Post on 13-Nov-2014
transcript
6.012 - Electronic Devices and CircuitsLecture 18 - Single Transistor Amplifier Stages - Outline
• AnnouncementsHandouts - Lecture Outline and Summary
Notes on Single Transistor AmplifiersExam 2 - Wednesday night, November 5, Room 10-250
Closed book; formula sheet provided; one crib sheet permitted• Review - Biasing and amplifier metrics
Current mirrors in emitter and source circuitsPerformance metrics: gains (voltage, current, power); input and output
resistances; power dissipation; bandwidth
• Mid-band analysisBiasing capacitors: short circuits above wLODevice capacitors: open circuits below wHIMidband: wLO < w < wHI
• Building-block stagesCommon emitter/source Common base/gateEmitter/source follower (also called common collector/drain)Series feedback (more commonly: emitter/source degeneration)Shunt feedback
Clif Fonstad, 11/03 Lecture 18 - Slide 1
LinearAmplifier
+ +
--vin
ioutiin
voutRest
ofcircuit
LinearAmplifier
+
-
itest
vtest
• Linear amplifier performance metrics:The characteristics of linear amplifiers that we use to compare
different amplifier designs, and to judge their performanceand suitability for a given application are given below:
Voltage gain, Av = vout/vinCurrent gain, Ai = iout/iinPower gain, Apower = Pout/Pin = voutiout /viniin = AvAiInput resistance, rin = vin/iin
Output resistance, rout = vtest/itest with vin = 0DC Power dissipation, PDC = (V+ - V-)(SIBIAS's)
Clif Fonstad, 11/03 Lecture 18 - Slide 2
• Linear equivalent circuits:pn diodes:
gd = q|ID|/kTCd = gdtd + Cdpl(VAB)
BJTs: (in FAR) gm = q|IC|/kT gp = gm/bFgo = |IC/VA| [or l |IC|]Cp = gmtb + Cdpl,be(VBE) [tb = wB
2/2De]Cm = Cdpl,bc(VBC)
MOSFETs: (in saturation) gm = K(VGS - VT) = (2K|ID|)1/2
gmb = hgm [h = {eSiqNA/2(|2fp| - VBS)}1/2/Cox
*]go = |ID/VA| [or l |ID|]Cgs = (2/3) WL Cox
*
Cgd: G-D fringing and overlap capacitance, all parasitic
Csb, Cgb, Cdb: depletion capacitances
Clif Fonstad, 11/03 Lecture 18 - Slide 3
+
-gp Cp
vp
b
e
Cm
gmvp go
e
c
+
- Cgs
vgs
g
s
Cgd
gmbvbs go
s
d
gmvgs
b
-
+vbs
CsbCdbCgb
+
-vds
gd Cd
b
a
• BJTs and MOSFETs biased for linear amplifier applications
IBIAS
-V
+V
IBIAS
-V
+V
IBIAS
-V
+V
IBIAS
-V
+V
npn pnp n-MOS p-MOS
Clif Fonstad, 11/03 Lecture 18 - Slide 4
V-
Q2 Q3
V+
RREFQ1
IC
• Examples of current mirror biased BJT circuits:
MOSFET MirrorIC ≈ (KQ3/KQ2) IREF
Clif Fonstad, 11/03 Lecture 18 - Slide 5
V-
V+
RREFQ1
Q2 Q3
IC
BJT MirrorIC ≈ (AQ3/AQ2) IREF
IBIAS
-V
+V
Above: Concept
Right: Implementations
Q1
A
BQ18
R1
Q5Q4Q3Q2
Q9
vIN2
+ 1.5 V
- 1.5 V
B
vIN1+-
Q6+-
Q7
Q19 B
Q12 Q13
Q21B B
Q11Q10Q8
Q20 Q22 Q24
Q15
Q16
Q17vOUT+-
B
A
Q14
Q23
R2 R3
• Looking at a complicated circuit: Lesson IFind the biasing circuitry and represent it symbolically
Consider the following example:
Clif Fonstad, 11/03 Lecture 18 - Slide 6
Circuitryprovidingthe VREF's
IBIAS1 IBIAS2 IBIAS3 IBIAS4
IBIAS5
IBIAS6
8 of the 24 transistors are "only" used for biasingthe other 16 transistors! If we get them out ofthe picture for awhile, the circuit looks simpler:
• Looking at a complicated circuit: Lesson I, cont.segregating out the biasing circuitry
Indicating the current sources symbolically lets youfocus on the action:
Clif Fonstad, 11/03 Lecture 18 - Slide 7
IBIAS1 IBIAS2 IBIAS3 IBIAS4IBIAS6
Q5Q4Q3Q2
Q9
vIN2
+ 1.5 V
- 1.5 V
vIN1+-
Q6+-
Q7Q12 Q13
Q11Q10Q8
Q15
Q16
Q17vOUT+-
Q14
R2 R3
IBIAS5
