SW-40 VFO-building gang,
Here's an experiment to try on your
freshly-minted VFO. It'll show some quirks of
oscillators, and how they differ from normal
amplifiers in the way they're biased.
Set your meter to "DC volts", and measure the base
voltage and emitter voltage of Q2. It gets kinda
crowded at the transistor pins, so you might want
to measure these voltages across R17(2.2k) and
Here's what I get:
Q2's base voltage (R16, 22k)........2.17v
Q2's emitter voltage (R17, 2.2k)....2.4 v
Ok, what's strange about this picture? The BASE is
lower than the EMITTER!!
How the heck can Q2 work when its base-to-emitter
voltage is reverse biased? In a normally working
amplifier, the base must always be about 0.6 volts
HIGHER than the emitter!
Now you may get slightly different results, because
your meter will "load down" the oscillator differently
than mine. But you will still likely see the strange
situation where Vb < Ve.
If you want to extend this experiment
further, turn off the power and tack-solder a short
jumper temporarily across the toroid (L1). This will
keep the circuit from oscillating.
Now power up and re-measure the DC base voltage
and emitter voltage:
Q2's DC base voltage.......2.13v
Q2's DC emitter voltage....1.5v
OK, that makes sense. The base is now about 0.6 volts
higher than the emitter. With 1.5v across the 2.2k
emitter resistor, the transistor is biased at 0.68 ma.
emitter current. Collector current is about the same.
Now its biasing the way an amplifier should...but its
not an oscillator anymore.
(Unsolder the short jumper now, before you forget)
So what's going on with the oscillator - how can it work
when the DC base voltage is less than the emitter?
Well, the key here is that there are large AC
waveforms (at 3MHz.) at the base and at the emitter too.
The AC amplitude at the base (3.8v p-p) is larger than
the amplitude at the emitter (1.9v p-p).
The base voltage DOES climb 0.6v higher than
the emitter, but only during a short part of the cycle:
right at the positive peak. For the rest of the cycle,
the transistor is biased off, because the emitter voltage
is lower than the base.
So the transistor conducts current in short pulses,
maintaining the oscillations by jamming a current pulse
from the emitter into the resonating L-C components
You don't see these pulses, because of the
"flywheel" effect of the 3 MHz. resonant LC components.
All the AC voltages look like sinewaves as a result. All the
transistor currents aren't sinewaves but are pulses.
Its also important to realize that AC voltages and DC voltages
co-exist in any amplifier: interpreting what your meter is
telling you can be tricky (especially when the AC volts are
bigger than the DC volts).
Glen VE3DNL [email protected]