(a) Describe briefly the functions of the three segments of n-p-n transistor. (b) Draw the circuit arrangement

Question

Q. (a) Describe briefly the functions of the three segments of n-p-n transistor.

(b) Draw the circuit arrangement for studying the output characteristics of n-p-n transistor in CE configuration. Explain how the output characteristics is obtained.

(OR)

Draw the circuit diagram of a full wave rectifier and explain its working. Also, give the input and output waveforms.

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Part (a)Concept understanding — Common Emitter Configuration

Common Emitter Configuration – First Principles

Imagine you have a water tap. A small turn of the handle produces a large flow of water. That's amplification — a small input controlling a large output. The common emitter configuration is the transistor's version of that tap, and it's the most widely used amplifier arrangement in electronics.

The Intuition: Why "Common Emitter"?

In any transistor circuit, one terminal must be shared between the input and the output. In the common emitter configuration, that shared terminal is the emitter. The input signal goes between base and emitter. The output is taken between collector and emitter. The emitter is common to both — hence the name.

Think of the emitter as the ground reference for the entire circuit. The base is the control terminal (the tap handle), and the collector is where the amplified output appears.

The Precise Statement

In a common emitter configuration:

Input: between base and emitter
Output: between collector and emitter
Emitter: common to both input and output circuits

The transistor is biased so that the base-emitter junction is forward-biased (like a diode conducting) and the collector-base junction is reverse-biased. This is the normal active region of operation.

What Makes It Special: Both Current and Voltage Gain

Here's the key insight that makes this configuration so powerful. A small change in base current ( $\Delta I_B$ ) causes a much larger change in collector current ( $\Delta I_C$ ). The ratio is the transistor's current gain:

\beta = \frac{\Delta I_C}{\Delta I_B}

Typical values of $\beta$ range from 50 to 300. So a tiny signal current at the base produces a large current at the collector.

But that's not all. The output is taken across a load resistor $R_C$ connected to the collector. The changing collector current flows through $R_C$ , producing a voltage change:

\Delta V_{out} = \Delta I_C \times R_C

Meanwhile, the input voltage change is just the small base-emitter voltage change (about 0.6 V for silicon, and the signal variation is only millivolts). The result is that the output voltage swing is much larger than the input voltage swing — you get voltage gain as well.

A_V = \frac{\Delta V_{out}}{\Delta V_{in}} = -\frac{\beta R_C}{r_{be}}

The negative sign means the output is inverted — when the input goes up, the output goes down. This 180-degree phase shift is a hallmark of the common emitter amplifier.

The Trade-Off …

Part (b)Concept understanding — Full Wave Rectifier

Full-Wave Rectifier: From Intuition to Precision

Imagine you have a tap that gives water in alternating bursts — forward, then backward, then forward again. You want a steady stream that always flows in one direction. A half-wave rectifier is like a one-way flap that only lets the forward bursts through, wasting the backward ones. A full-wave rectifier is smarter: it flips the backward bursts around so they also flow forward. Nothing is wasted.

That is the core idea. The AC voltage swings positive and negative. A full-wave rectifier takes both halves of the cycle and makes them contribute to a unidirectional (DC) output.

How It Works (The Intuition)

A full-wave rectifier uses either:

Two diodes and a centre-tapped transformer, or
Four diodes in a bridge configuration (the bridge rectifier — far more common).

In both cases, the diodes are arranged so that during the positive half-cycle, one pair conducts, and during the negative half-cycle, the other pair conducts — but the current through the load always flows the same way.

Tip

Think of the bridge rectifier as a "traffic roundabout" for current. No matter which direction the AC input pushes, the diodes steer the current so it always exits the same way through the load.

The Precise Statement

A full-wave rectifier is a circuit that converts the entire input AC waveform (both positive and negative half-cycles) into a pulsating DC output. The output voltage is always of the same polarity, and its ripple frequency is twice the input AC frequency.

For a sinusoidal input $V_i = V_m \sin(\omega t)$ , the output voltage (ideal diodes) is:

V_o = |V_m \sin(\omega t)|

That absolute value is the mathematical signature of full-wave rectification.

Key Result: Ripple Frequency

If the input AC has frequency $f$ (e.g., 50 Hz), the output ripple frequency is:

f_{\text{ripple}} = 2f

Why? Because each input cycle gives two output pulses — one from the positive half and one from the inverted negative half. So for 50 Hz mains, you get 100 pulses per second. This makes filtering much easier than in a half-wave rectifier (where ripple frequency equals $f$ ).

Important

Ripple frequency = $2 \times$ input frequency. This is a direct consequence of using both halves of the AC cycle.

Average (DC) Output Voltage

For a full-wave rectifier with a sinusoidal input of peak voltage $V_m$ , the average (DC) output voltage is:

V_{\text{dc}} = \frac{2V_m}{\pi}

Compare this to a half-wave rectifier, where $V_{\text{dc}} = V_m/\pi$ . The full-wave gives double the average output for the same peak input — another reason it is more efficient.

