11.3 Application of a transistor

  Semi-conductor Devices and Electronics

    1.0 Introduction
    2.0 Types of semiconductor
    3.0 Mass action law
    4.0 Electrical conductivity in semiconductor
    5.0 $p-n$ junction
    6.0 Breakdown voltage
    7.0 $I-V$ characteristics of a $p-n$ junction
    8.0 Rectifier
    9.0 Light emitting diode (LED)
    10.0 Zener diode
    11.0 Transistor
    12.0 Boolean identities
    13.0 Logic gates
    14.0 De Morgan's theorem

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11.3 Application of a transistor

The application of transistor are;

• Switch
• Common emitter amplifier
• Common base amplifier
• Oscillator

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11.3.1 Transistor as a switch

When the transistor is used in the cut off region or saturation region, it acts as a switch.

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11.3.2 Common emitter amplifier

In the common emitter transistor amplifier, the input signal voltage and the output collector voltage are $180^\circ $ out of phase.

DC current gain

It is defined as the ratio of the collector current $\left( {{I_C}} \right)$ to the base current $\left( {{I_B}} \right)$
$${\beta _{DC}} = \frac{{{I_C}}}{{{I_B}}}$$

AC current gain

It is defined as ratio of change in collector current $\left( {\Delta {I_C}} \right)$ to the change in base current $\left( {\Delta {I_B}} \right)$.
$${\beta _{DC}} = \frac{{\Delta {I_C}}}{{\Delta {I_B}}}$$

Voltage gain

It is defined as the ratio of output voltage to the input voltage.
$$\begin{equation} \begin{aligned} {A_v} = \frac{{{V_0}}}{{{V_i}}} \\ {A_v} = - {\beta _{AC}}\left( {\frac{{{R_0}}}{{{R_i}}}} \right) \\\end{aligned} \end{equation} $$

Where,
$R_o$: Output resistance
${R_i}$: Input resistance

Note: -ve sign represents that output voltage is opposite in phase with the input voltage.

Power gain

It is defined as the ratio of the output power to the input power.
$$\begin{equation} \begin{aligned} {A_p} = \frac{{{\text{Out power }}\left( {{P_o}} \right)}}{{{\text{Input power }}\left( {{P_i}} \right)}} \\ {A_p} = {\beta _{DC}}{A_v} \\\end{aligned} \end{equation} $$

Note: Voltage gain (in $dB$) $ = 20{\log _{10}}\frac{{{V_o}}}{{{V_i}}} = 20{\log _{10}}{A_v}$

Power gain (in $dB$) $ = 10{\log _{10}}\frac{{{P_o}}}{{{P_i}}}$

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11.3.3 Common base amplifier

In common base transistor amplifier, the input signal voltage and the output collector voltage are in the same phase.

DC current gain

It is defined as the ratio of collector current $\left( {{I_C}} \right)$ to the emitter current $\left( {{I_E}} \right)$.
$${\alpha _{DC}} = \frac{{{I_C}}}{{{I_E}}}$$

AC current gain

It is defined as the ratio of change in collector current $\left( {\Delta {I_C}} \right)$ to the change in emitter current $\left( {\Delta {I_E}} \right)$.
$${\alpha _{AC}} = \frac{{\Delta {I_C}}}{{\Delta {I_E}}}$$

Voltage gain

It is defined as the ratio of output voltage to the input voltage.
$$\begin{equation} \begin{aligned} {A_v} = \frac{{{V_o}}}{{{V_i}}} \\ {A_v} = {\alpha _{AC}}\left( {\frac{{{R_o}}}{{{R_i}}}} \right) \\\end{aligned} \end{equation} $$

Power gain

It is defined as the ratio of output power to the input power.
$$\begin{equation} \begin{aligned} {A_p} = \frac{{{\text{Output power }}\left( {{P_o}} \right)}}{{{\text{Input power }}\left( {{P_i}} \right)}} \\ {A_p} = {\alpha _{DC}}{A_v} \\\end{aligned} \end{equation} $$

Relationship between $\alpha $ and $\beta $
$$\begin{equation} \begin{aligned} \beta = \frac{\alpha }{{1 - \alpha }} \\ \alpha = \frac{\beta }{{1 + \beta }} \\\end{aligned} \end{equation} $$

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11.3.4 Transistor as an oscillator

An oscillator generates AC output signal without any input AC signal.

An oscillator is a self-sustained amplifier in which part of the output is fed back to the
input in the same phase. This process is also known as positive feedback.

The block diagram of an oscillator is shown in the figure.

The circuit diagram of the tuned collector oscillator is shown in the figure below.

The frequency of the oscillation is given by, $$f = \frac{1}{{2\pi \sqrt {LC} }}$$