Berkeley ELENG 105 - Lecture 37 Multistage Amplifier Frequency Response (10 pages)

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Lecture 37 Multistage Amplifier Frequency Response



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Lecture 37 Multistage Amplifier Frequency Response

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Pages:
10
School:
University of California, Berkeley
Course:
Eleng 105 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

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Lecture 37 Multistage Amplifier Frequency Response Summary of frequency response of single stages CE CS suffers from Miller effect CC CD wideband see Section 10 5 CB CG wideband see Section 10 6 wideband means that the stage operates to near the frequency limit of the device fT How to find the Bode plot for a general multistage amplifier can t handle n poles and m zeroes analytically SPICE develop analytical tool for an important special case no zeroes exactly one dominant pole 1 2 3 n V out Ao V in 1 j 1 1 j 2 1 j n the example shows a voltage gain it could be Iout Vin or Vout Iin EE 105 Fall 2001 Page 1 Lecture 37 Finding the Dominant Pole Multiplying out the denominator Vout Ao 2 n Vin 1 b 1 j b 2 j b n j The coefficient b1 originates from the sum of j i factors n 1 1 1 b 1 1 2 n 1 1 i 1 i Therefore if we can estimate the linear coefficient b1 in the demoninator polynomial we can estimate of the dominant pole Procedure see P R Gray and R G Meyer Analysis and Design of Analog Integrated Circuits 3rd ed Wiley 1994 pp 502 504 1 Find circuit equations with current sources driving each capacitor 2 Denominator polynomial is determinant of the matrix of coefficients 3 b1 term comes from a sum of terms each of which has the form RTj Cj where Cj is the jth capacitor and RTj is the Th venin resistance across the jth capacitor terminals with all capacitors open circuited EE 105 Fall 2001 Page 2 Lecture 37 Open Circuit Time Constants The dominant pole of the system can be estimated by 1 1 b1 n j R Tj C j 1 n 1 j 1 where j RTj Cj is the open circuit time constant for capacitor Cj This technique is valuable because it estimates the contribution of each capacitor to the dominant pole frequency separately which enables the designer to understand what part of a complicated circuit is responsible for limiting the bandwidth of the amplifier EE 105 Fall 2001 Page 3 Lecture 37 Example Revisit CE Amplifier Small signal model Apply procedure to each capacitor separately 1 C s Th venin



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