Thursday, December 28, 2017

Transistor oscillator

I have been looking for a simple, easy to understand oscillator circuit that uses a single transistor.  Well, I found one!  Twice, it seems.  The first place was in a classic vacuum tube circuit, as described in

Morecroft, Elements of Radio Communication, Wiley, 1929.

After suitable edits to make it transistorized, I then found the same circuit in a hand-drawn schematic that my father had squirreled away in the 1944 edition of the ARRL Handbook. Certainly not new!

For my and Edwin's benefit, I built the circuit using snap circuits.

Here's the schematic:

The different parts of the circuit are indeed easy to understand:
  • The tank resonator is an inductor-capacitor (LC) circuit, which sets the frequency.
  • The keying turns on and off the oscillator.
  • The bias ensures that the transistor is turned "on" and not saturated.
  • The transformer affords feedback from the tank to the input of the transistor.  It also contains the inductor part of the tank.  In the snap circuits version, we don't have much control over this (it's in a plastic package), but too little inductance will cause the circuit to fail to oscillate.
  • The 200 ohm feedback resistor sets the gain of the transistor amplifier.  Reducing the resistance increases the gain, which drives it harder.  Increasing the resistance makes for a cleaner signal, but can also stop the oscillations. It seems to stop around 1000 ohms or so, but with the 200 ohms it is noticeably overdriven.
  • The signal purity can be increased by selecting a higher voltage for the power supply.  I got it to work with 3 volts, but the output was more clipped.

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