ESE 372 / Spring 2013 / Lecture 15

Last time: BJT CE and CB biased by

Assume FA regime, then

I I  E , IQ  α  I , V  0.7V B β 1 C E BE

VC Calculate V and confirm it is > 0.2-0.3V, VB CE then BJT can be replaced with model for VE small signal analysis

1 ESE 372 / Spring 2013 / Lecture 15

Biased by current source and without RE

Output from Collector Input to itiinverting Base

Vin

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 -gmVin. 2 ESE 372 / Spring 2013 / Lecture 15 Biased by current source Start with bias DC analysis – make sure BJT is in FA, Output from then calculate small signal parameters for AC analysis. Collector *ignore rO for simplicity, then: noninverting Input to Emitter Vin No need in bypass cap Small !

Short circuit current gain

The circuit is current * when BJT rO buffer: delivers current can not be neglected – the circuit from source to load is said to perform an impedance transformation.

3 ESE 372 / Spring 2013 / Lecture 15 Common Collector Biased by current source Input to Start with bias DC analysis – make sure BJT is in FA, then calculate small signal parameters for AC analysis. Base Output from Emitter

No need in bypass cap

4 ESE 372 / Spring 2013 / Lecture 15 Common Collector Again.

No need for CE ! Bias current IE will determine gm, rπ and rO

Also R and C can be eliminated B C1 Redraw equivalent circuit in more convenient form

5 ESE 372 / Spring 2013 / Lecture 15 Common Collector amplifier

+

When

i.e. no voltage gain ! 6 ESE 372 / Spring 2013 / Lecture 15 Common Collector amplifier

Input impedance

Impedance transformation 7 ESE 372 / Spring 2013 / Lecture 15 Common Collector amplifier

Output impedance

Impedance transformation 8 ESE 372 / Spring 2013 / Lecture 15 Common Collector Amplifier = Emitter follower

Input to Start with bias DC analysis – make sure BJT is in FA, then calculate small signal parameters for AC analysis. Base Output from Emitter

Often R >> R and r >> R , then No need in B S O L bypass cap

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 ESE 372 / Spring 2013 / Lecture 15 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 ffACilfrequency of AC signal. Now – assume they have finite values.

1. Assume C1 is finite while C2 and CE are still infinite.

Depends on frequency.

Frequency independent. 10 ESE 372 / Spring 2013 / Lecture 15 Frequency response of Common Emitter amplifier

Role of the input coupling cap C1

Depends on frequency.

Voltage gain found before

Input voltage divider High found before. Pass GV0 – Filter net voltage gain found before for infinite caps.

11 ESE 372 / Spring 2013 / Lecture 15 Frequency response of Common Emitter amplifier

Role of the output coupling cap C2

2. Assume C2 is finite while C1 and CE are still infinite.

Again High Pass Filter but with

3dB frequency defined by C2

GV0 – net voltage gain found before for infinite caps.

12 ESE 372 / Spring 2013 / Lecture 15 Frequency response of Common Emitter amplifier

Role of the bypass cap CE

3. Assume CE is finite while C1 and C2 are still infinite.

13 ESE 372 / Spring 2013 / Lecture 15 Low Frequency response of Common Emitter amplifier

We have identified three HPF.

jf fLi Ti f  1 jf fLi C1  C2  CE 1μμ 1 f  R  R ~ kOhm  f 100Hz L1 2 π C R  R in S L1 1 in S R  R ~ 10kOhm  f  20Hz 1 out L L2 fL2  rπ  R S || R B ~ kOhm  fL3  kHz 2 π C2 R out  R L 1 fL3  LfLow frequency cut tffidtidbCoff is determined by C C E 2 π  E r  R || R β 1 π S B 14 ESE 372 / Spring 2013 / Lecture 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 Cπ and Cμ. 15 ESE 372 / Spring 2013 / Lecture 15 Short circuit current gain at high frequencies

Common emitter current gain defined earlier.

Negligible since << β0 for not extreme frequencies.

16 ESE 372 / Spring 2013 / Lecture 15 Short circuit current gain at high frequencies

Unity gain bandwidth fT.

Looks like it is supposed to iithbitbimprove with bias current because

However it does not. Why?

18