16 transistors left. In Lessons II and III we reduce thenumber to 5! Stay tuned…
• Three BJT single-transistor amplifiers
IBIAS
V-
V+
vout+
-vin+-
CE
CO
IBIAS
V-
V+
vout+
-
vIN
+
-
CO
CIIBIAS
V-
V+
vout+
-
vin+-
CO
vout
+
-vin+
-vout
+
-vin
+
-vout
+
-
vin
+
-
COMMON EMITTERInput: base
Output: collector Common: emitter
COMMON BASEInput: emitter
Output: collector Common: base
EMITT " " " "ER FOLLOWER " " " " "Input: base
Output: emitter Common: collector
Clif Fonstad, 11/03 Lecture 18 - Slide 8
vout
+
-vin+
-vout
+
-vin
+
-vout
+
-
vin
+
-
IBIAS
V-
V+
vout+
-vin+-
CE
CO
IBIAS
V-
V+
vout+
-
vin+-
CO
IBIAS
V-
V+
vout+
-
vIN
+
-
CO
CI
• Three MOSFET single-transistor amplifiers
COMMON SOURCEInput: gate
Output: drain Common: source
Substrate: to source
COMMON GATEInput: source; Output: drain
Common: gate; Substrate: to ground
SOURCE FOLLOWER " " " " "Input: gate
Output: source Common: drain
Substrate: to source
Clif Fonstad, 11/03 Lecture 18 - Slide 9
• Single-transistor amplifiers with feedback
Series feedback Shunt feedbackalso termed "emitter degeneration"
Clif Fonstad, 11/03 Lecture 18 - Slide 10
IBIAS
V-
V+
vout+
-vin+-
CE
CO
RF IBIAS
V-
V+
vout-vin
+-
CE
+CORF
vout
+
-vin+
-
RF
vout
+
-vin
+
-RF
IBIAS
V-
V+
vout+
-vin+-
CE
CO
• The "mid-band"concept: frequency range of constant gain and phase
Common emitter example:The linear equivalent circuit for the commonemitter amplifier stage on the left is drawnbelow with all of the elements included:
The capacitors are one of two types:Biasing capacitors: typically very large (in µF range)
(CO, CE, etc.) effectively shorts above some wLODevice capacitors: typically very small (in pF range) (Cp, Cm, etc.) effectively open until some wHI
Clif Fonstad, 11/03 Lecture 18 - Slide 11
gp
+
-vp
+
-
v in
v t+-
rt gmvp go
+
-
voutgLOAD
rIBIAS CE
COCm
Cp gnext
• The "mid-band"concept, cont.:At frequencies above some value (≡ wLO)The biasing capacitors look like shorts:
Clif Fonstad, 11/03 Lecture 18 - Slide 12
gp
+
-vp
+
-
v in
v t+-
rt gmvp go
+
-
voutgLOAD
rIBIAS CE
COCm
Cp gnext
At frequencies below some other value (≡ wHI)The parasitic capacitors look like open circuits:
gp
+
-vp
+
-
v in
v t+-
rt gmvp go
+
-
voutgLOAD
rIBIAS CE
COCm
Cp gnext
• The "mid-band"concept, cont.:If wLO < wHI, then there is a range where all of the capacitorsare either short circuits (the biasing capacitors) or opencircuits (the parasitics).
Clif Fonstad, 11/03 Lecture 18 - Slide 13
gp
+
-vp
+
-
v in
v t+-
rt gmvp go
+
-
voutgLOAD
rIBIAS CE
COCm
Cp gnext
We call the frequency range between wLO and wHI the "mid-band" range; for frequencies in this range our model issimply:
Valid for wLO < w< wHI, i.e. in the "mid-band" range.[where all bias capacitors are shorts and
all parasitic capacitors are open]
gl (= gLOAD+ gnext )
gp
+
-vp gmvp go
+
-v in
+
-voutv t
+-
rt
• Common emitter/source amplifiers
Commonemitter
IBIAS
V-
V+
vout+
-vin+-
CE
COMid-band LEC for common emitter
gl : conductance of "LOAD" andanything connected at "vout"
BJT MOSFETAv: -gm/(go + gl) -gm/(go + gl)
-gm(Ro||rl) -gm(Ro||rl)
Ai: -b gl/(go + gl) ∞@ -b
Rin: rp ∞
Rout: 1/go = ro 1/go = ro
A good workhorse gain stageClif Fonstad, 11/03 Lecture 18 - Slide 14
gp
+
-vp gmvp go gl
+
-v in
+
-voutv t
+-
rt
• Common base/gate amplifiers
Commongate
Mid-band LEC for common gategl : conductance of "LOAD" and anything
connected at "vout"The conductance of IBIAS can be neglected.
