Common Source FET Amplifier

OBJECTIVE: 1. To obtain the frequency response of the Common Source FET Amplifier

                         2. To find the Bandwidth.






A field-effect transistor (FET) is a type of transistor commonly used for weak-signal amplification (for example, for amplifying wireless (signals)). The device can amplify analog or digital signals. It can also switch DC or function as an oscillator. In the FET, current flows along a semiconductor path called the channel. At one end of the channel, there is an electrode called the source. At the other end of the channel, there is an electrode called the drain. The physical diameter of the channel is fixed, but its effective electrical diameter can be varied by the application of a voltage to a control electrode called the gate. Field-effect transistors exist in two major classifications. These are known as the junction FET (JFET) and the metal-oxide- semiconductor FET (MOSFET). The junction FET has a channel consisting of N-type semiconductor (N-channel) or P-type semiconductor (P-channel) material; the gate is made of the opposite semiconductor type. In P-type material, electric charges are carried mainly in the form of electron deficiencies called holes. In N-type material, the charge carriers are primarily electrons. In a JFET, the junction is the boundary between the channel and the gate. Normally, this P-N junction is reverse-biased (a DC voltage is applied to it) so that no current flows between the channel and the gate. However, under some conditions there is a small current through the junction during part of the input signal cycle. The FET has some advantages and some disadvantages relative to the bipolar transistor. Field-effect transistors are preferred for weak-signal work, for example in wireless, communications and broadcast receivers. They are also preferred in circuits and systems requiring high impedance. The FET is not, in general, used for high-power amplification, such as is required in large wireless communications and broadcast transmitters.

Field-effect transistors are fabricated onto silicon integrated circuit (IC) chips. A single IC can contain many thousands of FETs, along with other components such as resistors, capacitors, and diodes.


  1. Start TINA software. A blank circuit window will appear on the screen along with a component tool bar.
  2. Using component tool bar, place all the components on the circuit window and wire the circuit.
  3. Connect the circuit as shown in circuit diagram
  4. Apply the input of 10mV peak-to-peak and 1 KHz frequency sinusoidal, using AC Voltage Source.
  5. From drop down menu select Transient analysis in the analysis option.
  6. Input and output waveforms will be displayed. Measure the Output Voltage Vo (p-p)
  7. To find frequency response select AC analysis from drop down menu. Calculate the bandwidth and gain in the response displayed.


Input Voltage given Vin: 20mV


Av in dB

20 log (Vo/Vin)





















1. All the connections are to be connected properly.

2. Transistor terminals must be identified properly

RESULT: The frequency response of the common source FET Amplifier and Bandwidth is obtained.


  1. Students are able to
  2. Design and analyze CS amplifier using TINA software.
  3. Calculate voltage gain and bandwidth of CE amplifier from the observations made.


1. What are the differences between FET and BJT?


BJT is bipolar deviceFET is uni-polar device
BJT have junctionsFET doesn't have junctions
Current conduction due to majority and minority carriers in BJT ,which causes BJT is noisyCurrent conduction due to majority carriers only, which causes FET is less noisy Than BJT 
Current controlled deviceVoltage controlled device
The output impedance is highThe input impedance is high

2. Is FET an unipolar or bipolar device ?

Ans: FET is uni-polar device, because current conduction depends on only majority charge carriers.

3. Draw the symbol of FET?


Image result for symbol of FET

4. What are the applications of FET?

Ans: FETs are using low noise amplifiers, Voltmeters, Buffer circuits and used as a analog switch.

5. Is FET a voltage controlled or current controlled device?

Ans: FET is voltage controlled device, the input gate voltage control the output drain current.

6.Draw the equivalent circuit of common source FET amplifier?


Image result for Draw the h parameter equivalent circuit of common source FET amplifier

7.What is the voltage gain of the FET amplifier?

Ans: The voltage gain of FET  amplifier is less than unity.

8.What is the input impedance of FET amplifier?

Ans:The input impedance of FET is infinity  which is    -VGS/IG

9.What is the output impedance of FET amplifier?

Ans: The output impedance is RD=VDS/ID

10.What are the FET parameters?

Ans: FET parameters are gm is trans-conductance =iD/vGS , Amplification factor

\mu \equiv -\dfrac{\partial v_{DS}}{\partial v_{GS}}|I_{D} \approx \dfrac{\Delta v_{DS}}{\Delta v_{GS}}| I_D = \dfrac{v_{ds}}{v_{gs}}|I_D

     and rD output impedance  vDS/iD