# Differential Pair

Discussion in 'Homework Help' started by ac_dc_1, Jan 28, 2013.

1. ### ac_dc_1 Thread Starter Member

Jan 27, 2013
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0
This question came up in my exam and it states

Consider the diferential pair represented in the circuit.Determine VA,VCE,VC,IC for every transistor in the following conditions:

R1=R4=1K R2=13K R3=2.7K VBE=0.6V VCC=VEE=15V

a)V1=V2=0V
b)V1=V2=5V
c)V1=2V and V2=5V

I have written the following equation:

VR2+2VBE+(-20)=0,so VR2=18,6V

Then IR2=18,6/10K=1,86mA

Then VRC1=RC1*(1,86*10^-3)/2
VRC1=9,3V
For simetry reasons we have that PFR Q1=PFR Q2

VRE1=RE1*(1,86*10^-3)/2=0,465V

VRE1=0.6+0.465=1,065 V

VCEQ3=-1.065+20=18,935 V

ICQ3=1,85*10^-3 A

I did this analysis considering V1,V2 generic reference voltages.I am not seing how having different values of V1 and V2 affects the values of VA,VCE,VC,IC of the transistors.

Thanks

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2. ### WBahn Moderator

Mar 31, 2012
22,990
6,884
First off, this is NOT a current mirror.

Second, you are being sloppy with your definitions again. When you say VR2, what IS THAT? Even if we assume it is the voltage across the resistor R2, youi have given no indication of what the polarity of VR2 is. Where are you getting the 2*VBE from? Where is the -20 (assuming that means -20V) coming from?

Which path is this equation for.

Think about this -- if I were to present to you what you have presented to us, do you think you would be able to figure out where I was coming up with these equations?