ESE 372 / Spring 2013 / Lecture 15
Last time: BJT CE and CB amplifiers biased by current source
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 Common Emitter
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 Common Base 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 output impedance 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 amplifier 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.
jf fLi Ti f 1 jf 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?
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