ESE 372 / Spring 2013 / Lecture 14 Last time: Common Emitter Amplifier biased by current source.
Bias
Q * But make sure that BJT is in FA, i.e. VCE > 0.2-0.3V
Equivalent circuit based on Hybrid-π model, caps are short circuits for signals
1 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source.
Voltage gains
MAX Finite output resistance puts upper limit on amplifier AV0 gm rO voltage gain
2 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source.
Input/output impedances
Not high enough - problem
Net voltage gain
Depends on β – not too good 3 ESE 372 / Spring 2013 / Lecture 14 Common Emitter Amplifier biased by current source.
Short circuit current gain
Common emitter current gain
4 ESE 372 / Spring 2013 / Lecture 14 Example
npn - BJT with β 100 and VA 100V.
Bias current IE 1mA, VCC VEE 10V
R B 100k, R C 8k, R S 5k, R L 5k
1. Check if BJT is in FA regime I 1mA I E 10μ B β 1 101
VB 10μ 100k 1V VE 1V 0.7V 1.7V 10V.
VC 10V 1m8k 2V VCE 2V 1.7V 3.7V 0.3V 2. Calculate small signal parameters IQ 1mA mA 3. Find voltage gain g C 40 m R in R L Vth 25mV V G A V R R V0 R R β 100 in S L out rπ 2.5kΩ V gm 40mA V A g R || r 296 V0 m C O V VA 100V rO Q 100kΩ R in R B || r 2.4k, R out R C || rO 7.4k IC 1mA V V G V 0.32 296 0.4 38 V V
30mV p-p signal corresponds to about 10mV p-p variation of VBE. This produces about 1.2V p-p across load. 5 ESE 372 / Spring 2013 / Lecture 14 Example – output voltage swing
8 V
3.5 V
-1.5 To remain in FA
VCE > 0.2 V
Maximum amplitude of the undistorted sine wave at the output is limited by maximum negative voltage swing, i.e. by 3.5 V.
For 1mA bias current and 10 V power supply what could we do to increase the amplitude of undistorted sine wave?
What is the maximum value of this amplitude?
6 ESE 372 / Spring 2013 / Lecture 14
CE amp biased by current source and with RE.
Case 1
R E 0 R in R B || r r AV0 gm R C || rO
Case 2 (neglect rO)
R E 0
R in R B || r R E1 r R C AV0 r R E 1
Negative feedback resistor RE improves input impedance at the expense of gain
7 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier.
No need for CE !
Equivalent circuit for AC analysis
8 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier.
Redraw equivalent circuit in more convenient
form and neglect rO for beginning.
9 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier.
Voltage gain
Noninverting amplifier with open circuit voltage gain value similar to that of CE amp
Short circuit current gain
i.e. no current gain ! 10 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier.
Input impedance
Very small – PROBLEM for voltage amplifier
Output impedance
11 ESE 372 / Spring 2013 / Lecture 14 Common Base amplifier.
Net voltage gain
Could we guess this without analysis? Observe current buffer action of CB amp
Equivalent circuit of amp with current gain equal to one. 12 ESE 372 / Spring 2013 / Lecture 14 CB amplifier with BJT having finite output small signal resistance.
Voltage gain
13 ESE 372 / Spring 2013 / Lecture 14 CB amplifier with BJT having finite output small signal resistance.
Input impedance
Impedance When transformation
max
14 ESE 372 / Spring 2013 / Lecture 14 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
15 ESE 372 / Spring 2013 / Lecture 14 Common Collector amplifier
+
When
i.e. no voltage gain ! 16