A question about the output stage of a OPA

[wjxcom]

Hi, all: please look at this circuit which can be found in the book writed by Gray.
I do not know:
1. why this can improve the output swing,
2. why a large bandwith can prevent the crossover distortion?
3. why increase the quiescent current can reduce the crossover distortion?

Help me! thanx!!!

 

[mpig09]

Dear wjxcom :

i only answer first question.
the output only reduce a overdriver voltage, so the swing is increase.

Vout,max=vdd-Vod
Vout,min=vss-Vod



mpig09

 

[ptmsl]

1. Using this configuration the output transistors can be driven into saturation by the driver amplifier while the driver does not have to reach VDD or VSS. So with a light load the output can go full swing between VDD and VSS.

2. Feedback corrects crossover distortion as long as there is available gain for the feedback loop to use. When the harmonics produced by the crossover distortion are at a frequency that the opamp does not have enough gain feedback cannot correct it.

3. By increasing the quiescent current you move further into class AB and the point that each transistor turns off moves to a larger signal level and in comparisson the distortion is smaller. Remember that crossover distortion is at the point that one of the output transistors is about to turn off and its gain dramatically decreases.