common base (bjt) - common gate (mosfet) circuits
TRANSCRIPT
Sheet 1 of 3
Common-Base/Gate Circuits Common-Base BJT circuit The figure below shows the simplied ‘Pi’ model of a common-base BJT.
VoutVin
B
E C
rbe
rce
β.ib or gm.Vbe
ib(β+1)
ZoutRL
c
Vout
b
Vin
Zin
e
ib
VI e wher
gm1 r
1)(βi1).r(βi
IVR
T
CQbe
b
beb
IN
ININ ===
++
== gm
)rgm1 (as
VV
IV.
VI
rr
1)r(βr.β
1)r(βir.i.β
VVA ce
T
A
CQ
A
T
CQ
be
ce
be
ce
beb
ceb
IN
OUTV ===≈
+=
+==
( ) 1 1β
β )1βi
β.i I
I Ab
b
IN
OUTi ≈
+=
+==
Sheet 2 of 3
Vout
B
E C
rbe
rce
β.ib
ib
ZoutRL
c
Vout
b Vin
e
iT-ib
Vs
Rs
iT
iT+β.i
iT
To determine the Output impedance of the circuit we can connect a voltage source (Vs + Rs) to the base and ground the input ie the emitter. We then have to resistances in parallel connected to the current source β.ib.
( )
equation above into sub Rr
Ri i Also
r.iri.i V
iV R
sbe
sTb
bebcebTOUT
T
OUTOUT
+=
++=
=
β
sbe
bes
sbe
scece
T
OUTOUT
besce
sTce
sbe
sTTOUT
Rrr.R
RrRr.r
iVR
r.Rr
RirRr
Ri.i V
++
++==
++⎟⎟
⎠
⎞⎜⎜⎝
⎛+
+=
β
β
( ) cececeOUTs
ceOUTs
r.1 1r.r R then large R If
r R then 0 R If
+=++==
==
ββ
Sheet 3 of 3
Common-Gate MOSFET Circuit
Vin
s
gmVsg
g
d
rds
rdg
Io
Vout
RL
gate
source drain
Vout Vin
IIN = gmVsg
Vsg rIN
RL
Voltage Gain Av
( )
Rr.Rr
gm A //RrgmVsg V
Vsg V VV A
Ldg
LdgVLdgO
ININ
OV
⎟⎟⎠
⎞⎜⎜⎝
⎛
+==
==
Input Resistance
λI gm1
gmVgsVgs
IV R D
IN
ININ ====
Output Resistance As the source is low impedance ie close to ground for ROUT – rds appears to be connected across rds to ground.
dsdgO
OOUT //rr
IV R ==
Current Gain Ai Ai = 1