Configuration of BJT MCQ Quiz - Objective Question with Answer for Configuration of BJT - Download Free PDF

Explanation:

The high-frequency gain of a common-emitter amplifier is mainly affected by the collector-base junction capacitance and the emitter-base capacitance. Let's delve into the detailed explanation of why these capacitances play a crucial role and analyze the other options provided.

Correct Option: Option 2 - Collector-Base Junction Capacitance and Emitter-Base Capacitance

A common-emitter amplifier is a widely used configuration in electronic circuits. It provides good voltage gain and is commonly used for amplification purposes. However, at high frequencies, the gain of the amplifier is significantly influenced by certain capacitances inherent to the transistor. The two main capacitances that affect the high-frequency performance of a common-emitter amplifier are:

1. Collector-Base Junction Capacitance (C_cb): This is the capacitance between the collector and the base of the transistor. It is also known as the Miller capacitance because it gets multiplied by the gain of the amplifier in the feedback loop, making its effect more pronounced. This capacitance introduces a feedback path for high-frequency signals, reducing the overall gain of the amplifier at high frequencies.
2. Emitter-Base Capacitance (C_eb): This is the capacitance between the emitter and the base of the transistor. This capacitance affects the input impedance of the amplifier and influences the high-frequency response. It acts as a shunt capacitance at the input, causing the input signal to be partially bypassed to the ground at high frequencies, thus reducing the input signal amplitude and, consequently, the gain.

At high frequencies, these capacitances form low-impedance paths, which shunt the signal away from the intended path, leading to a reduction in gain. The combined effect of these capacitances is to create a low-pass filter, which attenuates the high-frequency components of the signal.

Mathematical Analysis:

The high-frequency response of the common-emitter amplifier can be analyzed using the hybrid-π model of the transistor. In this model, the capacitances C_cb and C_eb are explicitly included. The gain-bandwidth product (GBP) is a key parameter, which remains constant for a given transistor. The gain of the amplifier decreases as the frequency increases, and this relationship can be expressed as:

Gain (A_v) = A_v0 / (1 + jω/ω_T)

where A_v0 is the low-frequency gain, ω is the angular frequency, and ω_T is the transition frequency, which is inversely proportional to the sum of the capacitances.

PNP Transistor

PNP transistor is a three-terminal device i.e. base, emitter, and collector.

The main function of the PNP transistor is amplification.

For amplification, the base and emitter junction must be forward bias, and the collector and emitter junction must be reverse bias.

Operating modes of transistor

Additional Information

NPN Transistor:

• An NPN transistor is a Bipolar Junction Transistor consisting of two PN junctions in such as way that a P-type semiconductor sandwiching by two N-type Semiconductors.

• These back to back PN junction diodes are known as the collector-base junction and base-emitter junction.
• The base-emitter junction is always connected in forward bias in Active mode therefore it has a barrier voltage (VBE) of + 0.7 volts for Si and + 0.3 volts for Ge.
• The collector-base junction is always connected in reverse bias in Active mode.
• In this transistor, electrons are the majority carrier for the input current (IE) and holes are the minority carrier.

Transistors

• A transistor is an electronic device that has 3 terminals.
• The transistor is made up of semiconductor materials such as Silicon and Germanium.
• BJT and FET are examples of transistors.

​

Bipolar Junction Transistor (BJT)

• BJT is a semiconductor device that is constructed with 3 doped semiconductor Regions i.e. Base, Collector & Emitter separated by 2 p-n Junctions.
• BJTs are current-driven devices. The current through the two terminals is controlled by a current at the third terminal (base).
• BJT is a bipolar device (current conduction by both types of carriers, i.e. majority and minority electrons and holes) It has a low input impedance.

Junction Field Effect Transistor (JFET)

• JFET is a unipolar voltage-controlled semiconductor device with three terminals: source, drain, and gate.
• In JFET, the current flow is due to the majority of charge carriers. Hence, it is unipolar.

The correct answer is Both A and R is true, and R is the correct explanation of A..

Key Points

• Insulated Gate Bipolar Transistor (IGBT): An IGBT is a semiconductor device commonly used in power electronics. It combines the high input impedance and fast switching capabilities of a MOSFET with the low on-state conduction losses of a BJT.
• Assertion (A): An IGBT combines the advantages of BJTs and MOSFETs.
  • Explanation: This statement is true. IGBTs are designed to leverage the benefits of both BJTs and MOSFETs, making them highly efficient in various applications.
• Reason (R): An IGBT has high input impedance, like MOSFETs, and low on-state conduction losses like BJTs.
  • Explanation: This statement is also true. The high input impedance of IGBTs makes them easy to drive, similar to MOSFETs, while their low on-state conduction losses are akin to those of BJTs.

