Enhancement MOSFET is either used a switch or as amplifier. Usually the application of enhancement MOSFET is as switch in which case it is biased in the ohmic region using E-MOSFET ohmic biasing method. But E-MOSFET can also be used as amplifier and in this case it is biased in its active region. There are several methods to bias an enhancement MOSFET in its active region. Drain feedback bias is one of the method to bias an enhancement MOSFET in the active region. Other biasing methods that can be used to bias E-MOSFET in active region are the drain feedback bias method, E-MOSFET fixed gate bias method, and the E-MOSFET voltage divider bias method. Here it is explained how to bias an enhancement MOSFET in active region using drain feedback biasing method.
Drain Feedback Biased E-MOSFET Circuit Diagram and Operation
The following circuit diagram shows drain feedback biased enhancement MOSFET for use as an amplifier.
The drain feedback circuit has a gate resistor or the feedback resistor RG connected from the drain back to the gate, hence the circuit is called drain feedback biased circuit. The reason the feedback resistor is connected in drain feedback bias circuit is to automatically compensate any changes in the drain current so that it goes back to original drain current value so as to get a stable operating point. Suppose for any reason, such as environment temperature changes, the drain current changes(goes up or low), then this causes opposite change in the drain to source voltage(goes low or high). Because of the feedback connection, the gate to source voltage is equal to the drain to source voltage. Hence any changes in drain current will also have opposite change in the gate to source voltage. Due to this change in the gate to source voltage the drain current has opposite change and so compensates the original change. For example, let due to temperature increase, the original drain current of 10mA increases by 2mV and becomes 12mA. Due to increase in the drain current, the drain to source voltage which is originally 2V decreases by 0.2V to 1.8V. Since as aforementioned, in case of feedback biased circuit, the gate to source voltage is equal to the gate to source voltage and hence gate to source also changes from 2V to 1.8V. As the gate to source voltages goes down to 1.8V, it causes the drain to decrease. So while originally the drain current increased, due to the feedback effect, the drain current is decreased and in this way the drain current is stabilized.
Steps to bias drain feedback biased E-MOSFET amplifier
Here we will consider the N-channel enhancement MOSFET 2N7000. We will consider DC power supply \(V_{DD}=5V\), an input signal with amplitude of 100mV and frequency of 1KHz, load resistance of 1KOhm.
The steps to bias E-MOSFET with drain feedback biasing technique are as follows.
Step 1: Obtain gate to source threshold voltage \(V_{GS(th)}\)
For 2N7000, we use \(V_{GS(th)}=2.1V\)
Thus the drain current will flow only when \(V_{GS} >2.1V\)
Step 2: Set \(V_D\)
We have to set \(V_D\) higher than \(V_{GS(th)}=2.1V\) ,
Let, \(V_D=3V\)
For drain feedback biased circuit, \(V_D = V_{GS}=V_{DS}\)
And so, \(V_D = V_{GS}=V_{DS}=3V\)
Step 3: Determine \(I_D\)
Determine \(I_D\) when \(V_{DS}=V_{GS} = 3V\) either from the datasheet or the drain curve.
From the drain curve below, the drain current is \(I_D=25.3mA\)
Step 4: Calculate \(R_D\)
we have, \(R_D = \frac{V_{DD}-V_D}{I_D}\)
or, \(R_D = \frac{5V-3V}{25.3mA}=79.05\Omega\)
Step 5: Determine \(R_d\)
we have, \(R_d = R_D||R_L=\frac{R_D R_L}{R_D+R_L}\)
let \(R_L=1k\Omega\)
and therefore, \(R_d = \frac{79.05\Omega 1k\Omega}{79.05\Omega+1k\Omega}=73.26\Omega\)
Step 6: Determine k
we have, \(k=\frac{I_D}{(V_{GS}-V_{GS(th)})^2}\)
or, \(k=\frac{25.3mA}{(3V-2.1)^2}=0.031A/V^2\)
Step 7: Determine \(g_m\)
we have, \(g_m=2k(V_{GS}-V_{GS(th)})\)
or, \(g_m=2 \times 0.031A/V^2(3V-2.1V)=56.22mS\)
Step 8: Determine voltage gain, \(A_v\),
\(A_v = g_m R_d\)
or, \(A_v = 56.22mS \times 73.26\Omega = 4.12\)
Step 9: Calculate input impedance
\(Z_i = \frac{R_G}{1+g_m R_d}\) [assuming \(r_d >10R_d\)]
Let \(R_G=100k\Omega\) then, \(Z_i = 19.54k\Omega\)
Step 10: Calculate the input coupling capacitor value CC1,
\(CC_1 = \frac{10}{2\pi f Z_i}\)
Let the input signal frequency be 1KHz then,
\(CC_1 = \frac{10}{2\pi (1kHz)(19.54k\Omega)}=81.51nF\)
Step 11: Calculate the output impedance, \(Z_o\)
\(Z_o = R_d = 73.26\Omega\) [assuming \(r_d >10R_d\)]
Step 12: Calculate the output coupling capacitor CC2
\(CC_2 = \frac{10}{2\pi (1kHz)(50.29\Omega)}=21.74uF\)
Enhancement MOSFET Biasing And Amplifier Design Online Calculator
We can also use the online Enhancement MOSFET Biasing And Amplifier Design Calculator to obtain the component values and current and voltages. The following shows the input and output calculated using the online calculator.
Results
The circuit diagram of the drain feedback biased E-MOSFET with the component values calculated above is shown below.
Using circuit simulator like Proteus we can verify the drain current, drain voltage and gate to source voltage as shown below.
From the oscilloscope, we can read that the input signal has 2 division and the output signal has 9 division so the amplification is,
\(A_v=\frac{9 div}{2 div}=4.5\)
So the measured voltage gain from the circuit simulation is 4.5 which matches the calculated voltage gain of 4.12
Also we can see that the calculated input impedance is 19.54 KΩ while the calculated output impedance is 73.26 Ω. This shows that the drain feedback biased enhancement MOSFET has high input impedance and low output impedance.
