| 摘要 |
<p>1,231,500. Transistor amplifiers. STANDARD TELEPHONES & CABLES Ltd 28 Aug., 1968, No. 41144/68. Heading H3T. Overload protection in an amplifier has a capacitor C1 which changes to a voltage corresponding to the peak voltage across a resistor R1 carrying amplified current, and when the capacitor's voltage reaches a predetermined value a switch VT1, VT2 operates to reduce current in a stage VT3 of the amplifier. The transistor VT3 is the input stage and is connected through intermediate stages (not shown) to the output transistor VT4 whose collector resistor is R1. VT3 receives its current from the supply 1 through transistor VT2 which with VT1 forms a Schmitt trigger operable by a sufficiently negative voltage at 5, upon overload of the output, to cut off the supply to VT3. In a second embodiment (Fig. 2, not shown), the input transistor VT3 is in parallel with the transistor VT1 of the switching circuit, and is shorted out when a sufficiently positive voltage on the capacitor C1 is reached (indicating overload of the output). The capacitor (C1) is here connected through the diode (D1) to the emitter of one transistor (VT6) of a series pair (VT5, VT6) constituting the output stage. In both embodiments the diode D1 ensures a larger discharge than charge period and with adjustment of a resistor R2, a high off-on ratio can be achieved, whereby the transistors of the amplifier are subject to excess current only for very short periods, with long periods in which the junctions may cool down.</p> |
