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Physics 160
Lecture 6
R. Johnson
April 15, 2015
NPN Transistor Basic “Rules”
•
Collector
+
IC
•
Base
•
IB

The collector is more positive than the emitter
(by at least a few tenths of a volt at
“saturation,”
saturation, but usually much more).
The base-emitter junction is forward biased,
with the base about 1 diode drop (~0.6 to
0.7 V) higher than the emitter during normal
operation (for currents of a few mA).
The base-collector junction is normally
reverse biased during operation.
IE
E itt
Emitter
•
Since most of the electrons injected into the
base go to the collector
collector, not the emitter
emitter, then
I C  I B
with
ith  >>1,
1 ttypically
i ll ~50
50 tto ~250.
250
April 15, 2015
Physics 160
2
PNP Transistor Basic “Rules”
•
Emitter
+
IE
•
•
Base
The collector is less positive than the emitter
(by at least a few tenths of a volt at
“saturation,”
saturation, but usually much more).
The base-emitter junction is forward biased,
with the base about 1 diode drop (~0.6 V)
less than the emitter during normal operation.
The base-collector junction is normally
reverse biased during operation.
IB

IC
Collector
•
Since most of the holes injected into the base
go to the collector, not the emitter, then
I C  I B
with  >>1, typically ~50 to ~250.
April 15, 2015
Physics 160
3
NPN Emitter Follower
•
Vout=Vin minus 1 diode drop
•
Current
C
rrent gain
gain; no voltage
oltage gain
– Hi input impedance
– Low output impedance
– Power gain!
Vsupply
Input
NPN
Output
• Bias voltage and current
– IE=VE/R, typically a few mA in our circuits
– IC IE
– IB IE/100 Allowance must be made to
provide this small base current!
Of course,
course an emitter follower can also be made with
a PNP transistor.
April 15, 2015
Physics 160
4
Emitter Follower
•
•
Unity voltage gain
Current and power gain by 
•
Impedance buffer:
– Hi input impedance
– Low output
p impedance
p
 1
1 
Z in    


 RE RL 
scope
1
 1
1 

Z out  

 RE Rs  
1

1

Rs
What are the DC bias currents and voltages in this circuit?
Note that 2 supplies are used here, with the input source referenced
to ground
ground, between the supplies
supplies. The input source must supply the
bias current to the transistor base.
April 15, 2015
Physics 160
5
Emitter-Follower Simulation
• The base-emitter voltage is 0.68 V
• The emitter current is the emitter voltage
minus 15V divided by 5kohm.
• The collector current is very close to the
emitter current.
• The collector current is 176 times greater
than the base current (i
(i.e.
e 
=176)
176)
The gain is not quite
unity but is very close,
until it falls toward zero
at very high frequency
frequency.
Transistor
stops working
at very high
frequency
Z in  5000  176  880k
880
 0.999
881
April 15, 2015
Physics 160
6
Simulation with Very Large Source Impedance
Here the base current causes a
large voltage drop across the
source resistance.
Thi correspondingly
This
di l llowers th
the
emitter voltage, hence reducing
the emitter bias current.
• The voltage
g g
gain is only
y
about ½ now!
• This shows that the input
impedance of the emitter
follower is about 880kohm,,
resulting in a 50/50 voltage
division with the source
impedance.
• In fact the input impedance
should be RE=880 kohm.
April 15, 2015
Physics 160
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Exercise 2.5 (Page 71)
Design an emitter follower with 15 V supplies to operate over
the audio range (20 Hz to 20 kHz). Use 5 mA quiescent current
and capacitive input coupling.
April 15, 2015
Physics 160
8
Standard 5% Resistor Values
April 15, 2015
Physics 160
9
Standard 10% Capacitor Values
April 15, 2015
Physics 160
10
AC Coupled Source
Exercise 2.5
Offset due to base bias
current flowing through R1
AC
coupled
scope
• A resistor must be used to provide the base bias.
• Too large a resistor (R1) will make the bias voltage highly
d
dependent
d t on th
the b
beta
t off th
the transistor
t
i t (BAD design)!
d i )!
• Too small a resistor (R1) will make the input impedance of the
amplifier too low (BAD, especially for a source follower whose
g input
p impedance/low
p
output
p impedance).
p
)
raison-d’être is high
• What are the input and output impedances of the above amplifier?
April 15, 2015
Physics 160
11
Exercise 2.5
AC Coupled Input
Note that Zin is now dominated by R1
and is therefore ~27kohms.
April 15, 2015
Physics 160
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Offsetting the Input from Ground
This way we
need only a
single supply
supply.
15V
2.5 mA
~7 5V
~7.5V
f>100 Hz
•
•
•
•
•
Choose Vout to be biased to about one half of VCC.
Choose
C
oose RE to
og
give
e the
e des
desired
ed b
bias
as cu
current.
e
Current in the divider should be >> IB.
But if R1 and R2 are too small, the input impedance will be low.
C Rin must be large enough not to attenuate the lowest
C·R
frequencies of interest.
April 15, 2015
Physics 160
13
Emmitter-Follower Exercise
Design an emmitter-follower with a single 15 V supply and A/C
input coupling, to operate in the frequency range above 100 Hz.
The output should be biased at roughly half of the supply
voltage. Assume a 1 kohm source impedance. Design to a
2.5 mA quiescent current. Check that the gain with the stated
source impedance is at least 95%.
April 15, 2015
Physics 160
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Emitter-Follower Example (Single Supply)
What are the input and output impedances of this amp?
April 15, 2015
Physics 160
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AC Small-Signal Voltage Gain
The gain is less than
unity because of the
voltage division
between the source
impedance and the
parallel combination
of the bias resistors.
April 15, 2015
Physics 160
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