Lecture 23. OUTLINE BJT Differential Amplifiers (cont d) Reading: Chapter

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1 Lectue 23 OUTLINE BJT Diffeential Amplifies (cont d) ascode diffeential amplifies ommon mode ejection Diffeential pai with active load eading: hapte EE05 Sping 2008 Lectue 23, Slide Pof. Wu, U Bekeley

2 ascode Diffeential Pai out out = [ + g g m3 m3 ( O π 3 )] O3 + O ( O π 3 ) O 3 + O π 3 π 3 Half cicuit fo ac analysis A [ g ( ) ] v = gm out gm m3 O π 3 O3 + O π EE05 Sping 2008 Lectue 23, Slide 2 Pof. Wu, U Bekeley 3

3 Telescopic ascode Diffeential Pai Half cicuit fo ac analysis A v g [ g ( )] [ g ( )] π 3 m5 O5 O7 5 m m3 O3 O π EE05 Sping 2008 Lectue 23, Slide 3 Pof. Wu, U Bekeley

4 EE05 Sping 2008 Lectue 23, Slide 4 Pof. Wu, U Bekeley Example [ ] ( ) op O O m m v O O O m op g g A g ) ( π π π = + + = Half cicuit fo ac analysis

5 Effect of Finite Tail Impedance If the tail cuent souce is not ideal, then when an input common mode voltage is applied, the cuents in Q and Q 2 and hence the output common mode voltage will change. ΔV ΔV out, M in, M = 2g ( / 2) m + EE = g m + 2 EE ommon-mode gain should be small EE05 Sping 2008 Lectue 23, Slide 5 Pof. Wu, U Bekeley

6 Effect of Input M Noise Ideal Tail uent Thee is no effect of the input M noise at the output. EE05 Sping 2008 Lectue 23, Slide 6 Pof. Wu, U Bekeley

7 Effect of Input M Noise Non Ideal Tail uent The single ended outputs ae coupted by the input M noise. I TAIL = I + EE V P EE I TAIL 2 I TAIL Tail cuent, I TAIL, now changes with V P, and V P is affected by V M EE05 Sping 2008 Lectue 23, Slide 7 Pof. Wu, U Bekeley

8 ompaison Ideal Tail uent Non-Ideal Tail uent The diffeential output voltage signal is the same fo both cases. Fo small input M noise, the diffeential pai is not affected. EE05 Sping 2008 Lectue 23, Slide 8 Pof. Wu, U Bekeley

9 M to DM onvesion; gain A M DM ΔV If finite tail impedance and asymmety (e.g. in load esistance) ae both pesent, then the diffeential output signal will contain a potion of the input common mode signal. M ΔI = ΔV = BE ΔVM + 2 g m + 2ΔI EE EE ΔI = g m + 2ΔI EE ΔI ΔI ΔV ΔV ( / gm ) + 2EE EE05 Sping 2008 Lectue 23, Slide 9 Pof. Wu, U Bekeley ΔV ΔV ΔV out out 2 out = ΔV out M = ΔI = ΔI = out ( + Δ ) ΔV out 2 Δ = ΔI Δ

10 Example A M DM = g m Δ { g } + 2 [ + ( )] + m3 π3 O3 π3 EE05 Sping 2008 Lectue 23, Slide 0 Pof. Wu, U Bekeley

11 ommon Mode ejection atio M is the atio of the wanted amplified diffeential input signal to the unwanted conveted input common mode noise that appeas at the output. M A A DM M DM EE05 Sping 2008 Lectue 23, Slide Pof. Wu, U Bekeley

12 Diffeential to Single Ended onvesion Many cicuits equie a diffeential to single ended convesion. This topology is not vey good; its most citical dawback is supply noise couption, since no common mode cancellation mechanism exists. Also, we lose half of the voltage signal. EE05 Sping 2008 Lectue 23, Slide 2 Pof. Wu, U Bekeley

13 A Bette Altenative This cicuit topology pefoms diffeential to single ended convesion with no loss of gain. v v out v in in2 = ( ),, g m o NPN o PNP EE05 Sping 2008 Lectue 23, Slide 3 Pof. Wu, U Bekeley

14 Active Load With a cuent mio as the load, the signal cuent poduced by Q can be eplicated onto Q 4. This type of load is diffeent fom the conventional static load and is called an active load. EE05 Sping 2008 Lectue 23, Slide 4 Pof. Wu, U Bekeley

15 Diffeential Pai with Active Load The input diffeential pai deceases the cuent dawn fom L by ΔI, and the active load pushes an exta ΔI into L by cuent mio action; these effects enhance each othe. EE05 Sping 2008 Lectue 23, Slide 5 Pof. Wu, U Bekeley

16 Active Load vs. Static Load The load in the cicuit on the left esponds to the input signal and enhances the single ended output, wheeas the load in the cicuit on the ight does not. EE05 Sping 2008 Lectue 23, Slide 6 Pof. Wu, U Bekeley

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ECEN326: Electronic Circuits Fall 2017 ECEN36: Electonic Cicuits Fall 07 Lectue 4: Cascode Stages and Cuent Mios Sam Palemo Analog & Mixed-Signal Cente Texas A&M Univesity Announcements HW3 due 0/4 Exam 0/9 9:0-0:0 (0 exta minutes) Closed book