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 !