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Need help on this circuit

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fanrong

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Hi ,everyone :

Would anyone tell me the functions of the C4,C5,and bipolar transistor Q93 ?
This circuit is a buffer used in a LDO .



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Sorry , the image is too little . I post the link here :


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https://obrazki.elektroda.pl/10_1175502808.gif
 

hi!

i'm not quite sure but you can look for the book solid state pulse circuits by david bell. two capacitors in parallel add up resulting in a larger capacitance. capacitors attached to the base speed up the transistor. but excessive speeds may cause breakdown as this leads to large amounts of currents. it speeds up transistor switching on. i'm not sure if it speeds up the switching off as well.

- al
 

I think that under normal operation, Q93 doesn't do anything, since vin = vout. but if you were to bring vin high enough, the top branch of the circuit wouldn't be able to pull vout high enough since the max input voltage is something like 1 diode drop plus VGS-VT for the mosfet load. So I think they've added Q93 to allow the output to swing within a diode drop of the top rail.
 

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