# How does this oscillator in this schematic work?

I am becoming a radio transmission amateur soon, but there is still one thing I cannot really explain. In below schematic I see the oscillator but I cannot wrap my head around how it actually works from the moment the power is turned on. Why does it oscillate?

My thinking was that as soon as C2 gets charged both L1 and VC1 are already at max voltage, so what will oscillate into what? So either of them get at their max voltage first and the voltage over C2 causes the base voltage of Q1 be lower than the collector voltage turning of the transistor transiently returning to the original state?

• L1 and VC1 oscillates around +Vcc rail and C2 is positive feedback. C2 more opens the transistor (force more current) when collector is going down and more closes transistor when collector goes up. Commented Oct 24, 2023 at 18:19
• Has answers here. The circuits work similarly: electronics.stackexchange.com/questions/190311/… Commented Oct 24, 2023 at 18:21
• @MichalPodmanický this starts to make sense. So can we say that C2 an R1 actually make a voltage divider together, and collector voltage goes above base voltage periodically, turning Q1 to low current passage? Commented Oct 24, 2023 at 18:52

Well, in the first place have a look at the DC operation conditions. The microphone and R2 form a voltage divider and define the base voltage. This will be something around 4 V.

The emitter resistor R1 is chosen relative high in relation to R2 and the expected transistor current gain. So the base current will be small and the voltage across R1 will be around 3.5 V.

From the RF perspective the base "sees" GND via C1 and to modulate the transistor current an AC input signal at the emitter is required.

Starting from the established DC conditions we assume a small variation in the transistor current for whatever reason. If it is rising, the collector voltage will fall and the emitter voltage follows via C2.

This is an input signal and will increase the base current because C1 holds the base fixed. The initial litte transistor current rise gets amplified, collector and emitter voltage are falling, the magnetic field in L1 rises.

This current rise process ends because C2 will be discharged via the base current and can no longer hold the emitter voltage below the original DC conditions. So the process reverts, base current falling, collector voltage rising, via C2 the emitter voltage is also rising.

Finally the transistor is turned off and the resonant tank can swing. At the end of the period the again falling collector voltage will invoke the next transistor turn on.

The resonant tank L1/VC1 dominates the change rate and so the frequency, but C2 also has an influence. If chosen too low, the influence on the base current is too small and oscillation cannot be aceived. If chosen too high, the AC load on the tank is too big.

The desired FM modulation is not perfect linear but acceptable within, say, +/- 10 mV from the microphone. The audio signal changes the internal capacitances BE and more inportant BC and modulates the phase of the C2 feedback.

• Interesting explanation of frequency modulation... I was just wondering how the microphone (transistor) changes the frequency after the LC tank sets it. In my opinion, in the end, it changes (modulates) a parameter of the LC tank (most likely the total capacity). Commented Oct 25, 2023 at 8:11

This 1-transistor "wireless microphone" circuit took me back to my school days in the 1960s when I used to receive its signal on a large tube radio in the next room of our apartment. I remember when I opened the door, there was a whistle (acoustic feedback). Here is a quick intuitive explanation.

When the power is turned on, the LC tank gradually "swings" like a child's swing. The capacitor C2 transmits its "movement" like the parent's hand (the transistor) that "nudges the cradle" (LC tank) when it reaches its lowest point (i.e., the transistor turns on for a bit and adds a portion of energy to the tank). Here is a CircuitLab simulation (for now not so successful:-(

simulate this circuit – Schematic created using CircuitLab

An interesting phenomenon is that after the voltage across the LC tank changes its polarity, the collector voltage rises above the supply voltage, which at first glance seems impossible. The same is observed in voltage doublers.

• Super explanation, thank you! I also built this circuit, excepting my school days were two and a half decades later, so the receiver was a transistor radio 😁 Commented Oct 25, 2023 at 5:51
• @Reversed Engineer, As I can remember, it was the German Olympia This was my favorite transmitter circuit. As a little boy, I wondered what pranks to do with it. First I used it to eavesdrop on my grandparents:-) Then, I would go down in the evening in the dark to the yard, and watch how the rooms of the houses were lit up by the working TVs. I was starting to adjust the frequency until the TVs went out and it was completely dark :-) I also jammed the older boys' portable radios:-))) Commented Oct 25, 2023 at 7:10
• Shanks for sharing! "I also jammed the older boys' portable radios:-)))" - ha ha 😁 Commented Oct 25, 2023 at 14:33
• So why is this just fading out in like a few microseconds? I would expect it to continously oscillate but i cannot get it to work either.... Commented Jan 13 at 10:34
• I noticed R2 should be a capacitor, but then it behaves the same. I wonder if this has something to do with circuitlab, because this circuit works in reality. Commented Jan 13 at 10:39