Silicon

diodes

Fig, 10, Amplifier can be protected against excessive input signal voltage as shown here. Diodes are essentially open circuits at voltages less than threshold (about V2 volt) and have no effect on small signals. Above the thresholds diode becomes short-circuited and cuts off over voltage signal.

Signal circuits—a voltage peak in the applied signal is one of the most frequent causes for punch-through. The remedy is to incorporate voltage-limiting ahead of the transistor so that no input signal can exceed safe limits. Fig, 10 shows such a safety feature; the parallel diodes limit input voltage to a maximum oi half a volt and the rest of the amplifier is then designed to handle any voltage which can result from a half-volt input signal

Over-voltage spikes on the power supply lines may be prevented by extensive decoupling,, or by regulation of the power supply, A good battery is one of the best power sources since it is inherently regulated.

Heat is a critical factor for several reasons. If a transistor heats, its normal opera ting conditions are altered. The energy present as heat has the safe effect as ah increase in input signal and power-supply voltages, and all characteristics change.

In addition, when the inside gets hot enough, the crystal structure which is basic to transistor action begins to melt and change. This change in the crystal structure destroys the transistor and turns it into a useless hunk of semiconductor material.

I11 fact, the damage done by over-voltage is actually done by the heat which results from the over-voltage condition.

Some heat is always present in a transistor, since it does its amplification by changing the resistance of the collector-base junction. Current is passing through this resistance, and whenever current passes through a resistance heat is generated.

This resistance is primarily in the collector-base junction, so that the heat of the junction is the primary factor. Transistors are rated for power capability on the basis of their ability to withstand this heat, and to carry the heat away from the junction as it is generated. Power transistors have larger collectors, which are designed to carry the heat away more rapidly—and to operate at their maximum power ratings even these must be connected to "heat sinks" which help carry the heat away,

If the heat generated in the collector-base junction is not carried away fast enough, it acts to change the transistor's internal operating conditions just as would an increase in applied voltage. More current flows—and the more current, the greater the heat. As heat increases, still more current can pass. The condition is known as "thermal runaway" and leads to rapid destruction of the transistor, Once a transistor enters the runaway condition, nothing can stop it until it is using all the current available to it or has melted itself.

When 1 he junctions get hot enough, the base loses all control of operating conditions, this is the true "runaway" condition since no control is possible from the input circuit. Phe only control available is to prevent runaway from occurring.

The primary preventive measure we can use is to keep the transistor cool. This means using large enough heat sinks and assuring that enough air is available to keep the temperature within limits.

A secondary preventive measure is available by using an emitter resistor as shown in Fig. 11, Under normal operating conditions, the voltage appearing across the resistor is a part of the normal bias arrangement. When the transistor beats and leakage" current (that current due to heat and not controlled by the base) increases, the voltage across the resistor increases and thus subtracts from the original bias voltage. This reduces the controlled current flow; if the reduction is sufficient, then heating will

Fig. 11. Use of emitter resistor to control runaway depends upon its effect on base-emitter function bias. At far left is full circuit; at center is that part of the circuit seen by ac signalswhile that part at right is effective to dc levels* Drop across emitter resistor increases as transistor heats? reducing bias and cutting down on dc current flow. This limits maximum current.

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