The Common Emittor Amplifier

1
The Common Emittor Amplifier
The DC setup is nearly the same as the
emitter-follower, the only addition is the
resistor RC.
We will worry about a time Varying input voltage
vb.
VB is supplied
This will lead to a time varying Output signal, vc
We will consider time varying effects vb, ib,
vc, ic, ve, ie
2
The Common Emitter Amplifier
How is VE related to VB?
VE VB (0.6 to 0.7) V VB 0.65 V
How is VC related to VCC?
VC VCC IC x RC
How are Is related?
IE IB IC IC bIB IE IC
How are vb and ve related?
ve vb
ie ve/RE vb/RE vbieRE
What is ic in terms of ie?
vc -ic Rc -ie RC
ic ie
vc/vb
-ieRC/ieRE -RC/RE
GAIN
3
The Common Emitter Amplifier
Practical Considerations
Resitance re into emitter
RE looks like RE re
G - RC/(REre)
What is the lowest value of VEve?
so VBvb-0.65gt0
0V
Transistor turns off, vc0
How large can VBvb get?
Technically, to within 0.1V of VCC
Practically, Gainvb is within .1V of VCC
What quiet value of VC maximizes the range for
vc?
VC ½ VCC
4
The Common Emitter Amplifier
What is the output Impedance?
RC (resistance into collector)
What is Resistance into collector?
HINT Current Source
DV/DI infinity
Zout RC Large Value
What about input Impedance?
Rin (R1R2b(REre))
Controlled by R2
5
Frequency Filtering with Capacitors
C1 forms a HighPass Filter with R?
What is R? ?
Through the base b RE
To Ground R2
To VCC R1
C1
R? R1 R2 bRE
Dominated by smallest of the three!
6
Frequency Filtering with Capacitors
C2 Forms a High Pass Filter with R?
What is R? ?
Just the Load, RL
How small can RL be?
RE RL RL gt RE