Lecture 7b
EE-215 Electronic Devices and Circuits
Dr. Muhammad Anis Chaudhary
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
• is the most widely used MOS amplifier configuration
• Note that, here the biasing arrangement is not shown (the circuit in fig is the ac equivalent
circuit)
• the signal source has an internal resistance of Rsig and open circuit voltage of vsig
• let’s analyze this circuit to determine Rin , Avo , Ro and Gv
• Note that RD is part of the amplifier circuit, and when a load resistance RL is connected
– to the amplifier output, it appears in parallel with RD
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
• replacing MOSFET with its hybrid-π model
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 1 of 19
•
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
• this equivalent circuit can be used to determine the characteristic parameters (Rin , Avo , Ro ) of
the amplifier
• from the fig
– as gate current is zero =⇒ input resistance = Rin = ∞
vo
• by KCL =⇒ gm vgs + ro ∥R D
=0
vo
– vo = −gm vgs (ro ∥ RD ) =⇒ vgs = −gm (ro ∥ RD )
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 2 of 19
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
•
vo
• vgs = −gm (ro ∥ RD )
vo vo
• here vi = vgs =⇒ Avo = vi R =∞ = vgs R =∞ = −gm (ro ∥ RD )
L L
• if the channel length modulation can be ignored ro → ∞ (ro ≫ RD )
– =⇒ Avo ≈ −gm RD
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
• Now the output resistance Ro is the resistance seen looking into the output terminal with vi set
to zero
– vi = 0 =⇒ vgs = 0 =⇒ gm vgs = 0
– =⇒ Ro = RD ∥ ro
• thus for CS amplfier
– Rin = ∞ , Ro = RD ∥ ro , Avo = −gm (RD ∥ ro )
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 3 of 19
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier
• now the overall voltage gain Gv can be determined as follows
vo
– Gv = vsig
• As Rin = ∞ =⇒ vsig = vi = vgs
vo vo
• =⇒ Gv = vsig = vi = Av
• Note that Av , Gv are calculated with load resistor RL connected =⇒ vo = (−gm vgs ) (RD ∥ ro ∥ RL )
vo vo
– vgs = −gm (RD ∥ ro ∥ RL ) = vi = Gv = Av ∵vi = vgs
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
• here we have placed a resistance Rs in the source lead of the common-source amplifier
• Assuming λ = 0 i.e. ignoring channel length modulation
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 4 of 19
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
• the small-signal equivalent circuit can be drawn as (for λ = 0)
• here T-model is preferred over the π model, because it makes the analysis somewhat simple
• In general, whenever a resistance is connected in the source lead, the T model is preferred.
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
• As a consequence of the T-model, Rs appears in series with the resistance 1/gm
• as gate current is zero =⇒ Rin = ∞ =⇒ vi = vsig
• here only a fraction of vi appears between gate and source as vgs
(1/gm ) 1
• by voltage divider =⇒ vgs = (1/gm )+Rs vi = 1+gm Rs vi
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 5 of 19
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
•
(1/gm ) 1
• vgs = (1/gm )+Rs vi = 1+gm Rs vi
– thus the value of Rs can be used to control the magnitude of the signal vgs
– and thereby ensure that vgs doesnot become too large to cause
– unacceptably high non-linear distortion
• Another benefit is that Rs extends the useful bandwidth of the amplifier
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
• by ohm’s law,
– vo = −iRD
• As 1/gm and Rs are in series, the current i in the source lead can be given as
– i = 1/gmvi+Rs = 1+ggmm Rs vi
gm
• =⇒ vo = −iRD = − 1+gm Rs vi RD
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 6 of 19
vo gm −gm RD
• the voltage gain, Avo = vi R =∞ =− 1+gm Rs RD = 1+gm Rs
L
• =⇒ the resistance Rs reduces the voltage gain by the factor (1 + gm Rs )
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
gm RD
• Avo = − 1+g m Rs
• divide numerator and denominator by gm on R.H.S
RD
• Avo = − 1
gm +Rs
• =⇒ the voltage gain between gate and the drain is equal to the ratio of the total resistance in
the drain (RD ) to the total resistance in the source ( g1m + Rs )
•
Total Resistance in drain RD
• i.e. Voltage gain f rom gateto drain = − Total Resistance in source = − 1
gm +Rs
RD
• Note that setting Rs = 0 gives Avo of the CS amp= − (1/gm)
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
• the output resistance Ro can be determined by placing a test source at the drain
– and by setting vi = 0 =⇒ vgs = 0 =⇒ i = gm vgs = 0
• i = 0 =⇒ Ro = RD
• now when RL is connected at the output, Av can be given as
– Av = Avo RLR+R
L
o
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 7 of 19
gm RD
• As Avo = − 1+g m Rs
, Ro = RD
•
gm RD
• =⇒ Av = Avo RLR+R
L
o
= − 1+gm Rs RLR+R
L
D
Basic MOSFET Amplifier Configurations
The Common-Source (CS) Amplifier with a Source Resistance
gm RD gm
• Av = − 1+gm Rs RL +RD = − 1+gm Rs RRLD+R
RL RL
D
(RD ∥RL )
gm
• Av = − 1+gm Rs 1
1
+ R1
= − gm1+g m Rs
RD L
RD ∥RL
• Av = − (1/gm )+Rs
• Note that −Av is the ratio of the total resistance in drain to the total resistance in source
• As Rin is infinite =⇒ vi = vsig
•
vo vo
• and the overall voltage gain is Gv = vsig = vi = Av
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 8 of 19
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
• here common-gate amplifier is shown
• Note that the biasing circuit is not shown in figure
• the amplifier is fed with a signal source characterized by vsig and Rsig
• Note that Rsig appears in series with the source
– =⇒ it is more convenient to use the T model rather than the π model
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
•
• here channel length modulation is ignored