Hence, statement 1 is correct.

Hence, statement 2 is correct.

 Additional Information

• IGBT Characteristics:
  • IGBTs are used in applications like inverters, electric vehicles, and various power supply circuits due to their efficiency and reliability.
  • They can handle high voltages and currents, making them suitable for high-power applications.
• MOSFET Characteristics:
  • MOSFETs are known for their high switching speed and efficiency in low-voltage applications.
  • They are widely used in digital circuits and low-power electronics.
• BJT Characteristics:
  • BJTs are known for their ability to handle high currents and are often used in analog circuits.
  • They have low on-state voltage drop, which makes them efficient in high-current applications.

The correct answer is stability factor

Concept:
The maintenance of the operating point can be specified by: stability factor
The Stability Factor (S) of a transistor circuit is a measure of its ability to maintain the biasing conditions despite variations in temperature, transistor parameters, and other factors. A higher Stability Factor indicates better stability and less sensitivity to variations.
Stability factor (S):

It is the rate of change of collector current (IC) with respect to the reverse saturation current of collector junction (ICO), i.e.

\(S = \frac{{\partial {I_C}}}{{\partial {I_{CO}}}}\)       ---(1)

We know that collector current is given by:

IC = β IB + (1 + β) ICO     ---(2)

Differentiating equation (2) w.r.t IC and treating β as constant, we can write:

\(1 = \beta \times \frac{{\partial {I_B}}}{{\partial {I_C}}} + \left( {1 + \beta } \right)\frac{{\partial {I_{CO}}}}{{\partial {I_C}}}\)

\(\frac{{\partial {I_{CO}}}}{{\partial {I_C}}} = \frac{{1 - \beta \frac{{\partial {I_B}}}{{\partial {I_C}}}}}{{1 + \beta }}\)

\(\frac{{\partial {I_C}}}{{\partial {I_{CO}}}} = \frac{{1 + \beta }}{{1 - \beta \frac{{\partial {I_B}}}{{\partial {I_C}}}}}\)    ---(3)

From (1) & (3), we get:

Stability factor (S) will be:

\(S = \frac{{\partial {I_C}}}{{\partial {I_{CO}}}} = \frac{{1 + \beta }}{{1 - \beta \frac{{\partial {I_B}}}{{\partial {I_C}}}}}\)

Observations:

\( - 1 < \frac{{\partial {I_B}}}{{\partial {I_C}}} < 0\)

1 < S < 1 + β

For any BJT circuit, \(\frac{{\partial {I_B}}}{{\partial {I_C}}}\) lies between 0 & -1, therefore, the stability factor S lies between 1 and 1 + β.

A smaller stability factor indicates better stability in collector current. ∴ S should be smaller and ideally S = 1.

Common base configuration:

Input Terminal Emitter – Base (EB)

Output Terminal Collector – Base (CB)

Since for Amplification Application in BJT

EB Junction → Forward Biased (Low Impedance)

CB Junction → Reversed Biased (High Impedance)

Various transistor configurations and their characteristics are shown in the table

Common base configuration:

Input Terminal Emitter – Base (EB)

Output Terminal Collector – Base (CB)

Since for Amplification Application in BJT

EB Junction → Forward Biased (Low Impedance)

CB Junction → Reversed Biased (High Impedance)

The various characteristics of different configuration are shown 

Bipolar junction transistor

• The Bipolar Junction Transistor (BJT) is a three-terminal device i.e. Base, Emitter, and Collector.
• There are two main types of bipolar junction transistors the NPN and the PNP transistor.
• The emitter is a heavily doped region of the BJT transistor, providing the majority of carriers into the base region.
• The base region is a thin, lightly doped region sandwiched between the emitter and collector.
• The majority of carriers from the emitter pass through the base region and its flow can be externally controlled.

Working configuration of BJT

The correct answer is option 1) 5 mA

Concept:

The common base DC current gain is a ratio of the value of the transistor's collector current to the value of the transistor's emitter current,

i.e.  α= \(I_c\over I_e\) ---(1)

Also, the common-emitter current gain is the ratio of the value of the transistor's collector current to the value of the transistor's base current in a transistor,

i.e.  β= \(I_c\over I_b\) ---(2)

Using Equations (1) and (2), we get:

\(β =\frac{α}{1-α}\)

For a transistor, the relation between the collector, base, and emitter current is:

I_E = I_B + I_C

I_C = β I_B

I_E = (β + 1) I_B

Calculation:

With α_dc = 0.98,

β will be:

\(β =\frac{0.98}{1-0.98}=49\)

Now, the emitter current will be:

I_E = (49 + 1) 100 μA

I_E = 5000 μA 

I_E = 5 mA

Concept:

For a transistor, the base current, the emitter current, and the collector current are related as:

IE = IB + IC

where IC = αIE

α = Current gain of the transistor

Calculation:

Given,

IC = 0.95 mA

IB = 0.05 mA

∴ IE = IB + IC = 0.05 + 0.95 = 1 mA

From the above concept,

IC = αIE

\(\alpha=\frac{I_C}{I_E}=\frac{0.95}{1}=0.95\)

Region of operation of BJT:

In a given circuit, the region of the transistor can be decided through the following procedure.

