Viewing 3 reply threads
  • Author
    Posts
    • #6335
      LectronFan
      Participant

      Hello everyone !

      What would an oscillator section be without a Colpitts oscillator ?

      Well, our Lectron set has all it takes to build one in no time !

      A Colpitts oscillator consists of an amplifier stage (such as a transistor) with a feedback loop. The loop is usually made of an inductor with 2 capacitors or 2 magnetic coupled inductors (such as a transformer) with one capacitor.

      Both circuits will be published here and we will make some experiments with it.

      Let us start with the 1st variant : 1 inductor and 2 capacitors. Here’s the setup with a NPN transistor.

      Colpitts oscillator V1

      And here with a PNP transistor :Colpitts oscillator PNP

      When the battery is connected, you can hear a high pitch note of around 8,4 kHz.

      Because the inductor used here has a high inductance, the pitch of the oscillator is also high.

      Later on, we will use other inductors of our Lectron set to experiment with.

      Have you also noted that we used for C4 a value of 0,47µF ? If we would lower this value to 0,1µF, what pitch do you think the oscillator has ?

      The formula is :

      Colpitts form

      Adventurous Lectroneers can surely calculate the new frequency out of this formula !

      Greetings

    • #6338
      mwpeters75
      Keymaster

      Hi Frank,

      Thanks as always for a new topic!

      What is the advantage of this oscillator versus, for example, a Hartley oscillator?

      Thanks,
      Michael

      • This reply was modified 1 month, 1 week ago by mwpeters75.
    • #6353
      LectronFan
      Participant

      Hello Michael,

      The Colpitts oscillator has the advantage that is uses only one inductor and two capacitors in the tank circuit.

      You can clearly see these components (C4, C5, L1) in our circuit above.

      This is much easier to construct, since we don’t need some kind of transformer.

      A Hartley oscillator uses 2 magnetically coupled inductors and one capacitor.

      This kind of circuit is mostly used in radio circuits (remember the red oscillator transformer in radio’s ?)

      We will construct a Hartley oscillator later on with our Lectron set.

      But there are still some more cool experiments we can do with our Colpitts oscillator 😆

      Greetings,

      Frank

      • This reply was modified 1 month, 1 week ago by mwpeters75.
      • This reply was modified 1 month, 1 week ago by mwpeters75.
    • #6375
      LectronFan
      Participant

      Hi everyone !

      Ready for some more experiments with the Colpitts oscillator ?

      Well, today we are using a new coil in our circuit : the transformer block.

      It has 3 coils but only 2 coil are usable for our experiment.

      So, assemble the circuit as shown :

      Colpitts oscillator with transformer coil

      We will notice that the pitch of the tone becomes lower. This is because the inductance of the transformer coil is larger than the choke coil used in previous experiment.

      The choke coil has an inductance of 4,7mH (millihenry). The coil of the transformer is 9,4mH.

      Using the formula above, what do you think the new frequency will be ?

      As an experiment, turn the transformer so that we’ll use the smaller winding (0,2 mH). What do we hear now ?

      Can you explain what happens ?

      If you have an oscilloscope, make measurements at the collector of T1 and observe the waveform.

      We will now build something real cool !

      NOTE : This experiment can’t be performed with the Germanium transistor block with 100K resistor !

      Try this setup :

      Colpiits switched

      If you press S1 briefly, what happens ?

      Now, press S2 briefly, ain’t it neat ?

      Try to explain the working principle of the 2 switches and why the circuit acts like this.

      Some experiment tips :

      • Swap the straight line block (next to C4) with the choke coil block. Now we have 2 coils in series. Listen to the tone by pressing S1.
      • Try different values for the capacitors.

      Who can post here the working principle of this oscillator type ?

      Till now, we have uses the tank circuit (L1 or TR1, C4 and C5) connected from collector to base of T1.

      In following topic, we will use a different approach, from collector to emitter ! Yes, we can !

      Greetings

Viewing 3 reply threads
  • You must be logged in to reply to this topic.