Physics 160 Lecture 6

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
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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.
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Physics 160
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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
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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.
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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
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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.
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Physics 160
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Standard 5% Resistor Values
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Standard 10% Capacitor Values
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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?
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Exercise 2.5
AC Coupled Input
Note that Zin is now dominated by R1
and is therefore ~27kohms.
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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
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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%.
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Emitter-Follower Example (Single Supply)
What are the input and output impedances of this amp?
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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.
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