lecture 27lecture 27 amplifier configurations reading: ce...
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Lecture 27Lecture 27
Amplifier Configurations
Reading:
CE/CS: Jaeger 13.6, 13.9, 13.10, 13.11
CC/CD: Jaeger 14.1, 14.3
CB/CG: Jaeger 14 1 14 4CB/CG: Jaeger 14.1, 14.4
and Notes
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Amplifier Configurations
Voltage Amplifier: Voltage input and Voltage output
ifier
Inpu
t si
stan
ce
rce
Inpu
t si
stan
ce
Res
ista
nce
fier
Out
put
sist
ance
Am
pli
Res
Sour Res
Loa
d R
Am
plif
Res
Source LoadAmplifier•Any signal source has a finite “source resistance”, RS .•The amplifier is often asked to drive current into a load of finite
The controlled source is a Voltage-controlled-Voltage Source
A O Ci i V l G i b f d b l i
impedance, RL (examples: 8 ohm speaker, 50 ohm transmission line, etc…)
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Av=Open Circuit Voltage Gain can be found by applying a voltage source with Rs=0, and measuring the open circuit output
voltage(no load or RL=infinity)
Amplifier Configurations
Why is the input and output resistance important?
•Only the voltage vin is amplified to Avvin.
•Since Rs and Rin form a voltage divider that determines vin, you want Rin as large as possible (for a voltage amplifier) for maximum voltage gain.large as possible (for a voltage amplifier) for maximum voltage gain.
•Since RL and Rout form a voltage divider that determines vout, you want Routas small as possible (for a voltage amplifier) for maximum voltage gain.
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Amplifier Configurations
Current Amplifier: Current input and Current output
The controlled source is a Current-controlled-Current SourceThe controlled source is a Current controlled Current Source
Ai=Short Circuit Current Gain can be found by applying a current source with Rs= infinity, and measuring the short circuit output
current (No Load or R 0)current (No Load or RL=0)
•Only the current iin is amplified to Aiiin.•Since Rs and Rin form a current divider that determines iin, you want Rin as small as
ibl (f t lifi ) f i t i
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possible (for a current amplifier) for maximum current gain.•Since RL and Rout form a current divider that determines iout, you want Rout as large as possible (for a current amplifier) for maximum current gain.
Amplifier Configurations
Transconductance Amplifier: Voltage input and Current output
The controlled source is a Voltage-controlled-Current Source
Gm=Transconductance Gain can be found by applying a voltage source with Rs=0, and measuring the short circuit output current
(No Load or RL=0)(No Load or RL 0)
•Only the voltage vin is amplified to iout=Gmvin.•Since Rs and Rin form a voltage divider that determines vin, you want Rin as large as possible for maximum transconductance gain
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possible for maximum transconductance gain.•Since RL and Rout form a current divider that determines iout, you want Rout as large as possible for maximum transconductance gain.
Amplifier Configurations
Transresistance Amplifier: Current input and Voltage output
The controlled source is a Current-controlled-Voltage Source
Rm=Transresistance Gain can be found by applying a current source with Rs=infinity, and measuring the open circuit output
voltage (RL=infinity)voltage (RL infinity)•Only the current iin is amplified to vout=Rmiin•Since Rs and Rin form a current divider that determines iin, you want Rin as small as possible for maximum transresistance gain.
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p g•Since RL and Rout form a voltage divider that determines vout, you want Rout as small as possible for maximum transresistance gain.
Amplifier Configurations
Input Resistance
With the load resistance attached…
l i l d h i /iApply a test input voltage and measure the input current, Rin=vt/it
Or
Apply a test input current and measure the input voltage R v /iApply a test input current and measure the input voltage, Rin= vt/it
Output Resistance
With all input voltage sources shorted and all input current sources opened…
Apply a test voltage to the output and measure the output current, Rout=vt/it
Or
Apply a test current to the output and measure the output voltage, Rout= vt/it
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Final Summary of Transistor Amplifier Analysis1.) a.) Determine DC operating point. Make sure the transistors are biased into active mode (forward active for BJTs and Saturation for MOSFET. Do not (confuse the two terms as saturation means a completely different thing for a BJT)and b.) calculate small signal parameters gm, r, ro etc…2.) Convert to the AC only model.
