Thread: FET Questions
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Old 10th Dec 2013, 3:01 am   #62
Synchrodyne
Nonode
 
Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
Posts: 2,943
Default Re: FET Questions

James Bryant’s writings on the SL600 series were certainly very readable, even to the layman. And it was that 9 MHz IF strip with three SL612Cs in the signal path that I had in mind when I was thinking that post-main filter gain in consumer-level HF receivers could have been done by bipolar ICs as an alternative to discrete FETs.

Maybe I am drawing the wrong inference here, but I had the impression that Plessey specialized in particularly linear bipolar RF ICs (well,, aside, of course, from the logarithmic radar devices). As an example, the late-1970s SL1430 and SL1431/2 TV IF pre-amplifiers seem to fall into this category. They were designed to go between the tuner and the then new-fangled SAW filters (in which field Plessey was an early mover) to compensate in advance for the filter gain loss. Thus they operated in a relatively wideband part of the circuit. And the rationale (cross-modulation reduction) for using dual-gate mosfets in VHF TV tuners (at least in North American and Japanese reception conditions) would seem to apply equally to any following wideband IF stage. Plessey evidently addressed this by appropriate design, and did publish cross-modulation and intermodulation data for this family of ICs. I’d guess that one IC advantage here would have been the easy provision of a differential output for good matching to SAWFs. And the SL1431/2 added a wideband RF AGC loop, and idea that had also turned up in FM front ends and elsewhere in that time period.

Although discussion of RF/IF ICs may seem somewhat off-centre here, as previously observed, FETs and ICs appeared, in consumer equipment at least, at about the same time and to some extent competed with each other. Thus any broad discussion of FET applications reasonably includes those where they could have been used but were not because designers chose the IC pathway.

Cheers,
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