Source (Emitter) Followers and Source (Emitter) Degeneration

1
Source (Emitter) Followers and Source (Emitter)
Degeneration

• Dr. Paul Hasler

2
Follower Circuits
Vdd
Output Voltage Bias 3.0V
Vin
Vin
Vout
Vout
R1
R1
GND
GND
3
Basic Resistive Load Circuits
Vdd
Output Voltage Bias 3.0V
Vin
Vin
Vout
Vout
Iref (3V) / R1
R1
R1
GND
GND
BJT / Subthreshold VT
Above Threshold (Vd gt Vg - VT )
(3V) / R1 (K/2) (Vin - Vout - VT )2
(3V) / R1 Ico e(Vin - Vout)/UT
Vin Vout VT sqrt((6V)/(KR1))
Vin Vout UT ln ( (3V) / R1 Ico )
4
Small-Signal Modeling
V3
V3
I
I
V1
V1
V2
V2
gm
ro
rp
Av
BJT
(UT b) / I
I / UT
VA / I
VA / UT
Above VT MOSFET
2I /(V1-V2 -VT)
VA / I
2VA/(V1-V2 -VT)
?
Sub VT MOSFET
kI / UT
?
VA / I
kVA / UT
5
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
Compute Transconductance (gm)
6
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
Compute Transconductance (gm)
BJT / Subthreshold VT
gm I / UT (3V) / (R1 UT)
7
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
Compute Transconductance (gm)
BJT / Subthreshold VT
Above Threshold (Vdd gt Vin - VT )
gm I / UT (3V) / (R1 UT)
8
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
Compute Transconductance (gm)
BJT / Subthreshold VT
Above Threshold (Vdd gt Vin - VT )
gm 2I /(Vin 3V - VT) (6V) / (R1 (Vin -
3V- VT) )
gm I / UT (3V) / (R1 UT)
9
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout
Vin
rp
R1
gmV
10
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
(Vin - Vout ) / rp (Vin - Vout ) gm
Vout / R1
Vout
Vin
rp
R1
gmV
11
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
(Vin - Vout ) / rp (Vin - Vout ) gm
Vout / R1
Vout
Vin
rp
R1
gmV
(Vin-Vout )(1 rp gm) Vout (rp / R1)
Vout/Vin 1/(1 (rp / R1)/(1 rp gm))
12
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout / Vin 1 / (1 (rp / R1) / (1 rp
gm))
Vout
Vin
rp
R1
gmV
13
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout / Vin 1 / (1 (rp / R1) / (1 rp
gm))
Vout
Vin
rp
R1
rp gm b (large)
gmV
Vout / Vin 1 / ( 1 1 / (R1 gm) )
14
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout / Vin 1 / (1 1 / (R1 gm))
Vout
Vin
rp
R1
gmV
15
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout / Vin 1 / (1 1 / (R1 gm))
Vout
Vin
rp
Vout / Vin 1 / (1 UT/(3V))
R1
gmV
or
Vout / Vin 1 / (1 Vin-3V-VT /(3V))
16
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout/Vin 1/(1 UT/(3V))
or
Vout
Vin
rp
Vout/Vin 1/(1Vin-3V-VT /(3V))
R1
gmV
17
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
V -
Vout/Vin 1/(1 UT/(3V))
or
Vout
Vin
rp
Vout/Vin 1/(1Vin-3V-VT /(3V))
R1
gmV
Output Resistance Short the input to GND
18
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
Vout/Vin 1/(1 UT/(3V))
Vout
or
Vout/Vin 1/(1Vin-3V-VT /(3V))
1/gm
R1
rp
19
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
Vout/Vin 1/(1 UT/(3V))
Vout
or
Vout/Vin 1/(1Vin-3V-VT /(3V))
1/gm
R1
rp
20
Small-Signal Model Common Drain
Output Voltage Bias 3.0V
Iref (3V) / R1
Have bias Vin
gm (3V) / (R1 UT)
or
gm (6V) / (R1 (Vin-3V-VT) )
Vout/Vin 1/(1 UT/(3V))
Vout
or
Vout/Vin 1/(1Vin-3V-VT /(3V))
1/gm
R1
rp
Rout (1/gm) / (1 gm R1) 1/gm
21
Source Degeneration
Vdrain
Vin
Vin
Va
Va
R1
R1
GND
GND
22
Small-Signal Model Common Drain
Vdd
V -
Vin
Vin
Vout
Vin
Vout
rp
Vout
R1
gmV
R1
R1
GND
GND
Vout / Vin 1 / ( 1 1 / (R1 gm) ) R1 gm /
(1 R1 gm)
23
Small-Signal Model Common Drain
Vdd
V -
Vin
Vin
Vout
Vin
Vout
rp
Vout
R1
gmV
R1
R1
GND
GND
Vout / Vin 1 / ( 1 1 / (R1 gm) ) R1 gm /
