6.1 The BJTDifferential PairBasic Operation-l:Common-mode inputVco>The differential pair with a common-mode input signal VcM.RR>Two transistors are matched.>Current source with infinite outputresistanceO>CurrentIdivide equallybetweentwo+UCMtransistors.>The difference in voltagebetweenthe0.7UCAtwo collectoris zero.>Thedifferentialpairrejectsthecommon-mode input signal as long astwo transistors remain in active region-VEE(a)Microelectronic Circuits
Microelectronic Circuits 6.1 The BJT Differential Pair Basic Operation-1:Common-mode input ➢The differential pair with a commonmode input signal vCM. ➢Two transistors are matched. ➢Current source with infinite output resistance. ➢Current I divide equally between two transistors. ➢The difference in voltage between the two collector is zero. ➢The differential pair rejects the common-mode input signal as long as two transistors remain in active region
Basic Operation-2>The differential pair with aQIRCVCC“large" differential input signal.+IVoQ>Qi is on and Q2 is off.OftOn0.>Current I entirely flows in Qi+0.3VEE(b)Microelectronic Circuits
Microelectronic Circuits Basic Operation-2 ➢The differential pair with a “large” differential input signal. ➢Q1 is on and Q2 is off. ➢Current I entirely flows in Q1
Basic Operation-3Vcc>The differential pair with aVcaIRlarge differential input signal ofpolarity opposite to that in (b)QQ21VoOff>Q2 is on and Q, is off.OT10>Current I entirely flows in Q20VEE(c)Microelectronic Circuits
Microelectronic Circuits Basic Operation-3 ➢The differential pair with a large differential input signal of polarity opposite to that in (b). ➢Q2 is on and Q1 is off. ➢Current I entirely flows in Q2
Basic Operation-4:Difference-mode orDifference signalsIcC>The differential pair with asmall differential input signal V;RRlVeeRaAIRd>Small signal operation orRaVce=20AIRlinear amplifier.+AIRJAQ(small)>Assuming the bias currentsource I to be ideal and thus Iremains constant with thechange in VcM.>Increment in Q andVEEdecrement in Q2(d)Microelectronic Circuits
Microelectronic Circuits Basic Operation-4:Difference-mode or Difference signals ➢The differential pair with a small differential input signal vi . ➢Small signal operation or linear amplifier. ➢Assuming the bias current source I to be ideal and thus I remains constant with the change in vCM. ➢Increment in Q1 and decrement in Q2
Large-Signal Operation(VBI-V15e2E1α(VB2-VELinear regionIVTe~E2α1.01iElic2iclC11(VB2-VB1)+iE210.8/VTE11+e1iE20.6(VB1-VB2)ien+ie2/VT1+e0.4=I+ig2iEl0.21VEYH(VB2-VB1)/0VT-2028-8-6-44101+e106Vid1Normalized differential input voltage,E2(VB1-VB2)//VT1+eMicroelectronic Circuits
Microelectronic Circuits Large-Signal Operation ( ) ( ) ( ) ( ) ( ) ( ) T B B T B B T B B T B B T B E T B E V E v v V E v v E E V v v E E E V v v E E E V v v S E V v v S E e I i e I i i i I e i i i e i i i e I i e I i 1 2 2 1 1 2 2 1 2 1 1 1 1 1 1 1 2 1 1 2 1 2 2 1 2 1 2 1 − − − − − − + = + = + = + = + + = +