MHT-CET : Physics Entrance Exam

MHT - CET : Physics - Thermionic Emission Page 3

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5.

Construction and Working of a Triode

The triode consists of three electrodes:

1. a cathode
2. a plate or an anode and
3. a grid.

These electrodes are enclosed in an evacuated tube.

(b) Symbol of a high
vacuum triode

6.

Triode as an Amplifier

An amplifier converts a low input signal voltage into a high output signal voltage. It is necessary that the waveform of the signal remains unchanged.

In the triode, a small change in grid voltage can bring about a large change in voltage in the plate circuit. This property makes the triode a good amplifier.

Triode as an amplifier

The circuit consists of two parts:

1. Grid circuit: In this part of the circuit, the grid is given a suitable constant negative potential called grid bias (G.B.) with respect to cathode. In addition to this, a small alternating signal voltage which is to be amplified is also given to the grid.
2. Plate circuit: A plate is given a high positive voltage from the high tension (H.T.) battery. The filament is heated by a low tension (L.T.) battery.

A high load resistance RL is connected in series with the plate.

Working

The characteristic curve of a triode showing the variation of plate current (I
P) with the grid voltage (Vg) is as shown below:

1. Consider the straight-line portion ABC of the curve.
2. The input signal dvg which is alternating, is superimposed over the grid bias and produces variation in grid voltage.
3. During the positive half cycle of the input voltage, the grid becomes less negative, hence there is an increase in the plate current, IP.
4. During negative half cycle of the input voltage, the grid becomes more negative and hence there is a decrease in the plate current IP.
5. A large variation dIP is produced in the plate current. The variation in output voltage across RL is given by

dVO = dIP ´ RL

.

1. The large variation produced in the output voltage across RL is exactly similar to the smaller variation in the input signal.
2. In this way, amplification of the input signal voltage is produced which is given by

AV =

 output voltage input voltage

=

 dVO dVg

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