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Components and Circuits For discussions about component types, alternatives and availability, circuit configurations and modifications etc. Discussions here should be of a general nature and not about specific sets.

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Old 15th Jan 2015, 8:37 am   #101
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Default Re: FET Questions

A pair of 2SK125 was Icom's standard HF mixer for many years. It's only single-balanced, but it works quite well. THe IC765 receiver performance is still good.

You'll find the same circuit in most of their transcivers designed over a decade, It's there in the IC765 currently sitting upside-down opened on the table (Looks like CMOS logic in T/R switching fault) and it's there in the HF section of the R9000 stacked on the AR88s.

The IC7700 currently set up in the radio shack uses the grounded source SD5400 double-balanced arrangement, what was called 'H-Mode' by Colin Horrabin, and as he was the guy to publish it 20 years ago, I suppose he gets to name it.

I think 2SK125 have been obsolete for some time now, and can be hard to find.

David
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Old 19th Jan 2015, 4:29 am   #102
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Default Re: FET Questions

The 2SK125 jfet does seem to have been a device of choice for push-pull (single-balanced) mixers in Japanese HF receivers in the 1980s:

ICOM R-71 (1984): 1st mixer; also (push-pull) RF amplifier.

JRC NRD-525 (1986): 1st & 2nd mixers; also (push-pull) RF amplifier.

Kenwood R-5000 (1987): 1st & 2nd mixers; also post-1st mixer (push-pull) pre-filter IF amplifier; and 2 x 2SK125 in shunt cascode RF amplifier.

Yaesu FRG7700 (1981): 1st mixer; with single 2SK125 buffers ahead of and following it.


In its NRD-535 of 1991, JRC used 4 x 2SK125 in each of the RF amplifier and the 1st mixer positions. The RF amplifier was of parallel push-pull configuration, whereas the mixer was double-balanced, although I think still of the continuous quadratic function type and not of the switching type.

As already mentioned, Heathkit favoured the dual-gate mosfet for RF work, and it was used in the early stages of HF receivers of varying degrees of elaboration, FM receivers (following a dalliance with jfets), AM receivers and VHF TV receivers. Another example, from circa 1973, was the GR-110 NBFM scanning receiver covering 146 to 174 MHz, which range would have included the marine VHF RT channels and the NOAA weather radio channels. This used dual-gate mosfets in the 1st and 2nd RF amplifier positions, and for the mixer. It looks as if it had fixed RF tuning, with bandpass input and 1st interstage, and single-tuned 2nd interstage. So it was an example of mosfets being used in circuits that were not very sharply tuned. The 10.7 MHz IF was fully IC-based, so there were no fets therein. It used a pair of the FM amplifier/limiter/quadrature demodulator type, not identified on the schematic but with pinouts corresponding to the ULN2111A/MC1357 (which I think was the original of the species). The 2nd was deployed normally, providing quadrature demodulation. The 1st was used solely as an amplifier/limiter, but with the multiplier section providing additional amplification, in which role it [the multiplier section] acted more-or-less as a balanced cascode.

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Old 19th Jan 2015, 10:07 am   #103
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Default Re: FET Questions

Somewhat earlier than that "Sosin's Folly" the Marconi H2900 very high dynamic range receiver used 2N4391 JFETs as RF and IF amplifiers.

They are low Rds(on) FETs intended as analogue switches with high IDss. They arepretty big area devices, and I think they're symmetrical. Anyway, they do work at RF biased a highish currents and seem to work well in grounded gate circuits. Pretty similar to what the Japanese companies did with the 2SK125, but more than 10 years earlier.

Marconi would certainly have measured the living daylights out of them. Later on the transformer feedback trick on a moderate power CATV bipolar became the tool of choice.

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Old 5th Jul 2015, 9:44 am   #104
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Default Re: FET Questions

Back in post #94, I mentioned the 1979 GM patent covering the jfet-bipolar cascode RF amplifier, with agc applied to the upper, bipolar device, which was used for car radio AM sections. This came at a time when the roles of small-signal fets in consumer-type equipment, relative to bipolar transistors and to ICs, were relatively settled. And it was intended to displace dual-gate mosfet RF amplifiers in AM car radio receivers. That in and of itself was an inversion of sorts. Dual-gate mosfet RF amplifiers had displaced the bipolar kind, and in some cases there had been a two-stage transition, firstly from bipolar transistors to jfets, and then from jfets to dual-gate mosfets.

