Enhancement MOSFET is one type of MOSFET(Metal Oxide Semiconductor Field Effect Transistor) which is usually used for switching application. But it is sometimes also used as an amplifier device. It is used in the front end circuit of communication for small signal amplification because it has high input impedance and generates low noise. In order to use the E-MOSFET as an amplifier it must be biased to set a stable operating point so that the voltages and current does not drift in different environment. There are several methods to bias enhancement MOSFET such as E-MOSFET ohmic biasing method, drain feedback biasing, fixed gate bias and voltage divider biasing method. Here it is shown how bias a enhancement MOSFET using fixed gate bias to build an amplifier.
Fixed Gate Biased Enhancement MOSFET Amplifier Circuit Diagram and Operation
The following shows the circuit diagram of a fixed gate biased enhancement MOSFET amplifier.
In the above circuit, a fixed gate voltage VGG is applied to the gate of the E-MOSFET Q1. This applied fixed gate voltage VGG sets up a fixed gate to source VGS. This fixed gate to source voltage in turn sets up a drain current and drain to source voltage. The fixed gate voltage or equivalently gate to source voltage VGS must be higher than the threshold gate to source voltage so that drain current flows from the drain to source. For the selected gate to source voltage VGS which is equal to gate voltage VGG the drain current is found from the enhancement MOSFET datasheet. The drain resistor RD can be found DC power supply VDD, the drain current ID and the drain voltage VD. The drain voltage VD is equal to the drain to source voltage VDS.
To calculate the voltage gain Av of the amplifier we need to calculate the output drain resistance Rd, the transconductance gm. The voltage gain is equal to product of output drain resistance Rd and the transconductance gm. The output drain resistance Rd is the parallel combination of load resistor RL and the drain resistor RD. To calculate the transconductance gm, we need to calculate the k parameter of the enhancement MOSFET. The enhancement MOSFET device k parameter can calculated from the knowledge of applied gate to source voltage VGS, the drain current ID, and the gate to source threshold voltage VGSTH.
To calculate the coupling capacitors CC1 and CC2 values we need to determine the input and output impedances. The input impedance is equal to the gate resistor RG while the output impedance is equal to the output drain resistance Rd.
Fixed Gate Biased E-MOSFET Amplifier Example worked out
The following are the steps to bias 2N7000 enhancement MOSFET using fixed gate bias for building an amplifier.
Step 1: Obtain the gate to source threshold voltage, \(V_{GS(off)}\), gate voltage \(V_{GS}\), drain current, \(I_D\) from the MOSFET datasheet or drain or transfer curve graphs.
For 2N7000 E-MOSFET we use the following data.
\(V_{GS(th)}=2.1V\), \(V_{GS}=3.5V\), \(I_D=66.1mA\)
Step 4: Choose drain voltage \(V_D\)
Let's choose drain voltage, \(V_D=1.5V\)
Step 5: Calculate drain resistor, \(R_D\),
\(R_D=\frac{V_{DD}-V_D}{I_D}=\frac{5V-1.5V}{66.1mA}=52.95\Omega\)
Step 6: Calculate the output drain resistance, \(R_d\),
\(R_d = R_D||R_L = \frac{R_D R_L}{R_D+R_L}=\frac{52.95\Omega \times 1k\Omega}{52.95\Omega + 1k\Omega}=50.29\Omega\)
Step 7: Find k
\(k =\frac{I_D}{(V_{GS}-V_{GS(th)})^2}\)
or, \(k =\frac{66.1mA}{(3.5V-2.1V)^2}=\frac{66.1mA}{1.96V}=34mA/V^2\)
Step 8: Determine transconductance, \(g_m\)
\(g_m = 2k(V_{GS}-V_{GS(th)})\)
or, \(g_m = 2 \times 34mA/V^2(3.5V-2.1V) = 68mA/V^2 \times 1.4V=95.2mS\)
Step 9: Determine voltage gain, \(A_v\),
\(A_v = g_m R_d\)
or, \(A_v = 95.2mS \times 50.29\Omega = 4.7\)
Step 10: Calculate input impedance
\(Z_i = R_G\)
Let \(R_G=100k\Omega\) then, \(Z_i = 100k\Omega\)
Step 11: Calculate the input coupling capacitor value CC1,
\(CC_1 = \frac{10}{2\pi f Z_i}\)
Let the input signal frequency be 1KHz then,
\(CC_1 = \frac{10}{2\pi (1kHz)(100k\Omega)}=15.92nF\)
Step 12: Calculate the output impedance, \(Z_o\)
\(Z_o = R_d = 50.29\Omega\)
Step 13: Calculate the output coupling capacitor CC2
\(CC_2 = \frac{10}{2\pi (1kHz)(50.29\Omega)}=31.67uF\)
The above calculated values can be directly calculated using the online Enhancement MOSFET Biasing And Amplifier Design Calculator. Utilization of this calculator is shown below.
The following shows the E-MOSFET amplifier with the calculated values.
Circuit Simulation Results
The circuit simulation of the above designed amplifier is shown below.
As shown the drain current and the drain to source voltage matches the calculated values.
The following shows the input and output waveform of the E-MOSFET amplifier.
As can be seen the output signal is amplified, has the same frequency as the input signal and is out of phase of 180 degree.
Also from the calculated input and output impedance we see that the input impedance is much higher than the output impedance.
