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

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1 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 > V, then BJT can be replaced with model for small signal analysis E BE 1

2 Common Emitter Biased by current source and without R E Input to Base Output from Collector inverting V in Must have bypass cap to get AC gain. The circuit can have both voltage and current gains: Thevenin form, can be replaced with Norton form with current source -g m V in. 2

3 Input to Emitter Vin Short circuit current gain Common Base Biased by current source Output from Collector No need in bypass cap 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: Small! noninverting 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. 3

4 Input to Base Common Collector Biased by current source 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 4

5 Again. No need for C E! Common Collector amplifier 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 5

6 Common Collector amplifier + When i.e. no voltage gain! 6

7 Common Collector amplifier Input impedance Impedance transformation 7

8 Common Collector amplifier Output impedance Impedance transformation 8

9 Common Collector Amplifier = Emitter follower 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 9

10 Frequency response of Common Emitter amplifier 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. 10

11 Frequency response of Common Emitter amplifier 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 11

12 Frequency response of Common Emitter amplifier 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 12

13 Frequency response of Common Emitter amplifier Role of the bypass cap C E 3. Assume C E is finite while C 1 and C 2 are still infinite. 13

14 Low Frequency response of Common Emitter amplifier 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 C out 2 R R S R S C L E 1μμ ~ kohm f R L1 ~ 10kOhm f B ~ kohm f 100Hz L2 L3 20Hz khz Low frequency cutoff is determined by C E 14

15 Bandwidth of Common Emitter amplifier 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. 15

16 Short circuit current gain at high frequencies Common emitter current gain defined earlier. Negligible since << β 0 for not extreme frequencies. 16

17 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? 18

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

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.