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Unformatted text preview: Handout #20 EE 214 Winter 2009 Winter 2009 VGAs and Limiting Amplifiers B. Murmann and B. A. Wooley Stanford University Typical Design Goals ¡ Constant bandwidth across wide gain range ¡ Exponential gain control (“linear in dB”) preferred in many applications – VGA versus PGA (programmable gain amplifier, i.e. discrete steps) ¡ Low noise, low distortion, low power dissipation Poor Behavior Desired Behavior Barry Gilbert, “Considering Multipliers,” Analog Dialogue 42-11, November 2008 B. A. Wooley, B. Murmann EE214 Winter 2008-09 2 http://www.analog.com/library/analogdialogue/archives/42-11/multipliers.html Basic VGA Circuits [Xiao, JSSC 2/2007] Assuming a constant signal 2 2 inRMS m V G SNR α ∝ amplitude at the output: Attenuato 1 4 BW m L SNR kT f G R ∝ ⎛ ⎞ γ + ⋅ ⎜ ⎟ ⎝ ⎠ Attenuator ¡ In circuits (a) and (b), the output noise reduces for large inputs, but the distortion will degrade distortion will degrade ¡ In circuit (c), it is possible to maintain constant SNR, distortion and bandwidth – At the expense of lower baseline SNR, which may not be a problem if...
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- Winter '08
- Integrated Circuit