A high-gain and wide-band variable gain amplifier (VGA) is developed using 300-GHz InP-InGaAs double-heterojunction bipolar transistor (DHBT). Negative-R/sub E/ quad is used to enhance amplifier gain-bandwidth product. At maximum gain, the single-ended S/sub 21/ of 17 dB and the associated 3-dB bandwidth of 50 GHz are measured to produce a gain-bandwidth product of 354 GHz in a VGA including a Gilbert multiplier and an output driver. The gain-bandwidth product is twice the value measured from the VGA designed by single resistor degeneration in the same process. The circuit is designed in terms of detailed stability considerations and the experimental results show it to be unconditionally stable over 0.5-50 GHz. The linearity of the VGA is affected by nonlinear effects in DHBTs, and different design approaches are analyzed. An output interception point of the third harmonic of 16.2 dBm is measured.
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Lai et al. (2006) studied this question.
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