Single stage FET Amplifier in CS configuration and DC operating point.
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| Fig . 1 |
The field-effect transistor (FET) is type of transistor which uses an electric field to control the flow current. Like the old vacuum triode, the FET is a voltage-controlled device. It is a three terminal device . The lower terminal in the N material is called the source, and the upper terminal is called the drain; the two regions of P material, which are usually connected together externally, are called gates. The current in a FET is carried only by majority carriers and for this reason the FET is also known as unipolar transistor.
Working : So,what is amplifier ? It takes small signal and make it large . As it involves single transistor for amplification , it is called as single stage amplifier.Here, N- channel acts as an ohmic resistance through which current can pass between Drain and source terminal .Now applying the voltage at Gate and Source terminal the width of this channel or flow of electron can be managed. Width of the depletion region created between Gate and Source terminal can be varied using VGG . P-N junctions exist between the P and N materials, and in normal operation the voltage applied to the gates biases these junctions in the reverse direction. If the voltage applied to the gates is changed, the width of the transition region at the junction changes; thus the width of the channel changes, resulting in a change in the resistance between source and drain. In this way the current in the output circuit is controlled by the gate voltage so one can get desired amplification by choosing appropriate VGG and VDD sources .It has ability to provide a voltage amplification of about at frequencies up to 100 MHz.
CS configuration and DC operating point : Field-effect transistor (FET) has capability to amplify A.C.signals.Behavior of transistor can be studied with help of characteristics curve.Depending upon the configuration,JFET input and output characteristics can be studied.There are 3 types of configuration : Common source configuration,Common Drain configuration,common Gate configuration .
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| Fig. 2 |
Common Source FET amplifier : Figure 2 shows a common source N-channel FET amplifier . For biasing FET , voltage divider of R1 and R2 is used at Gate terminal of transistor . To bypass the input and output a.c. signals coupling capacitors ( Capacitor C1 and C3 ) are used . Here , C2 is used as bypass capacitor which keeps the source of FET at a.c. ground effectively.Output voltage at drain is 180 degree out of phase with input signal at gate . This phenomena is also known as phase inversion . When a.c. signal is applied to gate , it produces variations in the gate to source voltage . This results in variations in drain current . As gate to source voltage increases , drain current increases . Thereby voltage drop across the Rd also increases . This results in drop of drain voltage . Therefore positive half cycle of the input voltage produces negative half cycle at output and this causes phase inversion .
DC operating point : To use FET in any application , it is necessary to bias it . Proper biasing make sure to turn on the device and place it in region where it operates properly and maintain constant voltage gain. For this purpose we set fixed level of current and voltage values in transistor . This values define by the point at which transistor operates . This point is known as operating point (Q point) .Since the level of currents and voltages are fixed , that is why it is also called DC operation point .