Phototube Amplifiers

Methods of Current Amplification.—Since the current generated in most photocells is very low and is usually measured in microamperes, it is evident that for practical operation a way must be found to increase this feeble current, for practical relay operation.

Therefore in order to increase the power output of the photocell a method of amplification very similar to that associated with radio circuits is employed.

The amplifier tubes may be of the high vacuum or of the gaseous type. .When the photocell is connected to an amplifier circuit the current can be measured in milliamperes instead of in micro-amperes, as previously, i.e. the current is now one thousand times larger than before.

Two types of photo circuits are shown in figs. 1 and 2. Fig. 1 shows a simple circuit in which the current is measured in microamperes and fig. 2 shows an amplifier circuit where the current is measured with a milli-ampmeter.

The amplification can be arranged as required in various ways so as to give the necessary size of current variation for proper functioning of accessory apparatus.

Phototube Reverse Current

Fig. 1.—Photo-electric cell circuit. When the photocell is connected as shown, the current is too small for practical use.

BATTERY

Fig. 1.—Photo-electric cell circuit. When the photocell is connected as shown, the current is too small for practical use.

Phototube Reverse Current

Fig. 2.—Simple battery operated one stage amplifier circuit for use with a photo-electric cell. When the photo-electric cell is connected in this manner the current will operate a sensitive relay.

BATTERY

Fig. 2.—Simple battery operated one stage amplifier circuit for use with a photo-electric cell. When the photo-electric cell is connected in this manner the current will operate a sensitive relay.

For example there are single stage amplifiers of type as shown in fig. 2, multi-stage amplifiers, amplifiers designed for direct current operation, and amplifiers for A.C. operation.

The amplifier tubes may be connected to the photocell in such a way so as to increase or decrease the relay or output current when the amount of light on the phototube is varied. Sometimes a circuit connected to increase the output current of the amplifier tube when the light rays on the phototube increases, is called a forward circuit, and a circuit connected to decrease the current output of the amplifier tube when the illumination of the phototube increases is called a reverse circuit. See figs. 3 and 4.

PHOTOTUBE

GRID : RESISTANCE;

-vww ) AMPLIFIER TUBE

FILAMENT BATTERY

TO OUTPUT METER

Fig. 3.—Typical battery operated amplifier. Forward circuit for phototube operation.

PHOTOTUBE :

GRID • RESISTANCE:

AMPLIFIER TUBE

AMPLIFIER TUBE

Hjlp-f-

Fl LAMENT BATTERY

TO OUTPUT METER

Hill

Fig. 4.—Typical battery operated amplifier. Reverse circuit for phototube operation.

As seen from the diagrams there is a striking similarity between the two circuits. The forward circuit is more commonly

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