MIT 6 012 - Frequency Response of Amplifiers (10 pages)

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Frequency Response of Amplifiers



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Frequency Response of Amplifiers

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Lecture Notes


Pages:
10
School:
Massachusetts Institute of Technology
Course:
6 012 - Microelectronic Devices and Circuits
Microelectronic Devices and Circuits Documents

Unformatted text preview:

Lecture 21 Frequency Response of Amplifiers I Common Emitter Amplifier Outline Review frequency domain analysis BJT and MOSFET models for frequency response Frequency Response of Intrinsic Common Emitter Amplifier Effect of transistor parameters on fT Reading Assignment Howe and Sodini Chapter 10 Sections 10 1 10 4 6 012 Spring 2009 1 I Frequency Response Review Phasor Analysis of the Low Pass Filter Example R Vin C Vout Replacing the capacitor by its impedance 1 j C we can solve for the ratio of the phasors Vout Vin Vout 1 j C V in R 1 j C Vout 1 V in 1 j RC Vout Phasor notation 6 012 Spring 2009 2 Magnitude Plot of LPF Vout Vin Vout Vin 1 for low frequencies 0 for high frequencies Vout Vin log scale 1 Vout Vin Break point 0 3dB 0 1 0 01 1 0 707 2 1 dB 20 decade 0 001 0 0001 dB scale 20 40 60 0 01 RC 0 1 RC 1 RC 10 RC 100 RC 1000 RC 80 log scale a The break point is when the frequency is equal to o 1 RC The break frequency defines low and high frequencies dB 20 log x 20dB 10 40dB 100 40dB 01 At o the ratio of phasors has a magnitude of 3 dB 6 012 Spring 2009 3 Phase Plot of LPF Phase Vout Vin 0o for low frequencies Phase Vout Vin 90o high frequencies V out Vin Break point 0 45 90 135 180 0 01 RC 0 1 RC 1 RC 10 RC 100 RC 1000 RC log scale Transition region extends from o 10 to 10 o At o Phase 45o Review of Frequency Domain Analysis Chap 10 1 6 012 Spring 2009 4 II Small Signal Models for Frequency Response Bipolar Transistor C B C r V C gmV ro E E a MOS Transistor VSB 0 Cgd G D Vgs Cgs gmVgs ro S S b Replace Cgs for C Replace Cgd for C Let r 6 012 Spring 2009 5 III Frequency Response of Intrinsic CE Current Amplifier RS RL 0 Circuit analysis Short Circuit Current Gain Io Iin C Io Iin r V gmV C KCL at the output node Io gmV V j C KCL at the input node V Iin V j C where Z Z r 1 j C After Algebra j C j C 1 j gm r 1 o 1 gm gm z Io o Iin 1 j r C C 1 j r C C 1 j p gm Z C 6 012 Spring 2009 1 p r C C 6 Bode Plot of Short Circuit Current Gain Io Iin o gmr 1 C C C 1 r C C gm



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