Please read the description/textbox first. Circuit of a classical analog sine wave oscillator made with a Field Effect Transistor (FET), the BF 256 A (or B or C).
The variable voltage to the oscillator (= variable amplitude output) is set via the 10 K potentiometer, connected to the 36 V power supply with the help of a voltage dropping resistor. In this case a few paralleled resistors drop 36 Volt to 24 Volt: 6K8, 2K2,2K2 (watch the schematic). This was experimentally done to get to max. 24 Volt as being the proper/good supply voltage to the FET to get it into oscillation in all the frequency bands.
The circuit works in the a few Milli-Ampère range. This circuit is basic but always works.
Everything is told in the video. You can use this oscillator in radio applications (say as a Local Oscillator in a classic analog superheterodyne radio), but also as a test oscillator or an analog Sine Wave function generator.
In such a case a buffer stage could be necessary to prevent that the oscillator stops with a (heavy) load (impedance) at its output, via that 39 pF output capacitor.
The oscilloscope has a say endless (read: 1 Mega Ohm) impedance, so there is no damping of the oscillator, when its output is connected via that 39 pF capacitor to the scope.
I will (perhaps) talk about that in a next video.
To use it as a local oscillator in a superheterodyne no buffer stage is needed, the output level to the (radio) mixer can be aligned with a proper choice of the coupling capacitor value to the mixer stage, say in the 2 pF to 50 pF or even 80 pF range. Experiment!
With smaller inductance coils and by experimenting with the 150 pF and the 1000 pF capacitor in the gate-source circuit of the FET (leave the 150 pF capacitor as it is, only change the 1000 pF capacitor to a lower value, say 220 pf) you can get to much higher oscillator frequencies, surely to 9 MC.
The circuit was originally published in the ARRL Handbook in the 1970’s, I made it many times. In the past with the (obsolete) BF 245 A,B, or C, but now with the BF 256 A.
I am almost sure the circuit also will work with the B and C versions of the BF 256, the N-FET.
Correction to the video: on 5.53 I say that the capacitor is 1000 pF, but of course it is 100 pF.
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