Test: (Junction Field Effect Transistor) JFET - 2 - Electronics and Communication Engineering (ECE) MCQ

Advantage of FET over BJT are
i) Higher input impedance
ii) Good bias stability
iii) Lower noise figure.

Pinch off voltage = 5V
At cut off the gate to source voltage of JFET is equal to pinch off voltage
V_gs = V_P 
⇒ V_gs(off) = 5V.

Self bias eliminates the need of 2 power supply by connecting gate resistance to the supply voltage.

In order to design an amplifier the time constant and RC and voltage across specified terminals.

During high frequency the capacitance is due to combination of transmission capacitance and junction capacitance. The equation is given by F_H = 1/27R.

By converting FET into ac equivalent circuits and connecting them in parallel,
g_n = 2g_m and r_n = r_d/2.

In JFET equivalent circuit, the output current is controlled by the gate to source voltage and hence we can say it is a Voltage controlled current source.

During High frequency, the stray capacitance is the only source of capacitance in an amplifier circuit, the stray capacitance and junction capacitance.

The maximum gain of amplifier is 1000000.
When we consider this in decibel scale,
it gives 120dB
⇒ gain in dB = 20log10(100000)
= 120dB.

Concept:

• JEFT stands for Junction Field Effect Transistor.
• A field effect transistor is a voltage controlled device i.e. the output characteristics of the device are controlled by input voltage. There are two basic types of field effect transistors:

  1. Junction field effect transistor (JFET)

  2. Metal oxide semiconductor field effect transistor (MOSFET)

• A JFET is a three terminal semiconductor device in which current conduction is by one type of carrier i.e. electrons or holes.
• The current conduction is controlled by means of an electric field between the gate and the conducting channel of the device. To control the conduction of current from the source to the drain, the gate voltage must be more negative than the source voltage.
• JEFTs are futher divided into two types that n-channel JEFT and p-channel JEFT. The three leads of a JEFT are labeled source, gate and drain.