Some time back I came across an article from 2007 on this kind of hybrid jfet/bipolar cascode amplifier, which was suggested for use as an agc’d IF amplifier in amateur receivers. The article, by Hayward and Damm, is available here: http://www.ka7exm.net/hycas/hycas_200712_qst.pdf, and here: http://www.cqham.ru/forum/attachment...entid=159678&d...

Apparently the main benefit of the hybrid cascode, as compared with a jfet cascode, was that it was capable of operating on lower HT voltages, useful for battery-operated equipment. Otherwise it offered similar performance. Note though that unlike the GM case, here it was offered as an alternative to a jfet cascode.

An interesting comment towards the end of the article was: “We initially thought that the hybrid cascode circuit with a bipolar and a JFET was new. But we then discovered that it has been in use for a long time, especially in automotive equipment.” The associated reference was to US patent 4,277,757, Richard Kennedy, 1979, assigned to Delco/General Motors. It looks as if the authors did a lot of hard work on what they thought was a new idea only to find out afterwards that they had, more-or-less, re-invented the wheel. One may thus see the Hayward and Damm case as another application for the hybrid cascode, utilizing another potential benefit.

So far so good, but there is another connection, as it were. Subsequently I happened across the 1970 March Wireless World (WW) article “80-metre S.S.B. Receiver” by W.B. de Ruyter (http://www.americanradiohistory.com/...d_Magazine.htm).

What caught my eye was the opening comment: “Since f.e.t.s are now available at low-cost, it is possible to build a stable receiver with performance similar to valve receivers with the attendant advantages of low-power consumption and the absence of self-generated heat.” This of course was aligned with the theme of the time that the use of fets was the pathway to realizing the advantages of solid state circuitry without the performance sacrifice, as compared with valve circuitry, that came with small-signal bipolar devices. Whilst I was expecting that this receiver might have used dual-gate mosfets, a look at the circuit showed hybrid jfet-bipolar cascodes used for both the RF amplifier and the (single) IF amplifier. The mixer was a single jfet with both signal and oscillator on the gate, as was the product demodulator. Agc was applied to both the RF and IF stages, but in each case to the gate of the jfets, and not to the base of the bipolar device. So this was a point of departure from the later patented GM circuit, which applied agc to the base of the bipolar device. Still, the notion of applying agc bias to the upper element of a cascode was not unknown in 1970; it was implicit in the customary deployment of the dual-gate mosfet.

By way of yet another connection, the de Ruyter article also mentioned the Bowman amateur HF receiver described in WW 1969 July, August and September. I referred to the latter briefly upthread in post #87. In the context of this thread, it was notable – at the time - for its use of dual-gate mosfets in the RF amplifier and mixer stages. These were chosen for reasons that in due course became the “usual suspects”. In respect of the RF stage Bowman stated: “An investigation into various semiconductor devices shows that only the field effect transistor has a transfer characteristic of approximately square law. Bipolar devices are particularly poor in this respect. During some earlier work the author found that even f.e.t. cross-modulation performance is determined in part by the choice of drain current operating point. Very poor performance is likely if reverse a.g.c. is applied to a single gate device. This disadvantage can be overcome by using two f.e.ts in a cascode circuit applying a.g.c. to the common base stage.

“R.C.A. have recently marketed an integral cascode device which has the advantage of a somewhat lower h.t. requirement than separate devices, as well as a very low reverse transfer admittance value.


“These devices are marketed under an assortment of code numbers and vary in price from about 7s to 14s. The author tested the following types and at up to 30 MHz could find very little difference between them:-3N140, 3N141, TA7149 and 40500. (Since writing the MEM 564C has become available and is to be recommended since gate protection is incorporated).”

And in respect of the mixer:

“Any deviation from square law will introduce cross-modulation and therefore the dual gate f.et. is as equally applicable to mixers as amplifiers. It has the added advantage that the two signals can be fed to separate gate electrodes to provide considerable isolation between the local oscillator and the signal voltages.”

