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Enhancement MOSFET Voltage Divider Biasing to design Amplifier

Enhancement MOSFET are usually biased either using drain feedback bias or voltage divider bias. The depletion MOSFET can be biased using zero bias, fixed bias, self bias, voltage divider bias, two supply bias but the enhancement MOSFET cannot be biased using zero bias and self bias. Here it is shown how to design enhancement MOSFET based amplifier using voltage divider biasing method with example worked out calculations and online calculator.

VDB Biased E-MOSFET Amplifier Circuit & Operation

The following shows the circuit diagram of enhancement MOSFET biased using voltage divider biasing circuit.

voltage divider biased enhancement MOSFET amplifier circuit diagram


 Here the 2N7000 N-channel enhancement MOSFET is used as an example. The DC supply is 5V. The voltage divider circuit is made up of the resistors R1 and R2 which sets the gate bias voltage so that the Q-point or the biasing point is located in the active region(also called saturation region). When biased in the active region(saturation region) the circuit is used as an amplifier. The drain resistor RD and the source resistor RS are used to bias the output current and voltages. The capacitor C1 and C2 are the input and output coupling capacitor. The signal applied is 100mV with frequency of 1kHz which is coupled to the amplifier gate via the input coupling capacitor CC1. The output is coupled to the load resistor RL using the output coupling capacitor CC2. The CB is the bypass capacitor for resistor RS. 

Steps to design E-MOSFET amplifier using VDB method

1. Choose Q-point(the Bias Point)

 The first step to bias the amplifier is to select bias point or the operating point in the active region of operation. Following shows the selected bias point.

Q-point VDB E-MOSFET 2N7000

 The selected Q-point or the bias point is at,
VDSQ = 2V,  IDQ =3.3mA at VGS=2.5V and VGS(th=2.1V

2. Select Drain Voltage

Let us choose VD=2.5V (should be greater than VDSQ = 2V)

3. Calculate Source Voltage

VS=VDVDS

or, VS=2.5V2V=0.5V  

4. Calculate gate voltage

The gate voltage can be calculated using,

 VG=VGS+VS

 or, VG=2.5V+0.5V

that is,  VG=3V

5. Calculate drain resistor

 The drain resistor is,

RD=VDDVDID

or, RD=5V2.5V3.3mA=2.5V3.3mA 

that is, RD=791.14Ω

6. Calculate the source resistor

The source voltage is given by,

VS=ISRS

since IS=ID, the source resistor is,

RS=VSID 

or, RS=0.5V3.3mA 

that is, RS=158.23Ω 

7. Calculate voltage divider resistor

The gate voltage is given by the voltage divider rule as follows,

VG=R2R1+R2VDD

Rearranging we can write,

R1=R2(VDDVG1)

Let R2=10kΩ

then, R1=10kΩ(5V3V1)

that is, R1=6.67kΩ

8. Calculate Rd

Rd=RD||RL=RDRLRD+RL 

Let, RL=1kΩ 

or, Rd=791.14Ω×1kΩ791.14Ω+1kΩ 

thata is, Rd=441.70Ω

9. Calculate device k,

k=ID(VGSVGS(th))2

or, k=3.3mA(2.5V2.1V)2=19.75mA/V2

10. Calculate transconductance gm,

gm=2k(VGSVGS(th))  

or, gm=2×19.75mA/V2(2.5V2.1V)=15.80mS 

11. Calculate voltage gain Av,

Av=gmRd  

or,Av=5.8mStimes441.70Ω  

that is, Av=6.98  

12. Calculate input impedance Zi

Zi=R1||R2=R1R2R1+R2 

or, Zi=6.67kΩ1×10kΩ6.67kΩ+10kΩ 

that is, Zi=4kΩ

13. Calculate output impedance Zo

Zo=Rd 

that is, Zo=441.70Ω

14. Calculate input coupling capacitor

 The input coupling capacitor is,

CC1=12πf(0.1)Zi

or,CC1=12π(1kHz)(0.1)(4kΩ)

that is, CC1=398.09nF

15. Calculate input coupling capacitor

The output coupling capacitor is,

 CC2=12πf(0.1)Zo

or, CC2=12π(1kHz)(0.1)(441.7Ω)

that is, CC2=3.61μF

16. Calculate bypass capacitor

The bypass capacitor is,

 CB=12πf(0.1)RS

or, CB=12π(1kHz)(0.1)(158.23Ω)

that is, CB=158.23μF

Results

The complete voltage divider biased enhancement MOSFET amplifier with the calculated component value is shown below.

voltage divider biased enhancement MOSFET amplifier calculated values
Following shows the circuit voltage and current obtained using the calculated values in circuit simulator.

 
VDB enhancement MOSFET amplifier circuit simulation
The following shows VDB enhancement MOSFET amplifier input and output signals on oscilloscope.
 
VDB enhancement MOSFET amplifier signals on oscilloscope

As can be seen from the oscilloscope graph, the input and output signals are out of phase with that the output signal is amplified.
 

Using Enhancement MOSFET Biasing And Amplifier Design Calculator

The E-MOSFET amplifier can be readily designed, that is the component values calculated using the online Enhancement MOSFET Biasing And Amplifier Design Calculator. Below shows screenshot of the component values obtained from the calculator.

voltage divider biased enhancement MOSFET amplifier calculator



 

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