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Part (a): Emitter (heavily doped) injects electrons, thin lightly-doped base controls the flow, collector gathers them; CE output characteristics are $I_C$ – $V_{CE}$ curves at fixed $I_B$ with saturation, active and cut-off regions. Part (b): A full-wave rectifier (centre-tapped transformer + two diodes) rectifies both halves of the AC input, giving pulsating DC of average $\dfrac{2V_m}{\pi}$ and ripple frequency $2f$ .

Circuit arrangement for studying the output characteristics of an n-p-n transistor in CE configuration: base-emitter loop with V_BB, rheostat R_B and a microammeter (I_B) forward-biasing the base, and collector-emitter loop with V_CC, R_C, a milliammeter (I_C) and a voltmeter (V_CE) reverse-biasing the collector, with the emitter common to both loops.

Part (a)

Functions of the three segments

An n-p-n transistor is two p-n junctions sharing a common central p-region.

Emitter: most heavily doped so it can inject a large number of majority carriers (electrons) into the base. Its job is to supply charge carriers.
Base: made very thin and lightly doped. Only a tiny fraction of the injected electrons recombine here (giving a small base current); the rest are swept on to the collector. The base controls the collector current.
Collector: moderately doped, physically the largest region so it can dissipate the heat generated. It collects the carriers that diffuse across the base.

Output characteristics (CE)

Circuit: connect a variable supply $V_{BB}$ with a resistor $R_B$ to forward-bias the base–emitter junction, and a variable supply $V_{CC}$ with $R_C$ to reverse-bias the collector–emitter path. The emitter terminal is common to both loops.

Method: fix the base current $I_B$ using $V_{BB}$ ; then increase $V_{CE}$ in steps using $V_{CC}$ and measure the collector current $I_C$ each time. Repeat the whole run for several fixed $I_B$ values.

Result: plotting $I_C$ against $V_{CE}$ for each $I_B$ gives a family of curves:

Saturation region — for small $V_{CE}$ , $I_C$ rises sharply.
Active region — $I_C$ becomes almost independent of $V_{CE}$ and $I_C \approx \beta I_B$ ; the transistor works as an amplifier here. …

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Same / Similar Concept — real previous-year questions on the same or a closely similar concept, not this exact question.

CBSE 2025Set ANNUAL1 markMCQ
$Q. The function of a rectifier is$
(i) to convert AC to DC
(ii) to convert DC to AC
(iii) both
(i) and
(ii)
(iv) None of the above
›Reveal solutionSolution

A rectifier converts AC into DC.

A p-n junction diode conducts only when forward biased. In a rectifier this one-way behaviour allows current in only one direction during the AC cycle, so the pulsating output is unidirectional, i.e. direct current. Thus the function of a rectifie …

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CBSE 2024Set A1 markMCQ
$Q. In full wave rectifier, if input frequency is 50 Hz, then output frequency will be (A) 25 Hz (B) 50 Hz (C) 100 Hz (D) 200 Hz$
›Reveal solutionSolution

A full-wave rectifier doubles the input frequency, giving 2 × 50 = 100 Hz → option (C).

In a full-wave rectifier, both halves of each AC input cycle are converted into output pulses of the same polarity. So for every one input cycle there are two output pulses, and the fundamental frequency of the rectified output is twice the input frequency:

…

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CBSE 2024Set ANNUAL1 markMCQ
$\nu$
(a) $\nu/2$
(b) $\nu$
(c) $2\nu$
(d) $0$
›Reveal solutionSolution

A full-wave rectifier produces one output pulse per half-cycle of the input, doubling the ripple frequency.

A full-wave rectifier conducts during both the positive and the negative half-cycles of the input AC (using two diodes with a centre-tapped transformer, or a bridge of four diodes), producing one output current pulse for each half-cycle of the input. Since there are two half-cycl …

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CBSE 2023Set BS1 markMCQ
$Q. An N-P-N transistor is arranged as in the following figure. This circuit is of:$
i) common-base amplifier
ii) common-emitter amplifier
iii) common-collector amplifier
iv) none of these
›Reveal solutionSolution

The input is fed to the base–emitter and the output taken from the collector with the emitter shared by both loops — a common-emitter amplifier.

In a transistor amplifier the terminal shared between the input and output circuits names the configuration. Here the AC signal $V_i$ is applied across the base–emitter junction and the amplified output $V_o$ is taken across the collector load, while the emitter is common to both loops. This …

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CBSE 2019Set ANNUAL1 mark
$Q. Which type of transistor mode will you choose to use for maximum amplification, common base mode or common emitter mode?$
›Reveal solutionSolution

Common emitter (CE) configuration gives the maximum amplification because it has both current gain (β) and voltage gain greater than 1, unlike common base (CB) mode.

A transistor can be connected in three configurations — common base (CB), common emitter (CE), and common collector (CC) — depending on which terminal is common to the input and output circuits.

In the common base mode, the current gain $\alpha = I_C/I_E$ is always slightly less than 1 (since $I_C < I_E$ ), so although the voltage gain can be large, the overall power gain is comparatively modest.

…

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Q.(a) Describe briefly the functions of the three segments of n-p-n transistor.

Part (a)

Functions of the three segments

Output characteristics (CE)

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