| 摘要 |
<p>To prptect a transistor (T1) its emitter current is converted by a resistor (R1) into a voltage which by means of a voltage-current converter (6) is converted into a current which is proportional to the emitter current. The collector-emitter voltage is converted by means of a voltage-current converter (9) into a currentwhich is proportional to the difference between a knee voltage (VKN) which is applied to the converter (9), and the collector-emitter voltage, the converter (9) not producing an output current for collector-emitter voltages exceeding the knee voltage VKN. The difference between the output currents (1M) of the converters (6 and 9) is compared with the current (lo) carried by a current source (14). The protection circuit becomes operative when this difference current (IM) exceeds the current (ID) carried by the current source (14). A control amplifier (16) then drives the transistor (T,) in such manner that the difference current (IM) becomes equal to the current (ID). Above the knee voltage (VKN) a residual current which has a value proportional to the current (ID) carried by the current source (14) continuous to flow through the transistor T,. As a result thereof the value of the residual current depends to a lesser extent on variations, which reduces the risk of damage to the transistor due to overloading at high collector-emitter voltages.</p> |
