JFET MCQ Quiz in తెలుగు - Objective Question with Answer for JFET - ముఫ్త్ [PDF] డౌన్‌లోడ్ కరెన్

Junction Field Effect Transistor (JFET):

• It is a transistor that relies on the electric field to control the shape of the channel and therefore on the conductivity of the channel. Hence referred to as field-effect transistor.
• FET is a unipolar transistor that involves only one type of charge carrier in its operation.
• In the case of an N-channel JFET, the operation involves a flow of electrons.
• In the case of a p-channel JFET, the operation involves a flow of holes.
• JFET operation involves the flow of majority carriers.
• In JFET, the current flowing through channel b/w Drain in the source is controlled by the voltage applied at the Gate terminal, which is reverse biased.
• As it is reverse biased, the Gate current is practically zero as it has high input impedance.

The correct answer is 'option 3'

Solution:

• The JFET is a long channel of semiconductor material, doped to contain an abundance of positive charge carriers or holes (p-type), or of negative carriers or electrons (n-type).
• Ohmic contacts at each end form the source (S) and the drain (D).
• A p-n junction is formed on one or both sides of the channel, or surrounding it using a region with doping opposite to that of the channel, and biased using an ohmic gate contact (G).

The correct answer is 'option 2'

Solution:

• The high impedance of JFET is due to the reason that the Gate junction is reverse biased and because there is no minority carrier contribution to the flow through the device.
• The control element for the JFET comes from depletion of charge carriers from the n-channel. When the Gate is made more negative, it depletes the majority carriers from a larger depletion zone around the gate.
• This reduces the current flow for a given value of Source-to-Drain voltage.
• Modulating the Gate voltage modulates the current flow through the device.

\(\rm I_{DS}=I_{DSS}\left(1-\frac{V_{GSQ}}{V_P}\right)^2\)

\(\rm I_{DS}=40\left(1-\frac{(-5)}{(-10)}\right)^2\)

\(\rm I_{DS}=40\left(1-\frac{1}{2}\right)^2\)

I_DS = 10 mA

\(\rm R_s=\frac{-V_{GSQ}}{I_{DS}}\)

\(\rm R_s=\frac{(-5)}{10\times 10^{-3}}\)

R_s = 500 Ω

The junction FET, or JFET uses a reverse biased diode junction to provide the gate connection.

JFET (Junction Field Effect Transistor) is a uni-polar voltage controlled device that consists of three terminals called drain, source and gate

JFET has two diodes.

The structure consists of a semiconductor channel which can be either N-type or P-type.

A semiconductor diode is then fabricated onto the channel in such a way that the voltage on the diode affects the FET channel.

In operation this is reverse biased and this means that it is effectively isolated from the channel - only the diode reverse current can flow between the two.

The JFET is the most basic type of FET, and the one that was first developed. However it still provides excellent service in many areas of electronics.

Feedback Amplifiers:

Feedback amplifiers are the type of amplifiers in which a part of the output is given back to the input.

Feedback amplifiers are basically classified into 2 categories:

1.) Positive Feedback amplifier:

• It is a type of amplifier in which the source signal and the feedback signal are in the same phase.
• Thus, the feedback signal applied increases the strength of the input signal.

2.) Negative Feedback amplifier:

• In this type of amplifier source signal and the feedback signal is out of phase with each other. 
• Thus, the feedback signal is applied to decrease the strength of the input signal.

Classification of feedback amplifier on basis of input and output resistance:

From the above table, the input and output resistances of transconductance amplifier are ∞ and ∞ respectively.

MOSFET: Input impedance (typically of the order of >1010 Ω

BJT: input impedance is of the order of >104 Ω

JFET: Input impedance is of the order of >108 Ω

MESFET: Input impedance is of the order of >1012 Ω

Hence, option 4 is correct.

Note: At low or medium frequency MOSFET is offering high input impedance than JFET but at very higher frequencies, JFET has more input impedance than MOSFET. (Usually, We use MOSFET and JFET at low or medium frequencies. Hence, by default the Input impedance of MOSFET > Input impedance of JFET.

The difference between FET and BJT is explained in the following table:

JFET Parameters:

In a JFET, the drain current I_D is a function of V_GS and V_DS.

