For a class B amplifier which has a LC filter at the output, gives Pure sinusoidal Voltage and current waveforms. But if we check back the waveform at Drain, waveform for drain current remains clipped but voltage is pure sinusoidal.

Can anyone explain why voltage waveform gets copied from output to drain but not current?

Class B amp with LC filter


Can anyone explain why voltage waveform gets copied from output to drain but not current?

Nothing gets "copied", that's not how this works.

What does happen is that the FET feeds a distorted / non sinusoidal current into an impedance. That impedance is formed by L1, C1 and L3 and C3. Since you use ideal components for these (no series or parallel resistance) the impedance behaves as a bandpass filter. This means it will be a high impedance for a narrow frequency band. Other frequencies like harmonics are suppressed.

The high value of the impedance in the narrow pass band is also unrealistic. To make a circuit with realistic behavior you need to add series resistors to the inductors and the capacitors. Parallel resistors are usually not needed.

Why is the impedance important? Because the current coming out of the FET is multiplied by the impedance and that makes the output voltage: Vout = Z(f) * Iin

  • \$\begingroup\$ yes and 1nH is the inductance a short circuit of 1mm length or so, what is the ESL and ESR of 10uF, much more! \$\endgroup\$ Sep 23 '18 at 20:33
  • \$\begingroup\$ Thanks for your reply. But my doubt was little different. Once you add the LC filter and check the waveforms at Drain and output port 2. Voltage and current waveform at port 2 is not clipped which is perfect fine because we have added a filter. But when you check the waveforms at Drain, only current is clipped while voltage is pure sinusoidal. I was trying to understand this situation. Hope you got my question. \$\endgroup\$
    – Bth
    Sep 24 '18 at 1:00
  • \$\begingroup\$ Yes I got your question and this is exactly what I answered. The voltage is simply the current multiplied by the impedance. As the impedance is an LC filter, the clipped current signal is filtered. That filtering removes / suppresses the harmonics resulting in a purer sinewave. \$\endgroup\$ Sep 24 '18 at 11:59

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