COMMON DRAIN AMPLIFIER.
7 FEBRUARY 2006
COMMON DRAIN AMPLIFIER.
The output voltage is
Vout = Vin − Vgs
Vout = Vin − VT −
(1)
v
u
u Ibias
u
tµ C W
n
ox
(2)
2L
VT = f (VBS )
(3)
SMALL SIGNAL EQUIVALENT.
vgs = vin − vout
vbs = −vout
vds = −vout
(4)
(5)
(6)
(7)
Writing KCL at the source node
gm (vin − vout ) − gmbs vout − gds vout = 0
gm
vout
=
vin
gm + gmbs + gds
☛ Therefore gain is less than one.
☛ Typically gmbs varies from
gm
4
to
1
gm
2
(8)
(9)
Vdd
Vout
Vin
Figure 1: COMMON DRAIN AMPLIFIER
Vin
Vout
Figure 2: VARIATION OF Vout WITH Vin
vin
G
gmvgs
gmbsvbs
gds
S
vout
Figure 3: SMALL SIGNAL EQUIVALENT
2
G
-gm Vtest
-gmbs
Vtest
gds
S
Itest
Vtest
Figure 4: EQUIVALENT CIRCUIT TO FIND OUTPUT RESISTANCE
INPUT AND OUTPUT RESISTANCE.
Input resistance
Rin = ∞
(10)
Output resistance
To find the output resistance we apply a test voltage V test and find
the current Itest through it.
Itest = gm Vtest + gmbs Vtest + gds Vtest
Gout = gm + gmbs + gds
(11)
(12)
☛ It is mostly used as a buffer.
HIGH FREQUENCY ANALYSIS.
For High frequency analysis we have to include the effect of C gs,Cgd ,Csb
and Cdb
The transfer function is obtained as
Vout
gm + sCgs
=
Vin
gm + gmbs + gds + s(Cgs + Csb)
INPUT AND OUTPUT ADMITTANCE.
3
(13)
Vin
Iin
G
Cgd
Cdb
D
gm Vgs
Cgs
gmbsVbs
gds
S
Vout
Csb
Figure 5: HIGH FREQUENCY EQUIVALENT CIRCUIT
Vout
Vin
Cgs
Cgs +Csb
gm
gm+gmbs+gds
gm
Cgs
gm+ gmbs+ gds
Cgs + Csb
Figure 6: BODE PLOT FOR GAIN
4
Yin
-40db/decade
-20db/decade
gm
Cgs+Csb
ω
Figure 7: BODE PLOT OF OUTPUT ADMITTANCE
Output admittance
Yout = gm + gmbs + gds + s(Cgs + Csb)
(14)
Input admittance
Iin = (Vin − Vout )sCgs
gmbs + gds + sCsb
Iin = Vin
sCgs
gm + gmbs + gds + s(Cgs + Csb)
(15)
(16)
Approximating,
Iin
Yin
s2 CsbCgs
= Vin
gm + s(Cgs + Csb)
s2 CsbCgs
=
gm + s(Cgs + Csb )
(17)
(18)
☛ This shows it acts as a negative resistance and can be used in
oscillators
5