Lecture 22 ANNOUNCEMENTS Midterm 2 Th 11 15 3 30 5PM 3 30 5PM in Sibley Aud Bechtel Bldg HW 11 Clarifications revisions to Problems 1 3 4 were made OUTLINE Differential Amplifiers General considerations BJT differential pair Qualitative analysis Large signal analysis Large signal Small signal analysis Frequency response Reading Chapter 10 1 10 2 EE105 Fall 2007 Lecture 22 Slide 1 Prof Liu UC Berkeley Humming Noise in Audio Amplifier Consider the amplifier below which amplifies an audio signal from a microphone p If the power supply VCC is time varying it will result in an additional undesirable voltage signal at the output perceived i d as a humming h i noise i by b th the user EE105 Fall 2007 Lecture 22 Slide 2 Prof Liu UC Berkeley Supply Ripple Rejection Since node X and Y each see the voltage ripple their voltage difference will be free of ripple pp v X Av vin vr vY vr v X vY Av vin EE105 Fall 2007 Lecture 22 Slide 3 Prof Liu UC Berkeley Ripple Free Differential Output If the input signal is to be a voltage difference between two nodes an amplifier that senses a differential signal is needed EE105 Fall 2007 Lecture 22 Slide 4 Prof Liu UC Berkeley Common Inputs to Differential Amp The voltage signals applied to the input nodes of a differential amplifier p cannot be in phase p otherwise the differential output signal will be zero v X Av vin vr vY Av vin vr v X vY 0 EE105 Fall 2007 Lecture 22 Slide 5 Prof Liu UC Berkeley Differential Inputs to Differential Amp When the input voltage signals are 180 out of phase the resultant output p node voltages g are 180 out of p phase so that their difference is enhanced v X Av vin vr vY Av vin vr v X vY 2 Av vin EE105 Fall 2007 Lecture 22 Slide 6 Prof Liu UC Berkeley Differential Signals Differential signals share the same average DC value and are equal q in magnitude g but opposite pp in p phase A pair of differential signals can be generated among other ways by a transformer EE105 Fall 2007 Lecture 22 Slide 7 Prof Liu UC Berkeley Single Ended vs Differential Signals EE105 Fall 2007 Lecture 22 Slide 8 Prof Liu UC Berkeley BJT Differential Pair With the addition of a tail current an elegant and robust differential pair is achieved achieved EE105 Fall 2007 Lecture 22 Slide 9 Prof Liu UC Berkeley Common Mode Response Due to the fixed tail current the input common mode value can varyy without changing g g the output p common mode value VBE1 VBE 2 I C1 I C 2 I EE 2 V X VY VCC EE105 Fall 2007 Lecture 22 Slide 10 I EE RC 2 Prof Liu UC Berkeley Differential Response I C1 I EE IC2 0 VX VCC RC I EE VY VCC EE105 Fall 2007 Lecture 22 Slide 11 Prof Liu UC Berkeley Differential Response cont d I C 2 I EE I C1 0 VY VCC RC I EE VX VCC EE105 Fall 2007 Lecture 22 Slide 12 Prof Liu UC Berkeley Differential Pair Characteristics A differential input signal results in variations in the output currents and voltages g whereas a common mode input p signal g does not result in any output current voltage variations EE105 Fall 2007 Lecture 22 Slide 13 Prof Liu UC Berkeley Virtual Ground For small input voltages V and V the gm values are equal q so the increase in IC1 and decrease in IC2 are equal q in magnitude Thus the voltage at node P is constant and can be considered as AC ground I I C1 IC2 EE I 2 I EE I 2 VP 0 I C1 g m V I C 2 g m V EE105 Fall 2007 Lecture 22 Slide 14 Prof Liu UC Berkeley Extension of Virtual Ground It can be shown that if R1 R2 and the voltage at node A goes up by the same amount that the voltage at node B goes down down then the voltage at node X does not change vX 0 EE105 Fall 2007 Lecture 22 Slide 15 Prof Liu UC Berkeley Small Signal Differential Gain Since the output signal changes by 2gm VRC when the input signal changes by 2 V the small signal voltage gain is gmRC Note that the voltage gain is the same as for a CE stage but that the power dissipation is doubled 2 g m VRC Av g m RC 2 V EE105 Fall 2007 Lecture 22 Slide 16 Prof Liu UC Berkeley Large Signal Analysis I C1 IC2 EE105 Fall 2007 Lecture 22 Slide 17 Vin1 Vin 2 I EE exp VT Vin1 Vin 2 1 exp VT I EE Vin1 Vin 2 1 exp 1 VT Prof Liu UC Berkeley Input Output Characteristics Vout1 Vout 2 RC I EE tanh EE105 Fall 2007 Lecture 22 Slide 18 Vin1 Vin 2 2VT Prof Liu UC Berkeley Linear Nonlinear Regions of Operation Amplifier operating in linear region EE105 Fall 2007 Amplifier operating in non linear region Lecture 22 Slide 19 Prof Liu UC Berkeley Small Signal Analysis EE105 Fall 2007 Lecture 22 Slide 20 Prof Liu UC Berkeley Half Circuits Since node P is AC ground we can treat the differential pair as two CE half circuits vout1 vout 2 g m RC vin1 vin 2 EE105 Fall 2007 Lecture 22 Slide 21 Prof Liu UC Berkeley Half Circuit Example 1 v out 1 v out 2 g m rO v in1 v in 2 EE105 Fall 2007 Lecture 22 Slide 22 Prof Liu UC Berkeley Half Circuit Example 2 Av g m1 rO1 rO 3 R1 EE105 Fall 2007 Lecture 22 Slide 23 Prof Liu UC Berkeley Half Circuit Example 3 Av g m1 rO1 rO 3 R1 EE105 Fall 2007 Lecture 22 Slide 24 Prof Liu UC Berkeley Half Circuit Example 4 Av EE105 Fall 2007 Lecture 22 Slide 25 RC 1 RE gm Prof Liu UC Berkeley Differential Pair Frequency Response Since the differential pair can be analyzed using its half circuit its transfer function I O impedances locations of poles zeros are the same as that of its half circuit EE105 Fall 2007 Lecture 22 Slide 26 Prof Liu UC Berkeley
View Full Document
Unlocking...