Wideband u.h.f. amplifier using earthed-grid triodes

An untuned u.h.f. amplifier giving a mean gain of 25dB from 650 to 1000 Mc/s is described. Three discsealed triodes (type 416B) are cascaded in earthed-grid stages. Matching of interelectrode capacitances and the realisation of the broadband interstage transformers are obtained by simple transmission-line theory. The matching networks are constructed in slabline with a cylindrical inner conductor of a diameter suitable for axial mounting of the triodes. The noise figure varies from 7 to 10dB over the frequency band. Features of the design are the simplicity of the circuit configuration and the elimination of all tuning adjustments, with consequent ease of valve replacement. An extension of the circuit theory is briefly described, and it is shown that gain-bandwidth products in excess of the commonly accepted values can be achieved.