| 摘要 |
A switching circuit in which the signal path between a signal input and a signal output terminal includes at least two amplifier stages, the first one of which has its collector connected in a series circuit with a load impedance and the emitter-collector circuit of a switching transistor. The switching transistor is switched between a conductive and a non-conductive state by a switching voltage applied to its base. The base of a second amplifier stage is connected between the collector of the first stage and the load so that when the switching transistor is made non-conductive by preventing current from flowing in its emitter-collector circuit but also makes the second stage non-conductive by reducing its base current to zero. Making the first stage non-conductive keeps it from amplifying the signal and requires any leakage current to pass around the first stage by means of stray capacitance. Making the first and second stages non-conductive simultaneously prevents even the leakage current from being amplified and allows the leakage current to reach the output terminal only by way of a second stray capacitance, which further reduces the amplitude of such current. A cascode stage may be included between the first and second stages to be rendered non-conductive along with them as a further means of amplifying the signal when the transistors are operative and decoupling the signal when the transistors are non-conductive. Instead of a simple cascode stage, a differential amplifier stage may be included between the first and second stages to provide means for adjusting the gain of the circuit. |
