# How should I find the DC bias values for this circuit?

I would like to build this circuit, but I have to find the DC bias points first. Based on current requirements of total 11mA, I decided to set current through the differential amplifier stage to be 3mA, and 8mA through the gain and push-pull stage combined. I have chosen diodes with forward voltage of 0.7, and transistors with Vbe of 0.7 as well.

However, my understanding is that beta changes with temperature so I cannot have a fixed value for it. I'm not sure if I can assume Vout and Vin+ and Vin- to be 0 at the quiescent point also. How then should I proceed in the calculation?

• Where does the 11mA total current requirement come from? Dec 13, 2020 at 15:41
• The differential pair current is set by RE resistor value. Thus, if you want Ic1 = 1.5mA then Iee = 3mA and Re = (Vcc - Vbe)/Iee. And the VAS stage current Ic3 = (Vcc - Vbe)/Rc3
– G36
Dec 13, 2020 at 15:51
• The 11mA total current is from static power requirement of 0.2W with VCC = 9v and VEE = -9V Dec 13, 2020 at 16:09

I'm not sure if I can assume Vout and Vin+ and Vin- to be 0 at the quiescent point also. How then should I proceed in the calculation?

You pretty much can assume this because the DC gain of the circuit will be in the realm of maybe 1000 or even higher and, the effect of negative feedback via RF2 will ensure that the mid-point quiescent voltage on the output will be within a whisker of the ground voltage set by the lower end of RIN1.

How then should I proceed in the calculation?

By assuming what I said above is true.

BTW I would look to making most of the 11 mA current consumption in the quiescent condition pass through Q11 and Q12 to avoid unnecessary crossover distortion. If that means 10 mA for Q11 and Q12 and 1 mA for the rest of the circuit then so be it.

• Is it right to say then, if and only if my circuit is biased properly, will Vin-, Vin+ and Vout be 0 or very close to 0 at quiescent point? Also, in order to prevent the BJTs from overheating, I did not want to pass too much current through the push-pull stage. Dec 13, 2020 at 16:17
• When properly set up, Vin-, Vin+ and Vout will be within 10 mV (approximately) of 0 volts. If you don't want then to heat up then maybe you should reconsider whether a class AB stage is a valid design choice (unless of course your output load is fairly high impedance). Dec 13, 2020 at 16:24
• May I ask, why is it that the differential amplifier branch for Q1 has a resistor but Q2 doesn't? Shouldn't a differential amplifier be perfectly symmetrical to ensure the same current flows through both sides? Dec 14, 2020 at 20:47
• @cheapbrentcrude ideally yes but it won’t make a big difference given the high dc loop gain. Dec 14, 2020 at 21:41