L15Part1 BJT Cascode High Freq Method of Poles_1

# L15Part1 BJT Cascode High Freq Method of Poles_1 - EE...

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Zubair Rehman EE 311Lecture 15 & EEO 311 Lecture 14 Part 1 6.8 Cascode Amplifier BJT Cascode: High Frequency Response Method of Poles

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2 Review of High Frequency Calculations Two Methods 1. Pole-Zero 2. Open-Circuit Time Constants Pole-Zero Method 1. 2. 3. Cascode Miller’s Theorem Very Useful for Pole-Zero Method K = V o / V i Y1 = Y * (1 - K) Y2 = Y * (1 - 1 / K )
3 Important Special Case Y = j ϖ C Y1 = j ϖ C * (1-K) C1 = C(1-K) Y2 = j ϖ C * (1- 1 / K ) C2 = C(1- 1 / K ) Application To CE Circuit (or CS Circuit) C1 = C μ (1-K) C2 = C μ (1- 1 / K ) At Mid-Band K = V o / V π = -g m R L (We use Mid-Band Value of K)

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4 C1 = C μ (1+g m R L ) C2 = C μ (1+ 1/g m R L ) ≈ C μ The poles can be identified by inspection by determining the RC Time Constants. Pole = -1 / RC f BP = 1 / (2 π RC) At the Input: RC Time Constant = (R s ’ || r π ) * (C π + C1) f 1 = f in = 1 / [(2 π (R s ’ || r π ) * (C π + C μ (1+g m R L ))] At the output R L ’C2 Pole = -1 / RC = -1 / (R L ’C μ ) f 2 = f out = 1 / ( R L ’ C μ ) Almost always f in < f out
5 Cascode Amplifier Cascode and High-Frequency Equivalent Circuit

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L15Part1 BJT Cascode High Freq Method of Poles_1 - EE...

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