EECE 311: Electronic Circuits Part 2 AMPLIFIER FREQUENCY RESPONSE 2 Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits EECE 311 – Frequency Response 3 Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits n But let us consider capacitor impedance ̈ Changes with frequency : n infinite at DC n Decreases in magnitude as frequency increases n Approaches zero at very high frequencies EECE 311 – Frequency Response 4 Z C ( s ) = 1 sC , Z C ( j ω ) = − j ω C Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits n But let us consider capacitor impedance ̈ Changes with frequency : n infinite at DC n Decreases in magnitude as frequency increases n Approaches zero at very high frequencies n As w increases, we need to study the “Frequency Response” EECE 311 – Frequency Response 5 Z C ( s ) = 1 sC , Z C ( j ω ) = − j ω C Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits n But let us consider capacitor impedance ̈ Changes with frequency : n infinite at DC n Decreases in magnitude as frequency increases n Approaches zero at very high frequencies n As w increases, we need to study the “Frequency Response” ̈ Amplifier Input is EECE 311 – Frequency Response 6 Z C ( s ) = 1 sC , Z C ( j ω ) = − j ω C !"#$ % ! ! Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits n But let us consider capacitor impedance ̈ Changes with frequency : n infinite at DC n Decreases in magnitude as frequency increases n Approaches zero at very high frequencies n As w increases, we need to study the “Frequency Response” ̈ Amplifier Input is ̈ Amplifier Output is since the system is linear EECE 311 – Frequency Response 7 Z C ( s ) = 1 sC , Z C ( j ω ) = − j ω C !"#$ % ! ! !"#$ % ! " ! " + Frequency Response n At low frequency – close to DC ̈ Amplifier characteristics are not affected by frequency (no frequency- dependent elements) ̈ Capacitors are open-circuits and inductors are short circuits n But let us consider capacitor impedance ̈ Changes with frequency : n infinite at DC n Decreases in magnitude as frequency increases n Approaches zero at very high frequencies n As w increases, we need to study the “Frequency Response” ̈ Amplifier Input is ̈ Amplifier Output is since the system is linear ̈ How do A (magnitude) and f (phase) vary with frequency w ? EECE 311 – Frequency Response 8 Z C ( s ) = 1 sC , Z C ( j ω ) = − j ω C !"#$ % ! ! !"#$ % ! " ! " + MOSFET High-Frequency Small-Signal Model EECE 311 – High-Frequency Models 9 MOSFET High-Frequency Small-Signal Model EECE 311 – High-Frequency Models 10 MOSFET High-Frequency Small-Signal Model EECE 311 – High-Frequency Models 11 n At low frequency, capacitors are open circuits (infinite impedance) EECE 311 – High-Frequency Models 12 BJT High-Frequency Small-Signal Model EECE 311 – High-Frequency Models 13 S IDE N OTE : Unity-Gain Frequency Used to find capacitance values EECE 311 – High-Frequency Models 14 S IDE N OTE : Unity-Gain Frequency Used to find capacitance values ! " # ! " #$ #% # C ' ' ! = + n MOSFET EECE 311 – High-Frequency Models 15 S IDE N OTE : Unity-Gain Frequency Used to find capacitance values ! " # ! " #$ #% # C ' ' ! = + n MOSFET n BJT ! " # ! " # $ % % ! μ ! = + Example EECE 311 – Frequency Response 16 Example EECE 311 – Frequency Response 17 n Let us study the effect of just one capacitor on the gain of a voltage amplifier Example EECE 311 – Frequency Response 18 Example EECE 311 – Frequency Response 19 Example – How does capacitor affect amplifier gain? EECE 311 – Frequency Response 20