Colpitts Oscillator - Voltage Swing Amplitudes

Discussion in 'Homework Help' started by splatapus, Nov 28, 2015.

  1. splatapus

    Thread Starter New Member

    Nov 28, 2015
    Hi everyone,

    My telecommunications circuit assignment involved a design of a Colpitts oscillator with the base grounded. The schematic is shown:

    I've correctly calculated the values for C1, C2, and RL. The simulation results shows the voltage output at RL where the oscillating amplitude grows from near 0 to the desired oscillation amplitude:

    My question is about the amplitudes. I understand why the downswing is limited to roughly -4 V, due to the collector only being able to go down as low as the emitter voltage (5V Vcc - 0.8V Vemitter).
    I don't understand why the upswing can go as high as 6.7V though. I know that the inductor Lt stores energy which allows the collector to swing above the Vcc rail, but even then, shouldn't it only swing up by the same amount as the downswing (~4V)? Why does the upswing allow for 6.7V amplitude?
    Last edited: Nov 28, 2015