[Eeyore]

Trevor Wilson wrote:

"Eeyore" wrote
Trevor Wilson wrote:
"Arny Krueger" wrote

For one thing, modern output transistors have Ft up in the megahertz
range. Back in the 70s people were still struggling with output devices
with Ft in the KHz range.

**Nope. The Japanese had a goodly number of complementary devices, with
decent Voltage, current and SOA ratings which extended well into the tens
of MHz. By 1974, Hi-Rel had true Triple Diffused, TO3 devices with a 20MHz
fT and 20 Amps capacity.

Hi-Rel ?

**Yep, Hi-Rel. They were a small manufacturer back then.

I have a very vague idea I may have come across the name but in those
pre-internet (crikey - pre-fax even ! we still had a telex machine ) days one
was very constricted to using devices that local distributors carried. Typically
that meant RCA, Motorola and Thompson for highish power devices.


A mate discovered
them and used their devices for his amplifiers. Very impressive devices. I
have a spec sheet somewhere. When I locate it, I will scan and post, if you
like. Purely historical, of course. The devices have been unavailable for
quite awhile. FWIW, the numbers a ED203 and EB203.

I'd be interested to see those if you can find them thanks.


Exceptionally linear and very potent devices. I recall
them vividly, since Motorola had announced their intention to commit to
Epitaxial Base devices (ca. 2MHz), since it was impossible to produce
Triple Diffused devices (according to Motorola). Other manufacturers also
had
some impressive devices back then. The Marantz 2325 receiver used
2SA747/2SC1116 devices. These posessed an fT of 15MHz.

I recall being impressed by Sanken devices the first time I was designing
a serious amplifier around 1980. I settled for Hitachi lateral mosfets
instead. Obtainability was a serious issue back then.

**By 1980, Japanese transistors were much more easily available.

Not so easy IME.


The 2SD555 and it's complement were my favourites at the time.

I must check the data on that.


Having said all that, good gain linearity over a wide current range has
only been a relatively recent development.

With modern low-Z driver stages, current gain linearity is no longer an
issue the way it was.

**True enough, but, as you may recall, I have a preference for zero Global
NFB amplifiers. Every bit of linearity is helpful.

MY opinion is that zero global NFB is a classic example of navel gazing. There
is NO sensible basis for avoiding loop NFB totally. Indeed it you want very low
output Z it's the only way you can get it.

Personally, I use local and overall NFB in roughly similar amounts. It works
well.

Graham