However, Bowman chose not to use mosfets in the IF strip, saying: “Many circuit configurations were considered for use in the i.f. amplifier. The use of common emitter transformer coupled stages was avoided due to the high value of reverse admittance, making either circuit neutralization or low gain per-stage essential to ensure an adequate stability factor. The cascode arrangement of bipolar devices was investigated. It was decided that there was little advantage in using field effect transistors in the i.f. amplifier as the cross-modulation problem is minimal after the very narrow bandwidth filter. The cascode arrangement was found to exhibit high-gain with a very low reverse admittance. The circuit also lends itself to a.g.c. control rather in the same manner as the r.f. amplifier. The control voltage is applied to the common base connected stage. This in turn means that the r.f. and i.f. controlled sections can easily be coupled together. It was found that the cascode arrangement induced very much less de-tuning of the i.f. transformers and by using low Q single tuned circuits very little change in the overall i.f. response occurs with a.g.c. action.” As noted upthread, at the time, using fets rather sparingly in the critical front end, with bipolar devices in the IF strip, was typical of the era.

Returning to the de Ruyter 80-metre receiver, clearly using fets in the RF, mixer and IF stages was a primary objective, but less obvious is the reason for the choice jfet/bipolar cascodes for the RF and IF stages, particularly as these did not have agc applied to the upper, bipolar device. Possibly the answer is provided by this comment in the text: “The f.e.t./bipolar transistor r.f. stage, Tr1 and Tr2, does not require neutralizing if due care and attention with screening.” Thus it looks as if the designer chose a cascode solely in order to avoid the need for neutralization, in which case there was probably no compelling reason to use other than a bipolar device as the upper element. The same reasoning, I think, would have applied to the IF stage, which was a similar cascode. But reverse transfer might not have been such an issue with the mixer, where the IF output was well-separated from the signal and local oscillator frequencies. Perhaps that was why it was not cascoded.

The de Ruyter receiver was likely a very early application of the hybrid cascode, and preceded the GM patent by nearly a decade. Perhaps GM’s innovation was applying agc to the upper device of a hybrid cascode, although in a general sense, application of agc to the upper device of a cascode pair was also at least a decade old, probably older than that. Whether GM was even aware of the “prior art” is debatable.

At the end of their article, Hayward and Damm mentioned the possibility of using a differential pair for the upper unit of the hybrid cascode, and applying the agc bias to the side of the pair that was not in the signal path. It was noted that this technique was used in the Motorola MC1350 IC. And that connects back to the events of the late 1960s when both mosfets and ICs were in contention for use in TV IF strips, in order to overcome the problems with conventional discrete bipolar circuits.

Unknown is whether the application of agc to the upper device in a bipolar cascode pair was something that had been done previously, or whether it was spurred by the fact that it could be done with dual-gate mosfets. Certainly from the Bowman article one has the impression that it would not have been a commonly used technique; implied that he copied it from what was done with the dual-gate mosfet RF amplifier rather than drawing it from established practice.

Cheers,
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Old 5th Jul 2015, 10:25 am   #105
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Default Re: FET Questions

Various books covered FET cascades and most often included a comment that the upper device needed to have greater Gm than the lower. I noticed the remark, but never really understood it. Some of them suggested a bipolar upper stage as one means of ensuring this.

I suspect the real need wasn't the Gm directly but was for JFETS a means of partitioning the voltage drop favouring the device with the larger drain swing.

The Mullard blue transistor audio and radio circuits book has a bit on simple single bipolar mixers for broadcast radios, suggesting lower current consumption and hence better battery life.

The best mixers I know - best in terms of dynamic range and LO power consumption use FETs but do not need any square-law region. Discontinuous switches would be great.

If we look back at historic radio circuitry, there was great turmoil, many things appeared independently, many attempts were never published. Too much was happening in parallel and there were too many loop-backs for it all to ever be told as a linear story. At best we can hope to understand small areas at a time. The Big Picture will always be incomplete as much has been lost, or was never captured at the time. It can be fun trying, though.

David
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Old 7th Jan 2016, 11:47 pm   #106
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Default Re: FET Questions

Regarding the “big picture”, now and again one finds commentary from authoritative sources, and last year I found and read a couple such from early in the fet era.

The first was the Pat Hawker article “Communications Receivers” Wireless World 1970 June. The following three paragraphs from that article summarize the difficulties that were being encountered in the transition from valves to solid state devices, and the role that fets were playing.

“Not all design changes have been uniformly beneficial. Although the development of h.f. semiconductors (and more recently integrated circuits) has opened the way to compact receivers of extremely high stability and impressive "mean time between failures", these devices have posed serious "front-end" problems. These include limitations to dynamic range due to increased susceptibility to cross-modulation and inter-modulation, and damage from static charges and local transmitters. Other drawbacks are increased loading of tuned circuits, lower stage isolation and greater spread of characteristics. The availability, during the past few years, of single- and dual-gate field effect transistors, with near square-law transfer characteristics, and the increasing impact of hot-carrier (Schottky) diodes in wideband, double*balanced mixers are reducing these problems.

