FET Amplifier MCQ Quiz - Objective Question with Answer for FET Amplifier - Download Free PDF

Explanation:

FET (Field Effect Transistor)

Definition: A Field Effect Transistor (FET) is a type of transistor that uses an electric field to control the flow of current. It is one of the most fundamental components in electronics and is widely used in amplifiers, switches, and digital circuits. The current through the FET is controlled by the voltage applied to the gate terminal, making it a voltage-controlled device.

Working Principle: The FET operates based on the principle that the voltage applied to the gate terminal creates an electric field, which controls the conductivity of the channel between the source and drain terminals. By varying the gate voltage, the current flowing through the channel can be modulated.

In an n-channel FET, a positive gate voltage enhances the conductivity of the channel, allowing more current to flow. Conversely, in a p-channel FET, a negative gate voltage enhances conductivity. The ability to control current with voltage makes FETs highly efficient and versatile in various applications.

Advantages:

• High input impedance, resulting in minimal loading on preceding circuits.
• Low power consumption due to the absence of a direct current path between the gate and the source.
• Compact size and compatibility with integrated circuit technology.
• Wide range of applications, including amplifiers, oscillators, and digital logic circuits.

Disadvantages:

• Susceptibility to damage from static electricity due to the high input impedance.
• Limited output current capability compared to bipolar junction transistors (BJTs).
• Temperature sensitivity, which can affect performance in extreme conditions.

Applications:

• Used in amplifiers to boost weak signals in audio, radio, and communication systems.
• Serves as a switch in digital circuits, enabling or disabling current flow based on logic levels.
• Utilized in voltage regulators and power management circuits for efficient energy control.
• Commonly employed in radio frequency (RF) applications due to their high-frequency response.

Correct Option Analysis:

The correct option is:

Option 4: Voltage controlled device

FETs are classified as voltage-controlled devices because the current flowing through the device (between the source and drain terminals) is controlled by the voltage applied to the gate terminal. This characteristic differentiates FETs from current-controlled devices like bipolar junction transistors (BJTs), where the base current controls the collector current. The voltage-controlled nature of FETs allows for efficient operation with minimal power consumption, making them suitable for a wide range of applications, from analog signal amplification to digital switching.

Additional Information

To further understand the analysis, let’s evaluate the other options:

Option 1: Current controlled device

This option is incorrect because FETs are not current-controlled devices. In a current-controlled device like a BJT, the collector current is directly dependent on the base current. However, in FETs, the current flow is controlled by the voltage applied to the gate terminal, not by any current flowing into the gate.

Option 2: Magnetic device

This option is incorrect as FETs do not operate based on magnetic principles. Magnetic devices typically involve the use of magnetic fields for operation, such as transformers or inductors. FETs, on the other hand, rely on electric fields to control current flow.

Option 3: Power controlled device

This option is incorrect because the term "power controlled device" does not accurately describe the operation of FETs. While FETs are used in power electronics for switching and amplification, their operation is fundamentally based on voltage control rather than power control. Power control is a broader term that applies to various devices and systems, not specifically to FETs.

Option 5: (Not specified in the question)

As no specific description is provided for this option, it cannot be considered a valid choice for the classification of FETs.

Conclusion:

FETs are voltage-controlled devices that utilize an electric field to modulate the flow of current. This characteristic makes them highly efficient and versatile components in modern electronic circuits. Understanding the distinction between voltage-controlled and current-controlled devices is essential for selecting the appropriate component for a given application. The incorrect options highlight the importance of accurately identifying the operational principles of electronic devices to avoid misconceptions.

Concept

The forward current gain (α) of a power transistor is defined as:

\(α ={I_C\over I_E}\)

where, I_C = Collector current

I_E = Emitter current

For power transistors, the forward current gain (α) is typically high, usually in the range of 0.95 to 0.99. This indicates that most of the emitter current flows to the collector, with very little loss to the base current.

\(β ={α\over 1-α}\)

where β is the common-emitter current gain, having α close to 1 ensures that power transistors operate efficiently with high current gain.

Thus, the correct approximate value of forward current gain for power transistors is 0.95 to 0.99.

A field-effect transistor (FET) has 3 terminals. These are:

• Source (S): The terminal through which carriers enter the channel.
• Drain (D): The terminal through which carriers leave the channel.
• Gate (G): The terminal used to control the conductivity of the channel
   

Field effect transistor (FET)

• The field-effect transistor (FET) is a type of transistor that uses an electric field to control the flow of current in a semiconductor.
• FETs are devices with three terminals: source, gate, and drain. FETs control the flow of current by the application of a voltage to the gate, which in turn alters the conductivity between the drain and the source.
• FETs naturally contain two p-n junctions, hence it is a Bi-junction device.
• FETs use either electrons (n-channel) or holes (p-channel) as charge carriers in their operation, but not both.
• Field effect transistors generally display very high input impedance at low frequencies. 

Concept:

BJT (Bipolar Junction Transistor):

• It is a current-controlled device
• The input for BJT is a forward-biased diode, so the input impedance is low (Statement (a) is incorrect)
• It has a higher unity-gain bandwidth.