•DC Voltage sources are replaced with shorts to ground•DC Current sources are replaced with open circuits•Large capacitors are replaced with short circuits•Large inductors are replaced with open circuitsLarge inductors are replaced with open circuits
3.) Use a Thevenin circuit where necessary on each leg of transistor
4.) Replace transistor with small signal model5.) Simplify the circuit as much as necessary and solve for gain.6.) Solve for Input Resistance: With the load resistance attached… a.) Apply a test input voltage and measure the input current, Rin=vt/it or b.) Apply a test input current and measure the input voltage R = v /imeasure the input voltage, Rin= vt/it
7.) Solve for Output Resistance: With all input voltage sources shorted and all input current sources opened… a.) Apply a test voltage to the output and measure the output current R =v /i or b ) Apply a test current to the output and measure the output
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current, Rout=vt/it or b.) Apply a test current to the output and measure the output voltage, Rout= vt/it
Transistor Amplifier Configurations
Common Emitter and Common Source
Res
ista
nce
t nce
Res
ista
nce
t nce
Inpu
t R
Out
put
Res
ista
n
Inpu
t R
Out
put
Res
ista
n
O ll A lifi C fi ti
Can be modeled as a current amplifier, IC=IB, or a transconductance amplifier , iC=KvBE
Modeled as transconductance amplifier, iDS=KvGS
Overall Amplifier Configuration•Emitter/Source is neither an input nor an output•Input is between base-emitter or gate-sourceO t t i b t ll t itt d d i
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•Output is between collector-emitter and drain-source•Is a transconductance amplifier (see small signal models we have used in previous examples)
Transistor Amplifier Configurations
Common Emitter and Common SourcePortion due to bias circuitry
Portion due to transistor
Portion due to bias circuitry
Portion due to transistor
r gmvor gmvgsR
in r Rc
Rou
t
r is replaced with an open circuit for the MOSFET casecircuit for the MOSFET case
Previously, we have analyzed voltage gain. Now let us look at the amplifier input and output resistance (these are small signal
MOSFET for the 1||2BJT for the ||1||2 RRRinorrRRRin
parameters):
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MOSFET BJT for the || orRrRout co
Transistor Amplifier Configurations
Summary of Common Emitter and Common Source Characteristics
•Very Large Voltage Gain These propertiesVery Large Voltage Gain
•Inverting Voltage Gain (due to –gmro)
•High Input Impedance
These properties make the CE/CS configuration very good for high gain•High Input Impedance
•High Output Impedance
good for high gain stages of amplifiers.
Now let us consider the other two configurations of transistor amplifiers:transistor amplifiers:
•Common Gate/Common Base
C D i /C C ll t
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•Common Drain/Common Collector
Transistor Amplifier ConfigurationsCommon Collector and Common Drain
DC Ci itDC Circuit
B
E
CG
S
D
E
Collector (or Drain) is neither an input or output
Input is Base (or Gate)
Output is Emitter (or Source)
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p ( )
Transistor Amplifier ConfigurationsCommon Collector
DC Ci it t d t AC E i l t ( d d)DC Circuit converted to AC Equivalent (reduced)
DCDC Circuit
Note the extra ground due to C2
ACib CB
AC Circuit
ib ror
E
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Transistor Amplifier ConfigurationsCommon Collector
DC Ci it t d t AC E i l t ( d d)DC Circuit converted to AC Equivalent (reduced)
AC ib
ib
rr
CB
Circuit b ror
E
ib CB
AC Circuit (reduced)
ib
b
r
C
E ibE
vo
7||||41 RrRiv
b
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7||||41
7||||41RrRrRiv
RrRiv
oothbth
oboo
Transistor Amplifier ConfigurationsCommon CollectorAC V lt G iAC Voltage Gain
RRvv
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h
oothbth
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VV
RgRgA
Lm
Lmv 1
1
1Rg and RsR1||R2for But Lm
Gain is positive and ~1
Transistor Amplifier ConfigurationsCommon Drain Conversion from DC to AC Equivalent Circuit
DC Ci iCircuit
AC Circuit
ro
DG
R7gmvGS
CircuitS
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Transistor Amplifier ConfigurationsCommon Emitter and Common Source
DC Ci it t d t AC E i l t ( d d)DC Circuit converted to AC Equivalent (reduced)
AC DG
g vCircuit
ro
S
gmvGSR7
DG
AC Circuit (reduced)
DG
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Transistor Amplifier ConfigurationsCommon Drain AC Voltage Gain
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omGSoGSmGSth
oGSmo
7||||417||||47||||4
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vv
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11||2||
L
VV
RgRg
A Lmv 1
1
1RgandRsR1||R2for But Lm
