What are the function of Q176 and Q174 in this bias circuit?

[walker8764]

a bias circuit

Attached is a bias circuit, would like to discuss what the function of Q176 and Q174. In my opinion, when CD(chip disable) level is high, Vref is set to zero, and then Q172 and Q178 are cut off, so Q176 and Q174 and R85 form a Wildar current souce and supply current to Q170, so as to make sure Vref is set to zero.

Would like to know your opinion.

 

[paulux]

Re: a bias circuit

Q174& Q176 a startup circuit. Say thanks by pressing "help+5 points"

 

[walker8764]

Re: a bias circuit

ok, i think i understand. If the circuit works at the zero balance point, then Q174 will drive current from Q147, and the current is mirrored to Q143, and the working point will come to the normal point that we want.

Thanks.

 

[walker8764]

Re: a bias circuit

Another question: How to caculate the output resistance of terminal VB?

 

[fendy]

a bias circuit

The start-up CKT are not only Q174 & Q176...

 

[willyboy19]

Re: a bias circuit

First, the start-up circuit is Q176 and Q174, period.

Second, the output impedance of Vb can be simply calcuated as

R120 + 1/gm238.