ESE 372 / Spring 2011 / Lecture 19 Common Base Biased by current source

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Ethelbert Stewart
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1 ESE 372 / Spring 2011 / Lecture 19 Common Base Biased by current source Output from Collector Start with bias DC analysis make sure BJT is in FA, then calculate small signal parameters for AC analysis. *ignore r O for simplicity, then: Input to Emitter Vin No need in bypass cap Small! noninverting Short circuit current gain The circuit is current buffer: delivers current from source to load * when BJT output impedance r O can not be neglected the circuit is said to perform an impedance transformation. 1

2 Common Collector amplifier Again. No need for C E! Bias current I E will determine g m, r π and r O Also R B and C C1 can be eliminated Redraw equivalent circuit in more convenient form 2

3 Common Collector amplifier + When i.e. no voltage gain! 3

4 Common Collector amplifier Input impedance Impedance transformation 4

5 Common Collector amplifier Output impedance Impedance transformation 5

6 Common Collector (Emitter follower) Biased by current source Input to Base Output from Emitter Start with bias DC analysis make sure BJT is in FA, then calculate small signal parameters for AC analysis. No need in bypass cap Often R B >> R S and r O >> R L, then Short circuit current gain is almost the same as in case of CE amp, namely β+1. Impedances transformed The circuit is voltage buffer: delivers voltage from source to load 6

7 Low frequencies, i.e. BJT itself is fast enough 1. DC bias make sure BJT is in FA - AC analysis. Before assumed coupling caps are big enough to act as a short circuit for any frequency of AC signal. Now assume they have finite values. 1. Assume C 1 is finite while C 2 and C E are still infinite. Depends on frequency. Frequency independent. 7

8 Role of the input coupling cap C 1 Depends on frequency. Voltage gain found before G V0 net voltage gain found before for infinite caps. Input voltage divider found before. High Pass Filter 8

9 Role of the output coupling cap C 2 2. Assume C 2 is finite while C 1 and C E are still infinite. G V0 net voltage gain found before for infinite caps. Again High Pass Filter but with 3dB frequency defined by C 2 9

10 Role of the bypass cap C E 3. Assume C E is finite while C 1 and C 2 are still infinite. 10

11 Low frequency We have identified three HPF. T f f f i f L1 L2 L3 jf fli 1 jf f 2 π C 1 2 π C 2 Li 1 R in 1 R CE 2 π β 1 out 1 r π R S R R S L R B C R R r π 1 in out C R 2 S R R S C E 1 μ ~ kohm L f L1 ~ 10kOhm R B 200Hz f ~ kohm L2 f 20Hz L3 khz Low frequency cutoff is determined by C E 11

12 Bandwidth High frequency 3dB determines amplifier bandwidth. Amplifier bandwidth is determined by BJT high frequency capabilities determined by internal parasitic capacitances C π and C μ. 12

13 Short circuit current gain at high frequencies Common emitter current gain defined earlier. Negligible since << β 0 for not extreme frequencies. 13

14 Short circuit current gain at high frequencies Unity gain bandwidth f T. Looks like it is supposed to improve with bias current because However it does not. Why? 14

15 Frequency dependence of common base current gain 3dB frequency for α is equal to f T. C BC V CB C 1 1 V V 2 CB BC0 BC-junction depletion region capacitance bi Hence at f T electrons from emitter can not reach collector. C BE V BE C 1 1 V V 2 BE BE0 EB-junction depletion region capacitance bi Base transport time time of flight of electrons from emitter to collector. *There are also several parasitic caps associated with technology limitations 15

16 Base transport time and associated diffusion capacitance time of flight of electrons from emitter to collector. *Need thin base for high speed operation Effective velocity of diffusion electrons 16

17 Base transport time and associated diffusion capacitance time of flight of electrons from emitter to collector. *Need thin base for high speed operation Electron charge stored in base when current IC is flowing Charge storage capacitance Pn-junction depletion region capacitances and other parasitic caps 17

18 Unity gain bandwidth Total time delay Minimum possible time delay Ultimate limit for BJT speed 18

Last time: BJT CE and CB amplifiers biased by current source

Last time: BJT CE and CB amplifiers biased by current source Assume FA regime, then VB VC V E I B I E, β 1 I Q C α I, V 0. 7V Calculate V CE and confirm it is > 0.2-0.3V, then BJT can be replaced with