Vacuum tube amplifiers

Introduction

Any electronic device which rises the strength of weak electrical signal is known as amplifier where as,the process of rising the strength of a weak electrical signal is known as amplification.this phenomena of amplification is necessary throughout in communication system.for instance the radio signal picked up by the receiver antenna possess extremely small power of the order of milliwatts. This power is not adequate to operate the loudspeaker. Therefore,the signal received by the antenna is amplified to several times in order to increase its strength sufficiently to operate the loudspeaker of the radio receiver. Similarly,amplifiers find their wide implication in industrial and commercial fields. e.g. public address system, intercommunication system, television receiver, radio transmitters and receivers etc.

as discussed earlier vacuum tubes are capable to amplify the weak electrical signal.

Vacuum tube amplifiers

the amplifiers employing vacuum tubes for rising the strength of weak electrical signal are called vacuum tube amplifiers.

The vacuum tubes containing the control grid can act as amplifiers.when a weak electrical signal is applied between control grid and cathode and amplified output is obtained across the load connected in the plate circuit. In fact,in this tube a small change in great potential creates a large change in plate current which produce a large output voltage across the load resistor. Thus, an amplified electrical signal is obtained across the load.

The line diagram of a public address system using an amplifier is shown in figure.

Faithful amplification

The process of increasing the strength of weak signal without changing the shape of the signal is called faithful amplification.

Faithful amplification is the basic requirement of a vacuum tube amplifier. In order to achieve faithful amplification,the control grid of the amplifying tube should always be kept at negative potential with respect to cathode a respective the signal potential.to do show a negative growth potential is applied in the great circuit in addition to the signal voltage as shown in figure. This negative DC voltage applied to the grid is known as a grid biasand its magnitude is such that it always keep the great negative with respect to cathode regardless the polarity of signal.

Triode as an amplifier

A simple triode amplifier circuit is shown in figure when a weak signal eg is applied in the grid circuit it produces a large change in plate current. As the value of load resistor RL is quite high, a large voltage drop occur across it. Thus, abheek signal applied in the grid circuit appears in the simplified form in the plate circuit. The bias battery ec connected in the great circuit and sure to keep the grid always negative with respect to cathode.

The working of triode Asin amplifier can be summed up as under:

• With great bias Ecc and plate voltage Ebb when no signal is applied, DC plate current IP flaws in the plate circuit called Quiescent current.
• When signal eg is applied in the grid circuit, the plate current where is above And below IP according to the signal. This is a component due to signal voltage is represented as ip.
• The resultant plate current in under operating condition is the resultant of the two current

        ib=Ip+ip

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