Gain is positive and ~1
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Rg Lm1
Transistor Amplifier ConfigurationsCommon Collector/Drain Input Resistance
Input Resistance: With the load resistance attached apply a test input voltage and measure the input current, Rin=vx/i (where i=ix)
x Rrv
R 1 LoBJTin Rri
R 1,
Resistance in the emitter circuit is “multiplied” by transistor to increase the input resistance
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0,xx
MOSFETinv
iv
R
Transistor Amplifier ConfigurationsCommon Collector Output Resistance
ix
RL
Output Resistance: With all input voltage sources shorted and all input current sources opened, apply a test voltage to the output and measure the
Rv
Rrv
Rrv
rvi
Rrv
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L
x
th
xo
th
x
o
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x
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current sources opened, apply a test voltage to the output and measure the output current, Rout=vx/ix
4||1
1
11
, RrRwhere
RRrivR oL
Lth
x
xBJTout
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1o
Two resistors in parallel: RL, and Resistance in the base circuit is “multiplied” by transistor to decrease the output resistance
Transistor Amplifier ConfigurationsCommon Drain Output Resistance
gmvGS
ix
RL
Output Resistance: With all input voltage sources shorted and all input current sources opened apply a test voltage to the output and measure the
Rvvg
rvvgi x
xmx
GSmx
current sources opened, apply a test voltage to the output and measure the output current, Rout=vx/i
4||11
, RrRwhere
Rgi
vR
Rr
oL
Lm
x
xMOSFETout
Lo
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L
Two resistors in parallel: RL and inverse transconductance
Transistor Amplifier Configurations
Summary of Common Collector and Common Drain CharacteristicsSummary of Common Collector and Common Drain Characteristics
Th ti k th•Unity Voltage Gain
•Non-Inverting Voltage Gain
These properties make the CC/CD configuration very good for impedance transformation I ENon Inverting Voltage Gain
•Very High Input Impedance
L O t t I d
transformation, I.E. “buffering” high impedances to low impedances. CC/CD •Low Output Impedance peda ces. CC/Cconfigurations are good for output stages of amplifiers due to their very low output impedance, I.E., very little voltage drop in the output
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resistance of the amp.
Transistor Amplifier ConfigurationsCommon Collector/Drain Current Gain
Rth
ith
th
vth (o+1)ithi
oBJTi iiA , 1
th
MOSFETi
th
iiA
i
,
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th
Note: since R7 was originally defined as the load, the current gain should actually be (+1) (R4||ro)/(R4||ro+R7) using a current divider.
Transistor Amplifier ConfigurationsCommon Base and Common Gate
DC Ci itDC Circuit
Base (or Gate) is neither an input or output
Input is Emitter (or Source)
Output is Collector (or Drain)
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p ( )
Transistor Amplifier ConfigurationsCommon Base
DC Ci it t d t AC E i l t ( d d)DC Circuit converted to AC Equivalent (reduced)
DC Circuit
Note: Jaeger let’s ro go to infinity which
AC Circuit
g v ror
CBmakes the math dramatically easier
vo
gmv or
E
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Transistor Amplifier ConfigurationsCommon Base AC Equivalent (reduced)
AC vo
gmv ror
CBv
Circuit o
E
v
rogmvror gmv
v
vo
AC Circuit ( d d) r
v
VRvV 866704
(reduced)
ECE 3040 - Dr. Alan DoolittleGeorgia TechKRRR
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vV
sth
ss
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73.14||
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Transistor Amplifier ConfigurationsCommon Base Voltage Gain
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Transistor Amplifier ConfigurationsCommon Base Voltage Gain
1
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m
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Transistor Amplifier ConfigurationsCommon Base Voltage Gain
ththm
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gR
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arg110
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mth
Lmv gR
RgA
1Gain is positive and can be large
Transistor Amplifier ConfigurationsCommon Base Input Resistance
rogmv
iC
i
r
vvx
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-ix=iE
vx v vxx
Input Resistance: With the load resistance attached apply a test input l d h i R /ivoltage and measure the input current, Rin=vx/i
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L
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Transistor Amplifier ConfigurationsCommon Base Input Resistance
r
vr
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Input Resistance is very small!
