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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25
Lecture 25: CS, CD and CG circuits
Prof J. S. Smith
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Context
In today’s lecture, we will discuss the various ways that a transistor can be used as a component in active linear circuits, to amplify, act an impedance buffer, and so on.
These simple circuits will then be used as building blocks for building more complex circuits.
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Reading
For about a week, we will continue with chapter 8 in the text, single stage amplifiers. We will focus primarily on FET circuits, Common Source(CS) , Common Gate (CG), and Common Drain(CD), sections: 8.1,8.3, 8.4, 8.5, 8.8, 8.9
Following single transistor configurations, we will start discussing multistage and cascaded circuits, which are in chapter 9 of the text.
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Lecture Outline
MOS Common Source AmpCurrent Source Active LoadCommon Gate AmpCommon Drain Amp
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Common-Source Amplifier
Isolate DC level
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Configurations (CS)
Since the transistor is a three terminal device, we can make a two port device (input and output) but one of the terminals is going to have to be used for both the input and the output ports. The terminology follows…Common Source (CS)
– Provides current and voltage gain– Example: an NMOS device with the source grounded,
the input being a voltage between the gate and ground, and the output being a voltage between the drain and ground.
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CD
Common Drain (CD): high impedance input and low impedance output. Low voltage gainExample: An NMOS transistor with its Drain at +rail, with an input applied between the gate and (ground/+rail, the same for small signal analysis), and the output taken from the source with reference to ground.Example: A PMOS transistor with its Drain at ground, the input applied between the gate and ground, and the output taken from the source with reference to ground
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG
Common Gate, provides a low impedance input, and a high impedance output. (converts a weak current source to a strong current source)The gate is held at small signal ground, while the input is applied at the source, and the output is taken at the drain.Low current gain
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Common source
In the common source amplifier, the input is used to modulate the GS voltage, and the DS voltage is the output, giving a voltage gain depending on the load line:
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Load-Line Analysis to find Q
Q
D
DD outR
D
V VIR−
=
110k
slope =
0V10kDI =
5V10kDI =
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Small-Signal Analysis
inR = ∞
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
sv
sR
inRoutR LRm inG v
inv+
−
Two-Port Parameters:
Find Rin, Rout, Gm
inR = ∞
m mG g= ||out o DR r R=
Generic Transconductance Amp
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Two-Port CS Model
Reattach source and load one-ports:
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Maximize Gain of CS Amp
Increase the gm (more current)Increase RD (free? Don’t need to dissipate extra power)Limit: Must keep the device in saturation
For a fixed current, the load resistor can only be chosen so largeTo have good swing we’d also like to avoid getting to close to saturation
||v m D oA g R r= −
,DS DD D D DS satV V I R V= − >
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Current Source Supply
Solution: Use a current source!Current independent of voltage for ideal source
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CS Amp with Current Source Supply
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Load Line for DC Biasing
Both the I-source and the transistor are idealized for DC bias analysis
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Two-Port Parameters
From currentsource supply
inR = ∞
||out o ocR r r=
m mG g=
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
P-Channel CS Amplifier
DC bias: VSG = VDD – VBIAS sets drain current –IDp = ISUP
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Two-Port Model Parameters
Small-signal model for PMOS and for rest of circuit
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Common Gate Amplifier
DC bias:
SUP BIAS DSI I I= =
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG as a Current Amplifier: Find Ai
out d ti i i= = −
1iA = −
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG Input Resistance
At input:
Output voltage:
t outt m gs mb t
o
v vi g v g vr
⎛ ⎞−= − + + ⎜ ⎟
⎝ ⎠( || ) ( || )out d oc L t oc Lv i r R i r R= − =
gs tv v= −
( )||t oc L tt m t mb t
o
v r R ii g v g v
r⎛ ⎞−
= + + ⎜ ⎟⎝ ⎠
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Approximations…
We have this messy result
But we don’t need that much precision. Let’s start approximating:
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||1
m mbt o
oc Lin t
o
g gi r
r RR vr
+ += =
+
1m mb
o
g gr
+ >> ||oc L Lr R R≈ 0L
o
Rr
≈
1in
m mb
Rg g
=+
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG Output Resistance
( ) 0s s tm gs mb s
S o
v v vg v g vR r
−− − − + =
1 1 ts m mb
S o o
vv g gR r r
⎛ ⎞+ + + =⎜ ⎟
⎝ ⎠
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG Output Resistance
Substituting vs = itRS
1 1 tt S m mb
S o o
vi R g gR r r
⎛ ⎞+ + + =⎜ ⎟
⎝ ⎠
The output resistance is (vt / it)|| roc
|| 1oout oc S m o mb o
S
rR r R g r g rR
⎛ ⎞⎛ ⎞= + + +⎜ ⎟⎜ ⎟⎜ ⎟⎝ ⎠⎝ ⎠
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Approximating the CG Rout
The exact result is complicated, so let’s try tomake it simpler:
Sgm µ500≈ Sgmb µ50≈ Ω≈ kro 200
][|| SSombSomoocout RRrgRrgrrR +++=
][|| SSomoocout RRrgrrR ++≅
Assuming the source resistance is less than ro,
)]1([||][|| SmoocSomoocout RgrrRrgrrR +=+≈
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CG Two-Port Model
Function: a current buffer• Low Input Impedance• High Output Impedance
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
Common-Drain Amplifier
21 ( )2DS ox GS T
WI C V VL
µ= −
2 DSGS T
ox
IV V WCL
µ= +
Weak IDS dependence
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CD Voltage Gain
Note vgs = vt – vout||
outm gs mb out
oc o
v g v g vr r
= −
( )||
outm t out mb out
oc o
v g v v g vr r
= − −
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CD Voltage Gain (Cont.)
KCL at source node:
Voltage gain (for vSB not zero):
( )||
outm t out mb out
oc o
v g v v g vr r
= − −
1|| mb m out m t
oc o
g g v g vr r
⎛ ⎞+ + =⎜ ⎟
⎝ ⎠
1||
out m
inmb m
oc o
v gv g g
r r
=+ +
1out m
in mb m
v gv g g
≈ ≈+
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CD Output Resistance
Sum currents at output (source) node:
|| || tout o oc
t
vR r ri
= t m t mb ti g v g v= +
1out
m mb
Rg g
≈+
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Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith
CD Output Resistance (Cont.)
ro || roc is much larger than the inverses of the transconductances ignore
1out
m mb
Rg g
≈+
Function: a voltage buffer• High Input Impedance• Low Output Impedance
Department of EECS University of California, Berkeley
EECS 105 Spring 2004, Lecture 25 Prof. J. S. Smith