• A very low Rin, large Rout stage often used to complement other stages
IBIAS
V-
V+
vout+
-
vIN
+
-
CO
CIBJT MOSFET
Av: (gm+go)/(gl+go) (gm+gmb+go)/(gl+go)@ gm(rl||ro) @ (gm+gmb)(rl||ro)
Ai: (gm+go)/(gm+go+gp+gpgo/gl) 1@ 1
Rin: [gm+gp+go(gl-gm)/(gl+go)]-1 [gm+gmb+go(gl-gm-gmb)/(gl+go)]-1
@ 1/(gm+gp) = rp/(b+1) @ 1/(gm+gmb)Rout: ro[1 + (gm+go)/(gp+gt)] ro[1 + (gm+gmb+go)/gt]
@ (b+1)ro
Clif Fonstad, 11/03 Lecture 18 - Slide 15
(gm + gmb)vsg
go
+
-v in = v sg
gl
+
-voutv t
+-
rt
• Emitter/source followers
EmitterFollower
Mid-band LEC for emitter followergl : conductance of "IBIAS" andanything connected at "vout"
• A great outputbuffer stage withsmall Rout, big Rin
IBIAS
V-
V+
vout+
-
vin+-
CO
BJT MOSFET
Av: 1/[1 + (go+gl)/(gm+gp)] 1/[1 + (go+gl)/gm]@ 1 @ 1
Ai: b gl/(go+gl) ∞
Rin: 1/gp + (b+1)/(go+gl)= rp + (b+1) ro||rl ∞
Rout: [go+gl+(gm+gp)/(1 + gprt)]-1 [go+gl+gm]-1
@ (rt + rp)/(b+1) @ 1/gm
Clif Fonstad, 11/03 Lecture 18 - Slide 16
gp
+
-vp gmvp go
gl
+
-
v in
+
-vout
v t+-
rt
IBIAS
V-
V+
vout+
-vin+-
CE
CO
RF
• Series Feedback: emitter/source degeneration
Emitterdegeneration
Mid-band LEC emitter degenerationgl : conductance of "LOAD" and
anything connected at "vout"
BJT MOSFETAv: @ -rl/RF @ -rl/RF
Ai: @ b ∞
Rin: @ rp + (b+1)RF ∞
Rout: @ 1/go @ 1/go
Useful in discrete device circuit design; we use to understand common-mode gain suppression in differential amplifiers
Clif Fonstad, 11/03 Lecture 18 - Slide 17
gp
+
-vp gmvp go
gl
+
-
v in
+
-
voutv t+-
rt
RF
IBIAS
V-
V+
vout-vin
+-
CE
+CORF
• Feedback: shunt feedback elementShunt
feedback
Mid-band LEC for a shunted common-emittergl : conductance of "LOAD" and
anything connected at "vout"
BJT MOSFETAv: -(gm-GF)/(go+GF) -(gm-GF)/(go+GF)
@ -gmRF @ -gmRF
Ai: @ - gl/GF @ - gl/GF
Rin: 1/[gp +GF(1-Av)] RF/(1-Av) @ rp||RF/(1-Av)
Rout: @ (ro||RF) @ (ro||RF)
Used to stabilize high gain circuits and in transimpedance amplifiers; the same topology leads to the Miller effect "(Lec. 24).
Clif Fonstad, 11/03 Lecture 18 - Slide 18
gp
+
-vp gmvp go gl
+
-v in
+
-voutv t
+-
rt RF
• Summary of the stages (bipolar)
Clif Fonstad, 11/03 Lecture 18 - Slide 19
†
Voltagegain, Av
Currentgain, Ai
Inputresistance, Ri
Outputresistance, Ro
Common emitter -gm
go + gl[ ]= -gmrl '( ) -
b gl
go + gl[ ]rp ro =
1go
Ê
Ë Á
ˆ
¯ ˜
Common base gm
go + gl[ ]= gmrl '( ) ª1 ª
rp
b +1[ ]ª b +1[ ] ro
Emitter follower gm + gp[ ]gm + gp + go + gl[ ]
ª1 bgl
go + gl[ ]ª b rp + b +1[ ]rl ' ª
rt + rp
b +1[ ]Emitter degeneration
(series feedback) ª - rl
RF
ª b ª rp + b +1[ ]RF ª ro
Shunt feedback -gm - GF[ ]go + GF[ ]
ª -gmRF -gl
GF
1gp + GF 1- Av[ ]
ro RF =1
go + GF[ ]
Ê
Ë Á
ˆ
¯ ˜
6.012 - Electronic Devices and CircuitsLecture 18 - Single Transistor Amplifier Stages - Summary
• Mid-band analysisBiasing capacitors: typically in mF range
should/can be avoided completely in modern IC design (wLO = 0)Device capacitors: typically in pF range; goal is to make as small as possibleMidband: no capacitors in incremental analysis; gain and phase constant
want as wide as possible (we won't find wLO and wHI until Lec. 22)
• Building-block stagesCommon emitter/source: good voltage and current gain
large Rin and Rout good gain stage
Common base/gate: very small Rin; very large Rout unity current gain; good voltage gain
will find paired with other stages to form "cascode"Emitter/source follower: very small Rout; very large Rin
unity voltage gain; good current gain an excellent output stage or buffer
Series feedback: moderate voltage gain dependant on ratio of resistors
Shunt feedback: used in transimpedance amplifiersClif Fonstad, 11/03 Lecture 18 - Slide 20