ECE 3040 - Dr. Alan DoolittleGeorgia Tech
rro Input Resistance is very small!
Transistor Amplifier ConfigurationsCommon Base Output Resistance
v
Replace RL by a voltage source, vx
ror gmv
v
iCi
oib rorv
iB
iix
vx
vr
RthiE
ve
Result follows exactly after discussion in Jaeger pages 668-670 and 683-684
ECE 3040 - Dr. Alan DoolittleGeorgia Tech
Result follows exactly after discussion in Jaeger, pages 668-670, and 683-684.
Transistor Amplifier ConfigurationsCommon Base Output Resistance
eobox
erx
vriivvv
i rr
iCiB
v
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thxthxe Rr
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but
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rRr
ri
R
,
rge
rger
ECE 3040 - Dr. Alan DoolittleGeorgia Tech Very
Lar
Eve
n L
ar Output Resistance is HUGE!
Transistor Amplifier ConfigurationsCommon Base Current Gain
1 ol
i ii
A
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thi
Transistor Amplifier ConfigurationsCommon Gate Solution
The Common Gate solution can be found by recognizing that the following translations can be made in our small signal model:signal model:
r
oo 1
11
,,
thm
LmMOSFETv
ththm
LmBJTv Rg
RgA
rR
Rg
RgA
mMOSFETin
m
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rg
R 11
1,,
ththmoMOSFEToutth
tho
th
thmo
th
tho
th
thooBJTout RRgrR
RrRr
rRr
Rrgr
RrRr
rRr
RrR
1,,
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11 ,, oBJTioBJTi AA
Transistor Amplifier Configurations
Summary of Common Base and Common Gate CharacteristicsSummary of Common Base and Common Gate CharacteristicsThe input and output impedances are the opposite of what is
•High Voltage Gain
•Non-Inverting Voltage Gain
opposite of what is typically needed for a voltage amplifier. Thus, Common Emitter/Source Non Inverting Voltage Gain
•Very Low Input Impedance
V Hi h O t t I d
amplifiers are normally used instead of Common Base/Gate. The input and
•Very High Output Impedance output impedances are useful for current amplifiers but the current
i i t b t it Thgain is at best unity. Thus a current buffer is one useful application for the Common Base/Gate
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Common Base/Gate
Multistage Amplifier Configurations
You can combine or Cascade configurations to produce “High Performance” amplifiersYou can combine or Cascade configurations to produce High Performance amplifiers with High input impedance, low output impedance and huge voltage gains.
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Multistage Amplifier Configurations
For AC-Coupled amplifiers (capacitors between stages) the DC solution reduces to threeFor AC-Coupled amplifiers (capacitors between stages), the DC solution reduces to three parallel and independent circuits!
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Multistage Amplifier Configurations
For AC-Coupled amplifiers (capacitors between stages) the AC solution reduces to threeFor AC-Coupled amplifiers (capacitors between stages), the AC solution reduces to three circuits, each of which has a load dependent on the input resistance of the next stage! Continued….
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Multistage Amplifier Configurations
Continued (For AC-Coupled amplifiers (capacitors between stages) the AC solutionContinued….(For AC-Coupled amplifiers (capacitors between stages), the AC solution reduces to three circuits, each of which has a load dependent on the input resistance of the next stage!)
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Multistage Amplifier ConfigurationsContinued...
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ECE 3040 - Dr. Alan DoolittleGeorgia Tech
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Multistage Amplifier Configurations
AC-Coupled amplifiers (capacitors between stages) have one major limitation They doAC-Coupled amplifiers (capacitors between stages), have one major limitation. They do not amplify low frequencies or DC voltages. To accomplish this, we must DC-Couplethe stages as shown. Note: for this to be a DC coupled amp, C1 and C6 should also be replaced as shorts.
Since the bias here is usually (2/3)V (V =15V in this example) it is easier to use a
ECE 3040 - Dr. Alan DoolittleGeorgia Tech
Since the bias here is usually ~(2/3)Vcc (Vcc =15V in this example) , it is easier to use a PNP for the second stage so that VEB+IERE2 ~ (2/3)Vcc