What is the function of a bleed resistor?

[youyang]

bleed resistor

refering to the attachment that the left composite circuit is equivalent to the right PMOS, but has much higher current driving capability. My question is about the bleed resistor. Some book says the reason of existance of the resistor is twofold: one is to optimize the bias current in M3, the other is to speed up the turn-off of Q4 in high-frequency applications by allowing reverse base current to remove the base charge. I have no idea of the former reason, can anybody give a detailed explanation, and can anybody give a comprehensive explanation of the function of bleed resistor.
thanks in advance

 

[willyboy19]

Re: bleed resistor

Are you sure these two circuits are equivalent? I don't see how a PMOS transistor can be used here (left circuit) together with the NPN BJT. The current flow is not right. Can you double check?

 

[Miguel Gaspar]

bleed resistor

For Q4 to operate, Vbe is needed. So the bleed resistor is to provide the Vbe needed to the transistor to work.

 

[youyang]

Re: bleed resistor

Are you sure these two circuits are equivalent? I don't see how a PMOS transistor can be used here (left circuit) together with the NPN BJT. The current flow is not right. Can you double check?

Sorry, i just mismarked the source and drain of the pmos. Check the circuit agan. thanks
This(the left) is a class B or class AB output stage circuit in op amp. Adding npn is to strengthen the current driving capability when the output voltage is negative in respect of quiescent point in order to draw enough power from the load.Thus the pmos is operating as a source follower. You can check this circuit on page380 in P.R.Gray's ANALYSIS AND DESIGN OF ANALOG INTEGRATED CIRCUITS

Added after 8 minutes:

For Q4 to operate, Vbe is needed. So the bleed resistor is to provide the Vbe needed to the transistor to work.

With M3 operating, Bleed resistor is no necessarily needed to make the Q4 enabled because M3's drain current provides the biasd base current of Q4. My confusion is on the other functions of bleed resistor.