Re: 2sc1815 interesting stable bjt vfo schematic

ajparent1 <kb1gmx@...>
 

Hello,

Yes 'OZL is a fantastic and very active constructor. Indeed I've
been aware of his projects for many years now. The site for JF1OZL
is one of about 40 I have bookmarked as they represent both active
homebrewed (autoconstructed) and good application of theory.

www.qrp.pops.net is another worth looking along with www.k8iqy.com.

As to the 2SC1815, I do have a good quantity of them and it is a
very good and useful device. It only one of many I chose from
for various projects. I have an unusually good selection of new
devices as I used to provide service in commercial space for a
variety of electronic gear. I also have databooks for most major
makers of transistors as well as a detailed on line data sheet
archive. For BITX I don't think it is as good a choice for the
feedback amplifiers. For tuned amplifiers it is very good.

Most BJT devices can be used in common (grounded) base circuits
as the Alpha cutoff is much higher. I used to take advantage of
this ways back when transistors capable of HF or low VHF were very
hard to get.

Allison
KB1GMX

--- In BITX20@..., "Max" <m_orwell@y...> wrote:
JF1OZL in most of the constructions even on 50mhz is using this type
of trasistor. As with the increasing of colector current ft getting
higher. Also comon base ft is several times bigger than comon emiter
circuit in the contrarry with comon colector design.

More information about 2sc1815 and implementation of generall purpose
trasistors.

http://www.intio.or.jp/jf10zl/basic.htm


--- In BITX20@..., "ajparent1" <kb1gmx@a...> wrote:
The 2SC1815 is a good device but it's Ft is only 80mhz. For
HF VFOs that is adaquate but for the BITx if it may be a bit
weak. Other devices of the era are the 2sc784, 2sc930 and 2sc945
all good and easily found in salvaged equipment.

As to transistor capacitance in VFOs, it's is easy to bury the
device in configurations that overwhelm the devices capacitance.
By doing so you make the VFO less dependent on the device used and
it can aid stability. The series tuned Colpits VFO used in
orginal BIT-x is such an oscillator. The 500pf across Base to
Emitter is far larger tha the transistors internal capacitance.
The second 500pf capacitor from emitter to ground (collector is
at RF ground due to the .1uf from Collector to ground) swamps any
capacitance from Collector to Emitter.

One trick that I've used for VFOs that helps with stability is
a form of compensation. For most BJT VFOs there is a voltage
where the initial drift is neutral or nearly so. Often this is
easily found by using a LM317 variable voltage regulator and
starting at the design voltage and noting startup drift. Then by
adjustment of the voltage to the VCO it's possible to reduce or
even eliminate that drift. What it achieves is to find a point
where the transistors internal heating due to feedback and junction
heating is minimal or at least stable.

NOTE 1: This is not temperature compensation. There will still
be drift from warming or cooling of other components. The
inductor, capacitors and if used Varactor(Varicap) diodes are
still contributors to thermally drift. The Varactor diode is a
significant source of thermal drift in most VFOs.

NOTE 2: from testing I've found three terminal regulators
provide a more stable voltage than zener diodes due to temerature.


Allison
KB1GMX

--- In BITX20@..., "Max" <m_orwell@y...> wrote:
2sc1815 is toshiba pct process based transistor. so this mean
lower
capacitance and noise figure. this japanese design remind on
comon
base vfo used in the 70's in swan trx. schematics are in file
section.

73 de 4N1ZM

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