Lect.26: BJT amplifiers(1)

Lect. 26: BJT amplifiers- MOS amplifiers: Common-Source, Common-Gate, Source-Follower (Common-Drain)- BJT amplifiers: Common-Emitter, Common-Base, Emitter-Follower (Common-Collector)Common-Emitter AmplifierRio m π o m i oo∴ Aυo= = −viorv =− g vr=−g vrR=π= rovg rm oElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersFrequency response for CEOpen circuit time constant methodfτH≅12πτH( 1 )= C R + C ⎡ g R R R ⎤⎣+ +⎦+ C R' ' ' ' 'H π sig µm L sig L L Lf Hfor CE is reduced by Miller effectElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersCommon-Base Amplifierv sig v BE i C v OElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersInput Resistance for CBElectronic Circuits 1Riirobinυi=iυi= ii−rieυiυi= =r ( β + 1) re∴ i = i −υro i iv = i ⋅ r + i ⋅Ri ro o o Lυi=( i − ) r + ( i −υrπ) Rri o i i LeroRLvi(1 + + ) = ii( ro + RL)r reυi ro + RL ro + RL∴ Rin= = = rei rioRLRL1 + + re+ ro+r rβ + 1If r is very large, R ro in eeIf R is very large, Rin re( β + 1) = r πLππππHigh-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersOpen-circuit voltage gain for CBυi υi υii = , i = , i = β = gr r re b c m ieππ∴ v = v + g υ ⋅rAvoo i m i ovo= = 1+g rvim oυElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersOutput Resistance for CBv = ( i + g v)r + vx x m ov v 1 1= ix − , v( + ) = ix, v = ix⋅( Re|| rπ)R r R ree∴ v = i ⋅ r + g r i ( R || r ) + i ⋅( R || r )x x o m o x e π x evxRout = = ro [1 + gm ( Re || rπ)] + Re|| rix β roππππElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersBJT Cascodev i v Q2,E v oElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersR = r ( R for CE)inπ 1inR β r ( R for CB)out o outElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersAmplifier equivalent circuit modelVinR inA voR outV outCE r o1v in-g m1 r o1 v inr π1r π2CBβr o2Cascode open-circuit voltage gainv outg m2r o2v in2v = g r vout m2 o2 in2rv = ( −g r v )π 2in2 m1 o1inrπ2+ ro1r∴ v = g r ( −g r v )π 2out m2 o2 m1 o1inrπ2+ ro1rg r ( − g r v )π 2m2 o2 m1 o1inro1Assuming g = g , r = rvA = −βg rm1 m2 o1 o2outvo m1 o1vinElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersFrequency response of BJT cascodefτH1≅2πτH= C R + C R + C R + ( C + C )( R || R )H π1 π 1 µ 1 µ 1 π 2 c1 L µ 2 L outwhere R = R || rπ1 sig π1R = ( R || r )(1 + g R ) + Rµ 1 sig π1 m1 c1 c1⎡⎢R = r || ⎢r⎢⎢⎣c1 o1 e2⎤ro2+ R ⎥L⎥RLro2+ ⎥β2+ 1 ⎥⎦Using parameters given in Exercise 6.20 and 6.28,CE has f H= 75kHz, Cascode has f H=469KHz.Electronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersCommon-Collector or Emitter FollowerElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersCommon-Collector or Emitter Followeri bβi b( 1)[ ( || )] ( 1)( || )R = β + r + r R β + r Rin e o L o L( β + 1) roAvo = 1 (If β ro > Rsig)( β + 1)( r + r ) + Ro e sigElectronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersCommon-Collector or Emitter Followeri bβi b||[ ]RsigRsigRout = ro + reβ + 1 β + 1Electronic Circuits 1High-Speed Circuits and Systems Laboratory

Lect. 26: BJT amplifiersEmitter-Follower Frequency ResponsefH= 12π⎡⎣C R + C R'Rsig+ RL=RsigR1 + +r r'( β 1) Rµ = Rsig ⎡⎣rπ+ + RRππµ µ π π'Le⎤⎦L⎤⎦Electronic Circuits 1High-Speed Circuits and Systems Laboratory