COMMON SOURCE FET AMPLIFIER
AIM: 1. To obtain the frequency response of the common source FET
2. To find the Bandwidth.
N-channel FET (BFW11)
Resistors (6.8KΩ, 1MΩ, 1.5KΩ)
Capacitors (0.1µF, 47µF)
Regulated power Supply (0-30V)
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. Connections are made as per the circuit diagram.
2. A signal of 1 KHz frequency and 50mV peak-to-peak is applied at the
Input of amplifier.
3. Output is taken at drain and gain is calculated by using the expression,
4. Voltage gain in dB is calculated by using the expression,
5. Repeat the above steps for various input voltages.
6. Plot Av vs. Frequency
7. The Bandwidth of the amplifier is calculated from the graph using the
Where f1 is lower 3 dB frequency
f2 is upper 3 dB frequency
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1. All the connections should be tight.
2. Transistor terminals must be identified properly
RESULT: The frequency response of the common source FET
Amplifier and Bandwidth is obtained.