I'm curious about JFET/MOSFET circuit B class amplifier. When BJT is used we only have those (+/-)0.7V limiting the transistors and causing crossover distortion. But with adding FETs or MOSFETs to the circuit, things rapidly change:

  • FET class B amplifier: Here JFET in only limited by the pinch-off voltage (not by 0.7V like BJTs) so crossover distortion does not occur. If input voltage exceeds the pinch-off voltage then we only get a signal cutter, right?

  • MOSFET class B amplifier:

-- Enhancement mode MOSFET: Crossover distortion applies for this one, since we need to turn ON the transistor. But the disadvantage (my opinion) is that crossover distortion is quite wide/large, since the transistors threshold voltage varies from 2V to 4V.

-- Depletion mode MOSFET: Same as for JFET, only that here we can also enhance and deplete the channel, right?

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  • \$\begingroup\$ If the input voltage exceeds about 0.7 volts positive or negative, the other JFET will act as a diode. \$\endgroup\$ – Oskar Skog Apr 18 '17 at 17:53
  • \$\begingroup\$ @OskarSkog: That is obvious. And that this also means that n-channel and p-channel JFET can't be used as complementary pair as amplifier? \$\endgroup\$ – Keno Apr 18 '17 at 17:59
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    \$\begingroup\$ They can be used as a complementary pair for signals with peaks below 0.7 volts. And I think you could also use separate coupling capacitors for each JFET and connect a Schottky diode between gate and supply rail to short circuit before ~0.7 volts is reached. \$\endgroup\$ – Oskar Skog Apr 18 '17 at 18:04
  • \$\begingroup\$ @OskarSkog: Lets say I use a class JFET amplifier (n-channel). DC point is biased so quiescent point is in the middle of output line. But if the input AC signal exceeds 0.7V, what happens? JFET acts as a forward biased diode although DC point is biased properly? \$\endgroup\$ – Keno Apr 19 '17 at 19:43

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