ETE (Autumn – 2022)

(1)

(a) Differences

i. NPN and PNP Transistor (3 Marks)

Property	NPN Transistor	PNP Transistor
Structure	Two n-type and one p-type	Two p-type and one n-type
Current Flow	Electron flow	Hole flow
Biasing	Positive base voltage	Negative base voltage
Symbol	Arrow points out from emitter	Arrow points into emitter

ii. CB and CE Configuration of BJT (3 Marks)

Configuration	CB (Common Base)	CE (Common Emitter)
Input	Emitter	Base
Output	Collector	Collector
Gain	Low input impedance, high output	High voltage and current gain

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(b) Operation of a PNP Transistor (4 Marks)

A PNP transistor has two p-type materials (emitter and collector) and one n-type (base).

1. When the emitter is connected to positive voltage and base to negative, holes flow from the emitter to the base.
2. These holes combine with electrons in the base, creating a small base current.
3. Most holes pass to the collector, producing a larger collector current.

The base current controls the larger collector current, enabling amplification.

Neat Diagram:

• Label: Emitter, Base, Collector.
• Show current directions.

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(c) Calculations (3 Marks)

Given:

• $I_c = 5.2 \, \text{mA}$
• $I_{CBO} = \frac{X}{25} \, \text{mA} = \frac{11}{25} = 0.44 \, \text{mA}$

Formulas:

• $\beta = \frac{I_c}{I_b}$, $\alpha = \frac{I_c}{I_E}$, $I_E = I_c + I_b$.

1. Find $I_E$:
   $I_E = I_c + I_b = 5.2 + 0.44 = 5.64 \, \text{mA}$.

2. Find $\alpha$:
   $\alpha = \frac{I_c}{I_E} = \frac{5.2}{5.64} \approx 0.922$.

3. Find $\beta$:
   $\beta = \frac{\alpha}{1 - \alpha} = \frac{0.922}{1 - 0.922} \approx 11.82$.

Answer:

• $\alpha = 0.922$
• $\beta = 11.82$
• $I_E=5.64\text{mA}$

(2)

(a) Physical Reason for Stability in Emitter Biasing (3 Marks)

Emitter biasing provides more stability because it uses a resistor in the emitter leg, which creates negative feedback.

1. If the transistor's current ($I_C$) increases due to temperature, the emitter current ($I_E$) also increases.
2. This causes a higher voltage drop across the emitter resistor ($R_E$), reducing the base-emitter voltage ($V_{BE}$).
3. A lower $V_{BE}$ decreases the base current ($I_B$), which reduces $I_C$.

(4)

(a) i. FET is a Voltage Controlled Device, while BJT is Current Controlled (4 Marks)

• FET (Field-Effect Transistor): The operation of a FET depends on the voltage applied to the gate terminal. The gate voltage controls the flow of current between the source and drain, making it a voltage-controlled device.

• BJT (Bipolar Junction Transistor): The operation of a BJT depends on the current flowing into the base terminal, which controls the current between the collector and emitter. Therefore, a BJT is a current-controlled device.

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(b) Operation of N-Channel Depletion Type MOSFET (4 Marks)

An N-Channel Depletion-type MOSFET operates as follows:

1. Structure: It has an n-type channel between the source and drain, with a p-type gate.
2. Without Gate Voltage: When no voltage is applied to the gate, a small current can flow between the source and drain because the n-type channel is naturally conducting.
3. When Negative Gate Voltage is Applied: Applying a negative voltage to the gate causes the n-type channel to deplete of charge carriers (electrons), reducing the current flow between the source and drain.
4. Normally-On Device: Even with no gate voltage, it allows current to flow, but applying negative voltage can "turn it off."

OR

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(b) Operation of N-Channel Enhancement Type MOSFET (4 Marks)

An N-Channel Enhancement-type MOSFET operates as follows:

1. Structure: It has an n-type channel between the source and drain, but the channel is initially absent or very thin.
2. Without Gate Voltage: When no voltage is applied to the gate, no current can flow between the source and drain because the channel is non-conductive.
3. When Positive Gate Voltage is Applied: Applying a positive voltage to the gate attracts electrons into the channel, forming a conductive n-type channel.
4. Normally-Off Device: This MOSFET is "off" when there is no gate voltage and "on" when a sufficient positive voltage is applied to the gate.

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(c) Difference Between MOSFET and JFET with Respect to Their Construction (2 Marks)

Feature	MOSFET	JFET
Gate Material	Insulated gate (oxide layer)	Metal gate, directly connected to the channel
Channel Type	Can be n-type or p-type	Usually n-type or p-type
Gate Control	Voltage-controlled	Voltage-controlled (via depletion region)

(4)

(a) Hybrid Parameters of Small Signal Model of BJT (5 Marks)

The Hybrid Parameters describe the small signal behavior of a BJT. They are:

1. h₁ (Input Impedance)
   This is the ratio of the input voltage to the input current in the common-emitter configuration. It is given by:

   $$h_{in} = \frac{V_{in}}{I_{in}}$$

   Typically, it is measured in ohms (Ω) and is represented as h₁.

2. h₂ (Current Gain)
   This is the ratio of output current to input current in the common-emitter configuration. It is given by:

   $$h_{fe} = \frac{I_C}{I_B}$$

   This is also called current gain and is dimensionless.

3. h₃ (Voltage Gain)
   This is the ratio of output voltage to input voltage when the load is connected. It is given by:

   $$h_{oe} = \frac{V_{out}}{I_{in}}$$

   This parameter shows the voltage gain due to the transistor’s output resistance.

4. h₄ (Output Impedance)
   This is the ratio of the output voltage to the output current in the common-emitter configuration. It is typically represented by:

   $$h_{ob} = \frac{V_{out}}{I_{out}}$$

   This is a measure of the transistor’s output resistance.

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(b) Common-Emitter Hybrid Equivalent Circuit and Common-Base re Model (5 Marks)

(i) Common-Emitter Hybrid Equivalent Circuit

Given:

• $I_E = 2.5 \, \text{mA}$
• $h_{fe} = 140$
• $h_{infinity} = 20 \, \mu \text{S}$
• $h_{ob} = 0.5 \, \mu \text{S}$

The common-emitter hybrid equivalent circuit consists of:

• Input impedance $h_{in}$: $h_{in} = \frac{1}{h_{infinity}}$
• Output impedance $h_{ob}$: $h_{ob} = 0.5 \, \mu S$
• Current gain $h_{fe}$: $h_{fe} = 140$
• Transconductance: $g_m = \frac{I_C}{V_T} \approx \frac{2.5 \, \text{mA}}{26 \, \text{mV}} \approx 0.096 \, \text{S}$

The common-emitter hybrid model includes:

1. Input resistance: $h_{in} = 20 \, \mu \text{S}$
2. Output resistance: $h_{ob} = 0.5 \, \mu \text{S}$
3. Transconductance $g_m$: $g_m = 0.096 \, \text{S}$

The small signal model uses these parameters to predict the behavior of the circuit.

(ii) Common-Base re Model

The common-base re model is simpler and focuses on the transistor’s input impedance and current gain when the base is common to both input and output.

1. Transconductance $g_m$: $g_m = 0.096 \, \text{S}$
2. Input resistance: $r_e = \frac{V_T}{I_E} \approx \frac{26 \, \text{mV}}{2.5 \, \text{mA}} = 10.4 \, \Omega$

The common-base model shows how input and output impedance are connected and allows for modeling of current flow with respect to these parameters. The current gain in common-base configuration is typically low, while the output impedance is determined by $r_e$ and $g_m$.