Thread: FET Questions
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Old 23rd Dec 2013, 7:17 pm   #63
Synchrodyne
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Join Date: Jan 2009
Location: Papamoa Beach, Bay of Plenty, New Zealand
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Default Re: FET Questions

Mentioned previously in this thread was the fact that whereas use of dual-gate mosfets in VHF-TV and FM receiver front ends commenced circa 1968, it was not until the mid-1970s that they appeared in UHF TV front ends. Apparently the latter application awaited the development of (economic) devices that had enough gain towards 1000 MHz.

But an apparent oddity was that whereas in the VHF-TV and FM cases, dual-gate mosfets were adopted for both the RF amplifier and mixer positions, in the UHF-TV case they were used only in the RF amplifier position.

The resultant question is: was this because dual-gate mosfets were simply just not effective mixers at UHF, or were there other reasons.

An early example (circa 1976) of a UHF-TV tuner with a mosfet RF amplifier is the RCA KRK-226, schematic attached. This used a diode mixer. Now it seems possible that this choice could simply reflect the inertia of established US practice where UHF-TV tuners were concerned. The hitherto standard layout was a bandpass tuned RF input feeding a diode mixer, with a valve (such as the 6AF4A) or bipolar local oscillator. The UHF-TV tuner output was then fed into the VHF-TV tuner, passing through both the RF stage and then the mixer stage acting as an amplifier.

So it could be that the RCA UHF-TV tuner at interest was conceived as being more-or-less the standard format with the addition of a tuned RF amplifier stage, rather than a complete rethink. Against that, an interesting aspect of the RCA tuner was that it included what might be called an “IF preamplifier” in the form of a grounded base bipolar stage. Also, although it then fed into the corresponding VHF tuner (KRK-228), it bypassed the RF stage, being connected directly to the mixer stage acting as an amplifier. I suspect that this may have been done to simplify the switching; whereas with a turret tuner it was easy enough to have an extra position for switching the UHF tuner output through the whole tuner, doing that may have been more complex with a varactor tuner. Anyway, a possible take from RCA’s design choice is that the combination of a diode mixer with an IF preamplifier was better (more gain perhaps?) than a dual-gate mosfet mixer.

Some of the RCA TV receivers in which the KRK-226 and KRK-228 tuners were used had distributed selectivity IF strips based upon three dual-gate mosfets, probably unusual by the second half of the 1970s when ICs were dominant for this function. In the VHF case, the signal path through to the vision demodulator thus consisted of five consecutive dual-gate mosfets. In the UHF case, the active device sequence was something of a “sandwich”, namely one dual-gate mosfet, one grounded base bipolar and then four consecutive dual-gate mosfets. This departed from the somewhat conventional wisdom that the mosfets be used “up front” with bipolar stages reserved for later stages after the main IF selectivity.

There is an interesting parallel in that around the same time (1977?), Mullard made a step-change in its UHF-TV tuner range with the introduction of the U321. Like the RCA KRK-226, this used a diode mixer followed by a grounded base bipolar IF pre-amplifier. Previous Mullard UHF TV tuners had used a self-oscillating bipolar mixer, which was I think fairly standard and established UK practice, going back to valve days when self-oscillating triode (PC86) mixers were used. Where Mullard differed from RCA was in using a grounded base bipolar RF amplifier preceded by a PIN diode attenuator for AGC. I wonder if Philips/Mullard had an aversion to using mosfets at the time, even though it did adopt them for RF amplifiers a few years later, perhaps when they were better and/or cheaper than the early UHF-capable devices. Anyway, the Mullard U321 supports the notion that there was some merit in using a diode mixer followed by a gain stage at UHF rather than an active mixer with inherent gain.

Cheers,
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