“In some respects, the concentration on all-semiconductor designs came at an unfortunate time, when, for example, the availability of beam-deflection valves (7360, 6JH8 etc) for use as low-noise mixers made possible the elimination of signal-frequency amplification and offered a useful improvement in dynamic range; factors which have been exploited in only a very few designs. An exception was the Squires-Sanders SSRl receiver for the amateur market.

“Even today, in the lowest price ranges, it is usually possible to achieve a higher standard of front-end performance with valves than with semiconductor devices. The continuing demand for low-cost valve or "hybrid" designs of sufficient stability and low-enough tuning rate for s.s.b. reception has increasingly been met by Japanese firms. British, European and American firms tend to concentrate more on the professional user.”


Clearly, in the HF receiver world, there was pressure, real or perceived, to change from valve to solid-state technology, whether for “bragging rights” or for general benefits, before solid-state could fully match the core performance delivered by valves. Fets helped close the gap, and in the 1970s they were taken up particularly by the Japanese makers, who were then becoming dominant in the amateur and consumer HF receiver field. Although probably it was Heathkit in the USA who led the way in introducing fets to lower cost, consumer-oriented HF receivers.

Hawker also went on to say:

“At present, a more practical approach consists of using a special f.e.t. amplifier in conjunction with a wideband double*balanced diode ring mixer using hot-carrier diodes. Amplifiers of this type, capable of handling Iinearly signals up to over a volt, have been introduced by Comdel. Several current receivers use field-effect r.f. amplifiers employing the cascode arrangement, either with dual-gate m.o.s.f.e.t. devices or with two separate f.e.t. devices, since the junction f.e.t. appears to be less susceptible to static puncture than the dual-gate m.o.s.f.e.t.”

That last sentence was possibly inspired by Eddystone’s use of a jfet/single-gate mosfet cascode RF amplifier in its EC958 and following receivers, chosen because jfet robustness was desirable for the initial stage.

Later in the article, Hawker also said:

“Where bipolar transistors are used in r.f. amplifiers a useful extension of dynamic range can often be achieved by the use of r.f. overlay power transistors, an approach found in some recent Redifon receivers, which also make use of voltage-controlled diode attenuators in the input circuits. Manual attenuators are fitted in many semiconductor designs.”

That I think referred the use of CATV bipolar power devices such as the 2N3866, for example by Redifon as an RF amplifier. That approach may have become became more common than using small-signal fets in the professional HF receiver world, although the trend was towards high-level first mixers for which preceding RF amplifiers were either not required or seen as optional.

The second source was a 1973 paper by Prof. Gosling, “Integrated Circuits for Analogue Systems”.

Talking about professional applications, Prof. Gosling said:

“A variety of integrated circuit amplifiers for use in narrow-band i.f. and r.f. amplifiers are available. They mostly feature means of gain control for a.g.c., and usually have well-defined input and output conductances which facilitate the design of stable amplifiers. They are not, however, at present competitive with discrete component amplifiers in respect of ultimate noise or linearity performance. An exception is the dual-gate m.o.s. transistor, which may be regarded as an integrated m.o.s. cascode amplifier, and is the best low level r.f. amplifier currently available up to about 400 MHz. By contrast, there are few bipolar integrated r.f. amplifiers which can be operated beyond 100 MHz.”

That reminded me that at times RCA did describe its integrally gate-protected dual-gate mosfets as integrated circuits, so their inclusion in the paper was appropriate.

Gosling was quite specific in his comment, which referred only to dual-gate mosfets and only to small-signal devices. To put into context his comment about the relativity between discrete and integrated devices, this was in the section of the paper addressing professional applications. By 1973, the IC was taking over the IF amplifier function in consumer equipment, both TV and AM-FM, at least for non-portable items. But RF amplification, at least for TV and FM, remained largely discrete. Back in the later 1960s, RCA had originally offered its FM IC range for the RF job as well as IF, but these were soon displaced by mosfets for the RF functions.

The 400 MHz upper limited that Gosling quoted for dual-gate mosfets aligned with the published number for the ubiquitous RCA 40673. But in 1973, UHF-capable production mosfets were only a couple or so years away.

Generally, what has already been said in this thread appears to align reasonably with the Hawker and Gosling commentaries.

Cheers,
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