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 9 of 19
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
1
• the input resistance Rin can be given as Rin = gm
– as Rin is the resistance looking into the source and the gate is grounded
• typically 1/gm is a few hundred ohms
– =⇒ the CG amplifier has a low input resistance
• by ohm’s law
– vo = (−i) RD ,
– where i is also the source current
0−vi
– i.e. i = 1/gm = −vi gm
•
vo RD
• =⇒ vo = vi gm RD =⇒ Avo = vi = gm RD = 1/gm
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
• The output resistancee of the CG amplifier is
– Ro = RD
• the gain Av is
vo
– Av = vi = gm (RD ∥ RL )
– ∵ Avo = gm RD and as RL is in parallel with RD
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 10 of 19
•
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
• To determine the overall gain Gv
• first by voltage divider
(1/gm ) Rin
• vi = (1/gm )+Rsig vsig = Rin +Rsig vsig
vo vo vi
• Gv = vsig = vi vsig
vo vi (1/gm )
– Gv = vi vsig = Av vvsigi = Av (1/g m )+Rsig
• ∵ Av = gm (RD ∥ RL )
(1/gm )
– Gv = gm (RD ∥ RL ) (1/g m )+Rsig
RD ∥RL
– Gv = (1/gm )+Rsig
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 11 of 19
Basic MOSFET Amplifier Configurations
The Common-Gate (CG) Amplifier
RD ∥RL
• Gv = (1/gm )+Rsig
• Note that the overall voltage gain is simply the ratio of
– the total resistance in the drain circuit
– to the total resistance in the source circuit
• CG amplifier has excellent frequency response and can be used to
– amplify high-frequency signals that originate from sources
– with relatively low resistances
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• The common-drain amplifier is more commonly known as the source follower
• A common application of a CD amplifier is as a voltage buffer
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
The need for Voltage Buffers
• consider the situation shown in fig
• here a signal source with open-circuit voltage of vsig = 1V peak
– and internal resistance of 1MΩ is to be connected to a 1kΩ load resistance
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 12 of 19
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
The need for Voltage Buffers
• connecting the source to the load directly will result in
– severe attenuation of the signal
– i.e. the voltage appearing accross the load is
RL 1k
– vo = RL +Rsig vsig = 1k+1M vsig
1k
• As vsig = 1V =⇒ vo = 1k+1M vsig = 0.999mV ≈ 1mV
• thus for vsig = 1V peak , output voltage is only vo = 1mV peak
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
The need for Voltage Buffers
• now suppose, we have an amplifier as shown in fig
• this amplifier is used to connect the source to the load
• this amplifier has a unity voltage gain
– (here our signal is already of sufficient strength and increase in its amplitude is not re-
quired)
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 13 of 19
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
The need for Voltage Buffers
• this amplifier has a very large input resistance (Rin is very large)
– =⇒ vi ≈ vsig
• the amplifier has an output resistance of
– Ro = 100Ω
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
The need for Voltage Buffers
• here Ro = 100Ω , RL = 1kΩ , vi ≈ vsig , Avo = 1
• by voltage divider
RL 1k
– vo = RL +Ro Avo vi = 1k+100 (1) vsig = 0.9V
• =⇒ 90% of the input signal appears at the output
• such an amplifier circuit with Avo = 1 , Rin → ∞ and Rout → 0 is called a voltage buffer
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 14 of 19
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• The source follower can be used to implement the unity gain buffer amplifier
• the CD amplifier with the bias circuit omitted is shown here
• a signal generator (vsig , Rsig ) is applied between the gate and ground
• a load resistance RL is connected between the source terminal and ground
• As RL is connected to the source terminal, it is more convenient to use the T-model rather that
the hybrid-π model
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• here ro appears in parallel with RL . so channel length modulation effect can easily be incorpo-
rated.
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 15 of 19
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• if we ignore channel length modulation =⇒ ro = ∞
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• from figure
– Rin = ∞
– as no current flows in to the gate terminal
• =⇒ vi = vsig
• by voltage divider
vo RL
– vi = RL +(1/gm ) = Av
• and Avo can be given as
vo RL
– Avo = vi R =∞ = RL +(1/gm ) R =∞
L L
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 16 of 19
•
1 1
• Avo = 1+ g1m 1 = 1+0 =1
RL RL =∞
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
• the output resistance Ro is determined by setting vi = 0 and RL = ∞
– =⇒ vi = 0 =⇒ the gate terminal is grounded
1
– =⇒ Ro = gm
vo vo
• Gv = vsig = vi ∵ vi = vsig because of infinite Rin
vo RL
– Gv = vi = Av = RL +(1/gm )
– thus Gv will be lower than unity
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 17 of 19
•
• however, as 1/gm is usually a small value
– =⇒ voltage gain will be close to unity i.e Gv = Av ≈ 1
Basic MOSFET Amplifier Configurations
The Common-Drain Amplifier or Source Follower
vo
• Gv = vi = Av ≈ 1 and Avo = 1
• =⇒ the voltage at the source terminal will follow that at the input, hence the name source
follower
•
1
• thus the source follower has Rin = ∞ , Ro = gm (a small value) and Avo = 1, Av = Gv ≈ 1
• =⇒ the source follower is ideally suited for implementing the unity gain voltage buffer.
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Basic MOSFET Amplifier Configurations
EE-215 Electronic Devices and Circuits, Dr. M Anis Ch, Lecture 7b Page 19 of 19
