accommodates the application and can provide a current gain
of 19.9 dB, 3-dB bandwidth of 15 kHz, and an input range of
−0.4 to 0.4 mA, whereas the IRO and the noise current are
less than 96.6 pA and 37.6 pArms, respectively.