# Lect29 7.5.5 2010 BJTCascodeDiffAmpWithActiveLoad - EE 311...

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Unformatted text preview: EE 311 Lecture 29 Wednesday March 24, 2010 7.5 The Differential Amplifier With Active Load 1. BJT Systematic Input Offset Voltage 1. BJT Cascode Diff. Amp With Wilson Active Load 1. Lab 8 Active Loaded BJT Diff. Amp Employing a Folded Cascode Stage & Wilson Mirror Active Load 1. MOS Cascode Diff. Amp With Wilson Mirror Active Load Zubair Rehman Systematic Input Offset Current Because of finite β , I B does not equal 0 Q3 - Q4 PNP Current Mirror Current Transfer Ratio VBE3 = VBE4 IE3 = IE4 & IC3 = IC4 & IB3 = IB4 I C3 + I B3 + I B4 = α I / 2 = I C3 [1 + (2I B3 / I C3 )] α I / 2 = I C3 [ 1 + 2/ β ] I C3 = [ α I / 2 ] / [1 + 2/ β ] α I / 2 = I C4 + ∆ i = I C3 + ∆ i I C3 [1 + 2/ β ] = I C3 + ∆ i ∆ i = I C3 * 2/ β = [( α I/2) / (1 + 2/ β )] * 2/ β = [( α I/2) * [2/ β ] / (1 + 2/ β ) ∆ i = ( α I / β ) / (1 + 2/ β ) = α I / β for 1 >> 2 / β To reduce ∆ i to zero, apply V os to input |V os | = ∆ i / G m = ∆ i / g m |V os | = ( α I / β ) / g m = ( α I /...
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Lect29 7.5.5 2010 BJTCascodeDiffAmpWithActiveLoad - EE 311...

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