over years, i have visited kazuhiro's site. his work output continues to amaze me each time. he experiments fearlessly, then documents it all too. quite a volume of work. i find his designs interesting but they take minimalism to an extent where reproducability and performance become difficult to sustain.
my favourite from his site is the no-power AM transmitter which is literally powered by your speech. extreme qrp!
...
the general idea of using transistors with feedback is that the amount of gain extracted is much less than what the transistor is capable of. In such cases as BITX20, there are just two really important parameters to be considered: the Ft should be above 150 Mhz, the noise figure should be less than 6dB at low currents. Using a 1GHz transister will not make a difference as the gain of the stage is pegged at 16dB regardless of the actual Ft or the beta of the transister. It might fall below 16dB if the Ft is any lower, but wont exceed 16dB in anycase.
The inherent stability (and hence the reproducability) of the BITX is due to the fact that transistor types dont matter much, and the negative feedback assures that the individual stages dont self-oscillate.
- farhan
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On Fri, 26 Aug 2005, Max 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.
--- 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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