# Bias question in BOSOZ

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#### Peter Huang

I am not a profession in electronics. I don't understand the design of dc bias for the differential pair. I hope somebody to solve my question.

As shown in Fig. 3 of the project, when the regulated rails are 60 voltage and the voltage drop of R16 and R17 are neglected, the quiescent current through th mosfet is 40mA. The drains of the mosfet are 30V, the source pins are -4V and the gate pins are 0V when no inputs. Then, the voltage drop through R1 or R2 (750 Ohm) are 30V. Up to now, everything is ok. However, I find the voltage drop of R5 and R6 (or R3 and R4, 1500 Ohm totally) is only 56V, which is calculated by the voltage difference between the negative voltage supply(-60V) and the source pin of mosfet (-4V). Since the quiescent current is 40mA, I think the voltage drop through R5 and R6 (totally 1500 Ohm) should be 60V, not 56V. what is wrong in my thinking?

Thanks

#### jwb

There isn't anything wrong with your thinking. You are absolutely right, and the answer is that the quiescent current is not quite 40mA. It is 56V/1500Ω, or 37mA.

#### Peter Huang

Thanks a lot.

Now I know how to design.

If the voltage drop of R16 and R17 is considered, the quiescent current through two mosfets should be 56/(22.1+1500/2) A or 72.53 mA, and the drains of mosfets should be 60- 72.53/1000*(22.1+750/2)=31.2V. Anyway, in practice, the voltage difference between two drains of the two mosfets should be as small as possible.

I think the indication "+30V" in Fig.3 on the paper falls me in a mistake.

I will try to match resistors and mosfets as precision as possible, in addition, measure and listen to what is a difference.

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