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Well,the ability of the output transistors to pump a current over a very small resistive load (4/8/16 Ohms), to create a voltage will determine distortion.
Hence the output driver sizes are usually crucial because the output is a low impedance node. Hence you need a lot of current to cause an appreciable change in the output voltage.
edisonliu,
The main source of distortion is non-linearity in the output stage. For tube amplifiers, the output transformer also contributes to non-linearity.
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Class A output stages are the most linear, followed by class AB, class B and class D (Switching).
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Non linearity can be reduced by the use of negative feedback.
Regards,
Kral
Always check your output transistors are in the correct bias point (IDC=0 with Vbe just a little bit lower than Vbeon) to avoid crossover!!! This is one of the main sources of distortion.
1. effect of stray and coupling capacitance. At low frequency the coupling capacitance value is high and it appears in series with the signal source. Hence signal loss takes place.
2. Because of change in temperature the transistor Q-point changes which causes distortion in the output signal.
To reduce distortion of class AB amplifier:
1,try to achieve high small signal openloop gain and wide bandwidth.
2,try to increase output MOS size to get enough driver ability .
In fact I use 2 resistors as the feedback(beta) and there is only 10k load.But there is still a second harmotic in the output by test.Why I can not find the second harmotic by the simulation?
edisonliu,
Are you simulating the effects of crossover distortion in the open loop response?. The distortion due to the non-linear Vo/Vin at low output voltages can be severe for a class AB amplifiere. The distortion will be reduced, but not eliminated by the feedback.
Regards,
Kral
karl,
The distortion is larger both for small and high output voltage by testing.I will care the distortion more for higher output. The distortion may be reduced to 1/(beta*gain)/ (beta*gain) from Razavi's book. Is it right？
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