• Assume that the transistor is in the saturation region.
• Calculate Base current (IB) and collector current (IC) separately.
• Calculate the minimum Base current (IB)min required to operate the transistor in saturation.
• Compare Base current (IB) and (IB)min​

1. IB  ≥   (IB)min, Transistor is in the Saturation region.
2. IB < (IB)min, Transistor is in the Active region
3. IB  < 0, Transistor is in the Cut-off region.

Formula:

In saturation,

Base-Emitter voltage = VBE = 0.8 V

Collector-Emitter voltage VCE = 0.2 V

(IB)min = (IC)sat / β  ----(1)

(IC)sat  = Collector current in Saturation 

β = DC current gain

Calculation:

Assume transistor in Saturation region.

Apply KVL in the outer loop:

5 - I_C(Sat) x 5kΩ - V_CE(Sat) = 0

\(I_{C(Sat)}=\frac{4.8}{5k\Omega}\)

I_C(Sat) = 0.96 mA

From equation (1);

\(I_{B(Min)}=\frac{0.96mA}{100}\)

(IB)min = 9.6 μA

Now, Apply KVL in Emitter Loop; 

5 - 100I_B - 0.7 = 0

I_B = 43 μA

We observe that;

IB  ≥   (IB)min

Hence, BJT is in saturation region.

Concept:

The common base DC current gain is a ratio of the value of the transistor's collector current to the value of the transistor's emitter current, i.e.

\(α = \frac{{{I_c}}}{{{I_e}}}\)   ---(1)

Also, the common-emitter current gain is the ratio of the value of the transistor's collector current to the value of the transistor's base current in a transistor, i.e.

\(β = \frac{{{I_c}}}{{{I_b}}}\)   ---(2)

Using Equations (1) and (2), we get:

\(β =\frac{α}{1-α}\)

For a transistor, the relation between the collector, base, and emitter current is:

IE = IB + IC

IC = β IB

IE = (β + 1) IB

Calculation:

We have,

β = 49

From the above concept,

\(β =\frac{α}{1-α}\)

or \(49=\frac{α}{1-α}\)

or, 49 - 49α = α

or, 

Hence,50α = 49

α = 49/50 = 0.98

The correct answer is (option 4) i.e. holes flow in the emitter region

Concept:

The arrow on the symbol for bipolar transistors indicates the PN junction between base and emitter and points in the direction in which conventional current travels, i.e. the direction of holes.

NPN:

When the emitter-base is forward biased, holes from the base (p-type) start to flow to the emitter side (n-type) and electrons start to flow from the emitter to the base. The direction, however, represents the direction of the hole flow.

PNP:

When the emitter-base junction is forward biased, holes from the emitter starts to flow to the base. The direction of the arrow also indicates the same.

Concept:

Active mode:

In the active mode, one junction (emitter to base) is forward biased and another junction (collector to base) is reverse biased.

Method to check region of operation:

Step 1 : Assume BJT to be in saturation region (V_CE = 0.2 V), and calculate I_Csat from the circuit.

Step 2: Calculate I_Bmin = ICsat/ β 

Step 3: Calculate I_B from the given circuit.

Step 4: If,  IB > IBmin → Saturation region

                 IB < IBmin → Active region

Analysis:

Assume,

V_BE = 0.7 V

V_CEsat = 0.2 V

ICsat = ( V_CC - V_CEsat)/ R_C

⇒ ICsat = ( 9 - 0.2)/2 K

⇒ ICsat = 4.4 mA

IBmin = ICsat/ β = 4.4/50 = 0.088 mA

I_B = ( 9 - 0.7)/300K = 0.0276 mA

 IB < IBmin → Active region

Additional Information Saturation mode:

In the saturation mode, both the junctions of the transistor (emitter to base and collector to base) are forward biased.

Cutt-off mode:

In the cutoff mode, both the junctions of the transistor (emitter to base and collector to base) are reverse biased.

Inverted mode:

In the inverted mode ( Reverse active mode), one junction (emitter to base) is reverse biased and another junction (collector to base) is forward biased.