HO17_214W09_WidebandAmp-Part1_2pp

HO17_214W09_WidebandAmp-Part1_2pp - Handout #17 EE 214...

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B. Murmann, B. Wooley EE214 Winter 2008-09 1 Wideband Amplifiers (Part I) B. Murmann and B. A. Wooley Stanford University Handout #17 EE 214 Winter 2009 B. Murmann, B. Wooley EE214 Winter 2008-09 2 Wideband, or broadband, lowpass amplifiers are typically designed to provide a moderate amount of desensitized gain over bandwidths that are much larger than can be achieved using an operational amplifier. Typical specifications may include: Moderate gain (10 - 40 dB) Large –3dB bandwidth Low gain sensitivity to changes in device parameters that result from variations in IC fabrication or temperature Noise Linearity (harmonic distortion)
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B. Murmann, B. Wooley EE214 Winter 2008-09 3 Gain-Bandwidth Product Gain ! Bandwidth is the principal measure of performance in the frequency domain. Consider a simplified model for a single common-emitter stage: In a cascade of identical stages, the equivalent load resistance is R L = r ! ||R C while the load capacitance is C t r ! C t g m v i + v i + v o R C B. Murmann, B. Wooley EE214 Winter 2008-09 4 At the “output” node of the stage v o = ! g m v i 1 L + sC t " # $ % & R L C t g m v i + v i + v o R L C t The voltage gain for the stage is therefore A(s) ! v o (s) v i (s) = ! g m R L 1 + sR L C t
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B. Murmann, B. Wooley EE214 Winter 2008-09 5 The low-frequency (“midband”) voltage gain for the stage is A 0 ! A(0) = g m R L and the –3dB bandwidth is ! B ! frequency, ! , at which A(j ! ) = A 0 2 = 1 R L C t Thus, for the simplified single stage GB ! gain ! bandwidth = A 0 ! " B = (g m R L ) 1 R L C t # $ % & ( = g m C t B. Murmann, B. Wooley EE214 Winter 2008-09 6 Observations: • In a multi-stage cascade amplifier, the bandwidth over which gain can be obtained is less than f T • By reducing R L , the bandwidth can be increased in exchange for a reduction in gain • The collector-base capacitances in a cascade of common-emitter stages cause interaction among the stages that results in pole- splitting and a lowering of the bandwidth. The interaction is small when R L is small In an idealized case where C t is dominated by minority carrier storage in the intrinsic base region (C t ~ C B ) GB ! g m C B = 1 " T = # T = 2 $ f T
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B. Murmann, B. Wooley EE214 Winter 2008-09 7 Cascaded Multi-Stage Amplifiers In a multi-stage cascade there is generally a loss of bandwidth and gain- bandwidth product per stage, even without interaction between stages In a cascade of N stages, define A 0N = total midband voltage gain " BN = overall –3dB bandwidth A figure-of-merit used to assess the extent to which stages in a cascade are used effectively is the equivalent per stage gain-bandwidth product , defined as GB N ! (A 0N ) 1/N !" BN B. Murmann, B. Wooley EE214 Winter 2008-09 8 Bandwidth Shrinkage Consider a cascade of N identical stages, each with the single- pole response A(j ! ) = A 0 1 + j ! !
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HO17_214W09_WidebandAmp-Part1_2pp - Handout #17 EE 214...

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