FET (Field Effect Transistor):

• It is a voltage-controlled device.
• It has a high input impedance as the gate is Reverse biased for JFET's and because of the insulating layer in the case of MOSFET's.
• It has low gain bandwidth.

UJT (Unijunction Transistor):

• The device has a unique characteristic when it is triggered.
• It exhibits a negative resistance characteristic so it can be employed as an oscillator (Statement (c) is correct)
• It cannot be used as an amplifier.

Configuration of MOSFET amplifier

1.) Common drain amplifier 

• In the Common Drain configuration, the input is applied to the Gate and its output is taken from the Source. 
•  The input signal is applied to the gate through a coupling capacitor, and the output signal is coupled to the load resistor through the other capacitor.
• A common-drain amplifier is also called a source-follower. Self-biasing is used in this particular circuit.

​2.) Common source amplifier 

• In the Common source configuration, the input is applied to the Gate and its output is taken from the drain.
• This configuration provides a 180° phase shift between input and output. 

3.) Common gate amplifier 

• In the Common gate configuration, the input is applied to the source, and its output is taken from the drain.

Properties of MOSFET amplifier

FET:

As the input circuit of FET is reverse biased, FET exhibits a much higher input impedance ( in the order of 100 M 12 ) and lower output impedance and there will be a high degree of isolation between input and output.

So, FET can act as an excellent buffer amplifier but the BJT has low input impedance because its input circuit is forward biased.

Common Drain configuration:

• In the Common Drain configuration (similar to common collector), the input is applied to the Gate and its output is taken from the Source.
• The common drain or “source follower” configuration has the highest input impedance and low output impedance.

The voltage gain is unity, although the current gain is high. The input and output signals are in phase.

Common Source configuration:

• In the Common Source configuration (similar to common-emitter), the input is applied to the Gate and its output is taken from the Drain as shown.
• This is the most common mode of operation of the FET due to its high input impedance and good voltage amplification and as such Common Source amplifiers are widely used.
• The common source mode of FET connection is generally used audio frequency amplifiers and in high input impedance pre-amps and stages.
• Being an amplifying circuit, the output signal is 180o “out-of-phase” with the input.

Common Gate configuration:

• In the Common Gate configuration (similar to common base), the input is applied to the Source and its output is taken from the Drain with the Gate connected directly to the ground (0v) as shown.
• The high input impedance feature of the previous connection is lost in this configuration as the common gate has a low input impedance, but a high output impedance.
• This type of FET configuration can be used in high-frequency circuits or impedance matching circuits were a low input impedance needs to be matched to high output impedance. The output is “in-phase” with the input.

Explanation:

• FET is a device that is usually operated in the constant-current portion of its output characteristics. But if it is operated on the region prior to pinch-off (that is where VDS is small, say below 100 mV), it will behave as a voltage-variable resistor. 
• It is due to the fact that in this region drain-to-source resistance R_DS can be controlled by varying the bias voltage V_GS.
• In such applications the FET is also referred to as a voltage-variable resistor or volatile dependent resistor. It finds applications in many areas where this property is useful.

Important Points

Concept

The small signal model of a common source MOSFET amplifier is:

The input capacitance for a common source(CS) MOSFET amplifier is given by:

\(C_{in}=C_{gs}+(1+A_V)C_{gd}\)

where, C_gs = Gate to source capacitance

C_gd = Gate to drain capacitance

A_V = Voltage gain

Calculation

Given, Cgs = 5 pF

Cgd = 3 pF

AV = 3

\(C_{in}=5+(1+3)3\)

C_in = 17 pF

Concept:

The output Resistance of FET is defined as

\({r_o} = \frac{1}{{\lambda {I_D}}}\)

Where λ = channel length modulator parameter

I_D = drain current

Calculation:

Given

λ = 0.05 V^-1

I_D = 10 mA Important Point:

\({r_o} = \frac{1}{{\lambda {I_D}}} = \frac{1}{{0.05 \times 10 \times {{10}^{ - 3}}}} = 2K{\rm{\Omega }}\)

NOTE: 

If MOSFET does not have a channel length modulator then ⇒ λ = 0

\({r_{ds}} = \frac{1}{{\lambda {I_{DS}}}} = \infty \) (of P resistance)

Concept:

BJT (Bipolar Junction Transistor):

• It is a current-controlled device
• The input for BJT is a forward-biased diode, so the input impedance is low (Statement (a) is incorrect)
• It has a higher unity-gain bandwidth.

FET (Field Effect Transistor):

• It is a voltage-controlled device.
• It has a high input impedance as the gate is Reverse biased for JFET's and because of the insulating layer in the case of MOSFET's.
• It has low gain bandwidth.

UJT (Unijunction Transistor):

• The device has a unique characteristic when it is triggered.
• It exhibits a negative resistance characteristic so it can be employed as an oscillator (Statement (c) is correct)
• It cannot be used as an amplifier.

