Download Biasing of Discrete MOSFET Amplifiers

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Biasing of Discrete
MOSFET Amplifiers
VDD
VDD
RD
R1
+
+
R2
ID
VDS
VGS
-
-
It is difficult to give “rule-ofthumb” values that will always
work well for MOSFET DC
bias design.
But, try this procedure, but
check your results to ensure
success!
RS
1.
Given the desired value of ID, make source voltage
Vs  VDD 4 , i.e. set the source resistor RS to:
RS 
V
Vs
 DD
ID
4ID
(1)
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2. Now determine the required value of VGS . Since
ID  K (VGS Vt )2 , we find that VGS should be:
VGS 
ID
Vt
K
3. Set the required value of gate voltage VG .
Note that:
VG  VGS VS
Thus, we can add the results of the previous two steps to find
the required value of the gate voltage VG .
To set the gate voltage to this value, we must select the proper
values of resistors R1 and R2 . Since the gate current is zero
(iG  0 ), we find from voltage division that:
 R2 
VDD
VG  VDD 


 R1  R2   R1   1
 R2 
(2)
Note this equation determines the ratio of resistors R1 and R2 ,
but not the resistors themselves. We need a second equation
to explicitly determine the resistors values—the sum of the two
resistances, for example.
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We find that making the resistances R1 and R2 as large as
possible is very desirable! This will typically maximize the
amplifier input resistance, as well as result in minimum power
dissipation.
As a result, we make the resistors as large a practicable. For
example:
(3)
R1  R2  250 K
4. Set the required value of DC drain voltage VD .
Recall that:
a) we require VD
VDD  L to avoid cutoff mode.
b) And, we require that VDS
Note that VDS
VGS Vt to avoid triode mode.
VGS Vt means that:
VDS VS  VGS Vt VS
But, VDS VS  VD and VGS VS  VG , so that we find the drain
voltage VD must be:
VD
VG Vt  L
to avoid entering the MOSFET triode mode.
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Summarizing, we require that the DC drain voltage should be:
a) much less than VDD =L+ to avoid cutoff.
b) much greater than (VG Vt ) =L- to avoid triode.
Solution: set the drain voltage VD to a value half-way between
VDD and VG Vt !
In other words, set the DC drain voltage to be:
VD 
VDD  VG Vt 
2
To achieve this, we must select the drain resistor RD so that:
RD 
VDD VD VDD  VG Vt 

ID
2ID
Thus, use equations (1), (2), (3), and (4) to determine the
standard DC bias design (i.e., R1, R2, RS , and RD ) for MOSFET
amplifiers.
If I were you, I’d make sure I understood
this material well enough that I could also
bias a non-standard MOSFET amplifier
problem.
It’s not enough to simply know how, you must
also know why!
(4)