Design of A Low Voltage Class AB Variable Gain Amplifier (VGA)

M. Idzdihar Idris, M.N.Shah Zainudin, M. Muzafar Ismail, R. Abd Rahim


A variable gain amplifier (VGA) is one of the most
significant component in many applications such as analog to
digital  converter  (ADC). In communication receiver, VGA is
typically employed in a feedback loop to realize an automatic
gain control (AGC), to provide constant signal power to
baseband analog-to-digital converter (ADC) for unpredictable
received signal strengths. Gain range, power consumption and
bandwidth of ADC are strongly influenced by the performance of
operational amplifier. VGA is the key element for amplifying
process in ADC. However, current class AB VGA is experiencing
the limit of bandwidth, which is not suitable for high speed
automatic gain control AGC. In order to overcome these
limitations a high linearity and wide bandwidth of VGA is
indispensable. The aim of this research is to get higher gain and
larger bandwidth for VGA. In this research, a low cost, low
power voltage and wide bandwidth class AB VGA is designed to
mitigate this constraint. Superiority of the proposed VGA has
been confirmed by circuit simulation using CEDEC 0.18-μm
CMOS process with the help of tools from Mentor Graphics in
designing a 100-MHz VGA under 1V supply voltage draining
total static power consumption less than 125uW. The results
show that the circuit is able to work with high linearity and wide
bandwidth by varying Rf and Rs.  Therefore, the frequency
response (Gain) and the wide bandwidth of this class AB VGA is
better than previously reported class AB VGA. Consequently,
this modified class AB VGA is appropriate for high speed



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ISSN: 2180-1843

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