Concept:

1) The thermal runway is not possible in FET because as the temperature of the FET increases, the mobility decreases, i.e. if the Temperature (T) ↑, the carries Mobility (μn or μ­p) ↓, and Ips↓

2) Since the current is decreasing with an increase in temperature, the power dissipation at the output terminal of a FET decreases or we can say that it’s minimum.

So, there will be no Question of thermal Runway at the output of the FET.

• The thermal runaway takes place in a BJT.
• Thermal Runway in BJT is a process of self-damage of BJT because of overheating at the collector junction due to an increase in Ic with Ico
• If T↑, then Ico (Reverse separation current) ↑, which results in an increase in the collector current, i.e. Ic ↑.
• Power dissipation at the collector junction increases in the form of heat which again raises the temperature and the cycle continues.
• If the above cycle becomes repetitive then the collector junction gets overheated and thereby thermal runway takes place.

Concept:

The small-signal model of a CS MOSFET is drawn as:

In common source configuration, C_gd will be present between the Input node gate and output node drain. It can be therefore replaced with C_m and C_n using Miller’s theorem as:

C_m = C_gd (1 - A_v)

\({C_n} = {C_{gd}}\left( {1 - \frac{1}{{{A_v}}}} \right)\)

C_m appears parallel with C_gs.

∴ The net capacitance between the gate and the source ‘or’ the net input capacitance will be:

C_in = C_gs + C_m ; C_in

C_in = C_gs + C_gd (1 - A_v)

Calculation:

Given: C_gs = 4 pF, C_gd = 1 pF, Av = 5

Since the voltage gain of a common source Amplifier is always Negative, therefore Av = -5

Also, since C_in = C_gs + C_gd (1 - A_v), we get:

C_in = 4 + 1 (1 + 5)

C_in = 10 pF

Special note:

C_n appears parallel to C_ds. ∴ the net output capacitance will be:

\({C_0} = {C_{ds}} + {C_{\begin{array}{*{20}{c}} {n\;}\\ \end{array}}}\)

\({C_0} = {C_{ds}} + {C_{gd}}\left( {1 - \frac{1}{{{A_v}}}} \right)\)

Miller effect occurs in CE configuration in BJT and CS configuration in MOSFET, it increases in input capacitances.

Configuration of MOSFET amplifier

1.) Common drain amplifier 

• In the Common Drain configuration, the input is applied to the Gate and its output is taken from the Source. 
•  The input signal is applied to the gate through a coupling capacitor, and the output signal is coupled to the load resistor through the other capacitor.
• A common-drain amplifier is also called a source-follower. Self-biasing is used in this particular circuit.

​2.) Common source amplifier 

• In the Common source configuration, the input is applied to the Gate and its output is taken from the drain.
• This configuration provides a 180° phase shift between input and output. 

3.) Common gate amplifier 

• In the Common gate configuration, the input is applied to the source, and its output is taken from the drain.

Properties of MOSFET amplifier

In AC model analysis all DC sources will be shorted.

For M₂ MOSFET-

So gate and source of M₂ are shorted.

V_GS = 0

So output resistance \({R_{out}} = \dfrac{{{V_x}}}{{{I_x}}} = {r_{02}}\) 

So M₂ will act as a resistor with resistance r_o2.

Here M₂ will act as a Resistor whose Resistance will be \({r_{{0_2}}}\)

Replacing M₁ with small-signal model.

\({V_0} = \left( {{r_{{0_1}}}\parallel {r_{{0_2}}}} \right) \times (- g_{m_1}\;Vgs)\)

V_in = Vgs

\(\frac{{{V_{out}}}}{{{V_{in}}}} = - g_{m_1}\left( {{r_{{0_1}}}\parallel {r_{{0_2}}}} \right)\)

The composite FET (Two FET connected in parallel can be drawn as)

\({I_D} = {I_{{D_1}}} + {I_{{D_2}}}\)

\({g_m} = \frac{{{I_D}}}{{{v_{gs}}}} = \frac{{{I_{{D_1}}}}}{{{v_{gs}}}} + \frac{{{I_{{D_2}}}}}{{{v_{gs}}}}\)

\( = {g_{{m_1}}} + {g_{{m_2}}}\)

∵ Both FET are identical \(\left( {{g_{{m_1}}} = {g_{{m_2}}}} \right)\)

Overall g_m = 2g_m

Now, 

\({r_d} = \frac{{{v_{ds}}}}{{{I_d}}} = \frac{{{v_{ds}}}}{{{I_{{d_1}}} + {I_{{d_2}}}}} = \frac{1}{{\left( {\frac{{{I_{{d_1}}}}}{{{v_{ds}}}}} \right) + \left( {\frac{{{I_{{d_2}}}}}{{{v_{ds}}}}} \right)}}\)

\(\frac{1}{{\left( {\frac{1}{{{r_{{d_1}}}}} + \frac{1}{{{r_{{d_2}}}}}} \right)}}\)

\({r_d} = {r_{{d_1}}}\parallel {r_{{d_2}}}\)

∵ both are parallel \(\left( {{r_{{d_1}}} = {r_{{d_2}}}} \right)\)

\({r_d} = \left( {\frac{{{r_d}}}{2}} \right)\)