| 摘要 |
<p>In a pulse width modulated signal amplifier employing a tetrode valve S1, current to the grid G1 and screen G2 is controlled by constant current sources 19 and 24 respectively. Each constant current source comprises a switch S3 a current sensor R1, R2 and an inductor L1, L2. Each switch S3 S4 is switched on and off at a set frequency which, in the illustrated system, is the same as the pulse repetition frequency of the signal to be amplified. Each time the switch S3 or S4 is closed the current in the associated inductor L1, L2 builds up until a set limit, determined by sensor R1, R2 is reached whereupon the sensor opens the switch S3, S4. Thus, the period for which switches S3 and S4 remain closed varies to maintain the currents to G1 and G2 at the desired levels. By employing this technique the inductors L1 and L2 do not need to be as large as would be required using a known technique described in UK Patent Specification 1566095. Stray capacitances and resulting distortion are thus minimised. Also, since the switching frequencies of the switches S3 and S4 are identical the generation of harmonics of these frequencies, resulting in further interference with the signal to be amplified, is also avoided.</p> |
