Design of Efficient Broadband Variolossers

T h e gain control of broadband high quality transistor IF amplifiers has been a difficult problem for many years. Control of gain by variation of bias on the gain stages causes severe bandshape distortion, so it is customary to use one or more semiconductor diode variolossers for this purpose. T h e use of a variolosser, which introduces a large attenuation to the signal, can impair the noise figure of the amplifier; for this reason several variolossers of limited range are imbedded between suitable gain stages to minimize the effect on the noise figure. Several types of variolossers have been used but all of them haAre one or more of the following disadvantages. ( i ) Complicated circuitry, making adjustment difficult and thermal stability hard to achieve. (ii) L a r g e minimum loss, which requires additional gain stages and adds to the noise figure problem. {Hi) L a r g e bias power. (iv) Parasitic reactances degrading the performance at high frequencies. 1687 1688 THE BELL SYSTEM TECHNICAL JOURNAL, JULY-AUGUST 1969