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5/14/2017 582793487 1/4 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) 5/14/2017 582793487 2/4 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. 5/14/2017 582793487 3/4 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. 5/14/2017 582793487 4/4 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)