Dynamic drain resistance (r_d): This is the ratio of the change in the drain to source voltage to the change of drain current keeping the gate to source voltage constant, i.e.

\({r_d} = \frac{{\partial {V_{DS}}}}{{\partial {I_D}}},\) at constant V_GS

Transconductance (g_m): It is the ratio of the change in drain current to the change in the gate to source voltage keeping drain to source voltage constant, i.e.

\({g_m} = \frac{{\partial {I_D}}}{{\partial {V_{GS}}}},\) at constant V_DS

Amplification factor (μ): It is the ratio of the change in drain voltage to the change in the gate voltage at a constant I_D, i.e.

^{\(\mu = \frac{{\partial {V_{DS}}}}{{\partial {V_{GS}}}},\)} at constant I_D

The above can be written as:

\(\mu = \frac{{\partial {V_{DS}}}}{{\partial {I_D}}} \times \frac{{\partial {I_D}}}{{\partial {V_{GS}}}} = {r_d} \times {g_m}\)   

Application:

Given μ = 40, g_m = 100 μs

Since μ = r_d × g_m

40 = r_d × 100 μ

r_d = 4 × 10⁵ Ω

A thyristor is not a type of Field Effect Transistor (FET).

Thyristor

• A thyristor is a four-layer semiconductor device, consisting of alternating P-type and N-type materials (PNPN).
• A thyristor usually has three electrodes: an anode, a cathode, and a gate, also known as a control electrode. The most common type of thyristor is the silicon-controlled rectifier (SCR).

Field Effect Transistor(FET)

• The field-effect transistor is a type of transistor that uses an electric field to control the flow of current in a semiconductor.
• FETs have three terminals: the source (S), the drain (D), and the gate (G). It works in two modes: depletion and enhancement type.
• Enhancement FET does not conduct at 0 volts, as there is no channel in this type to conduct. Depletion FET conducts at 0 volts as the channel is already present in it.

Amplification Factor (μ) of Junction-FET:

The amplification factor is defined as the ratio of change of drain voltage (δVDS) to change of gate voltage (δVGS) at a constant drain current (ID = Constant).

\(μ = \frac{{{\delta _{DS}}}}{{\delta {V_{GS}}}}\ at\,constan t\,{I_D}\)

There is a relation between transconductance (gm) and dynamic output resistance (rd) and that can be established in the following way.

\(μ = \frac{{{\delta _{DS}}}}{{\delta {V_{GS}}}} = \frac{{\delta {V_{DS}}}}{{\delta {I_D}}} × μ = \frac{{\delta {I_D}}}{{\delta {V_{GS}}}}\)

⇒ μ = r_d × g_m

Additional InformationTransconductance (gmo): Transconductance is the ratio of change in drain current (δID) to change in the gate to source voltage (δVGS) at a constant drain to source voltage (VDS = Constant).

\(g_m = \frac{{{\delta {I_D}}}}{{\delta {V_{GS}}}}at\,cons\tan t\,{V_D{_S}}\)

This value is maximum at VGS = 0.

This maximum value (gmo) is specified in a JFET data sheet.

The trans-conductance at any other value of gate to source voltage (gm) can be determined as follows.

The expression of drain current (ID) is

\({I_D} = {I_{DSS}}{\left[ {1 - \frac{{{V_{GS}}}}{{{V_{GS}}_{\left( {off} \right)}}}} \right]^2} \)

By partial differentiating the expression of drain current (ID) in respect of gate to source voltage (VGS)

\({g_m} = \frac{{\delta {I_D}}}{{\delta {V_{GS}}}} = \frac{{2{I_{DSS}}}}{{{V_{GS\left( {off} \right)}}}}\left[ {1 - \frac{{{V_{GS}}}}{{{V_{GS\left( {off} \right)}}}}} \right]\)

At VGS = 0, the transconductance gets its maximum value and that is

\({g_{mo}} = \frac{{2{I_{DSS}}}}{{{V_{GS\left( {off} \right)}}}}\)

Therefore, we can write,

\({g_m} = {g_{mo}}\left[ {1 - \frac{{{V_{GS}}}}{{{V_{GS\left( {off} \right)}}}}} \right]\)