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Common emitter amplifier
What is IQ?
What is the gain?
Are the biasing resistor properly chosen?
Can we increase the gain to 200?
200
ac flows here
ac gain = -20k/200 = 100 (better answer later)
Look at where ac flows
Ebers Moll equation for transistor
Ic
I s (e
eVBE KT
Ic ~ I se
Ic
1)
eVBE KT
VBE
rE
dVBE
dI C
kT e
IC
If Ic is measured in mA:
rE
25
:
IC
e.g. Ic =1 mA, rE = 25 :
Not that small!
Ic ~ I se
eVBE KT
rE
25
:
IC
60 mV GVBE
per decade GIC
Current mirror
NPN
C
PNP
E
iC
B
iB
B
iB
E
VBE>0.6
IB: into B
Ic: into C
Ic = E IB
C
VEB>0.6
IB: out of B
Ic: out of C
Ic = E IB
iC
Internal resistance revisited
Rth
Vth
Rload
Vout = Vin
Rload
Rth + Rload
Ideal voltage source: Rth small or large?
Ideal current source: Rth infinite
10M:
10V
Rload
(a) Rload = 10: , I = ?, V = ?
(b) Rload = 1 k: , I = ? V = ?
(c) Rload = 10 M:, I = ? V = ?