(1 R1 gm)
R1 ltlt 1/gm
Vout / Vin (R1 gm)
(Resistor has a small effect)
24
Small-Signal Model Common Drain
Vdd
V -
Vin
Vin
Vout
Vin
Vout
rp
Vout
R1
gmV
R1
R1
GND
GND
Vout / Vin 1 / ( 1 1 / (R1 gm) ) R1 gm /
(1 R1 gm)
R1 ltlt 1/gm
R1 gtgt 1/gm
Vout / Vin (R1 gm)
Vout / Vin 1
(Resistor has a small effect)
(Resistor sets gm)
25
Source Degeneration
Vdrain
Vdrain
ro
Vin
V -
Vin
Va
Vin
Va
Va
rp
R1
R1
R1
gmV
Vdrain
GND
GND
R1 gm
Gm gmV gm(Vin - Va ) gm(1 -
) Vin
1 R1gm
26
Source Degeneration
Vdrain
Vdrain
ro
Vin
V -
Vin
Va
Vin
Va
Va
rp
R1
R1
R1
gmV
Vdrain
GND
GND
R1 gm
Gm gmV gm(Vin - Va ) gm(1 -
) Vin
1 R1gm
(ignore ro here)
27
Source Degeneration
Vdrain
Vdrain
ro
Vin
V -
Vin
Va
Vin
Va
Va
rp
R1
R1
R1
gmV
Vdrain
GND
GND
R1 gm
Gm gmV gm(Vin - Va ) gm(1 -
) Vin
1 R1gm
(ignore ro here)
28
Source Degeneration
Vdrain
Vdrain
ro
Vin
V -
Vin
Va
Vin
Va
Va
rp
R1
R1
R1
gmV
Vdrain
GND
GND
R1 gm
Gm gmV gm(Vin - Va ) gm(1 -
) Vin
1 R1gm
(ignore ro here)
gmVin
29
Source Degeneration
Vdrain
Vdrain
ro
Vin
V -
Vin
Va
Vin
Va
Va
rp
R1
R1
R1
gmV
Vdrain
GND
GND
R1 gm
Gm gmV gm(Vin - Va ) gm(1 -
) Vin
1 R1gm
(ignore ro here)
gmVin
Vin /R
30
Source Degeneration
Vdrain
Vin
Vin
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout
31
Source Degeneration
Vdrain
Vin
Vin
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout
32
Source Degeneration
Vdrain
Vdrain
Vdrain
gmVa
ro
Vin
Vin
Va
Va
Va
rp // R1
R1
R1
GND
GND
Gm 1 / R1
Solve for Va Va / (rp // R1) gmVa (Vdrain -
Va)/ ro
Rout
33
Source Degeneration
Vdrain
Vdrain
Vdrain
gmVa
ro
Vin
Vin
Va
Va
Va
rp // R1
R1
R1
GND
GND
Gm 1 / R1
Solve for Va Va / (rp // R1) gmVa (Vdrain -
Va)/ ro
Rout
34
Source Degeneration
Vdrain
Vdrain
Vdrain
gmVa
ro
Vin
Vin
Va
Va
Va
rp // R1
R1
R1
GND
GND
Gm 1 / R1
Solve for Va Va / (rp // R1) gmVa (Vdrain -
Va)/ ro
Rout
Va ro (gm (1/(rp // R1)) ) Vdrain
35
Source Degeneration
Vdrain
Vdrain
Vdrain
gmVa
ro
Vin
Vin
Va
Va
Va
rp // R1
R1
R1
GND
GND
Gm 1 / R1
Solve for Va Va / (rp // R1) gmVa (Vdrain -
Va)/ ro
Rout
Va ro (gm (1/(rp // R1)) ) Vdrain
Va Vdrain /rogm
36
Source Degeneration
Vdrain
Vdrain
Vdrain
gmVa
ro
Vin
Vin
Va
Va
Va
rp // R1
R1
R1
GND
GND
Va Vdrain /rogm
Gm 1 / R1
Solve for Current I Va / (rp // R1)
Rout
I Vdrain /rogm(rp // R1)
Rout rogm(rp // R1)
37
Source Degeneration
Rin (conductance is zero for a MOSFET)
Vdrain
Vin
Vin
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
38
Source Degeneration
Rin (conductance is zero for a MOSFET)
Vdrain
Vin
Vin
Reflect R1 through the base
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
39
Source Degeneration
Rin (conductance is zero for a MOSFET)
Vdrain
Vin
Vin
Reflect R1 through the base
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
40
Source Degeneration
Rin (conductance is zero for a MOSFET)
Vdrain
Vin
Vin
Reflect R1 through the base
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
Rin bR1 rp b R1(1 (1/(gmR1) ) )
41
Source Degeneration
Rin (conductance is zero for a MOSFET)
Vdrain
Vin
Vin
Reflect R1 through the base
Va
Va
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
Rin bR1 rp b R1(1 (1/(gmR1) ) )
Rin b R1
42
Source Degeneration
Vdrain
Vin
Vin
Vin
Vdrain
V -
Va
Va
GmV
Rin
Rout
R1
R1
GND
GND
Gm 1 / R1
Rout rogm(rp // R1)
Rin b R1
43
Source Degeneration
Vdrain
Vin
Vin
Vin
Vdrain
V -
Va
Va
GmV
Rin
Rout
R1
R1
GND
GND
Gm 1 / R1
Voltage Gain Gm Rout rogm(1 // (R1/rp ) )
Rout rogm(rp // R1)
Rin b R1