# Precision reference voltage: buffering with OpAmp and BJT

I'm designing a reference voltage generator for the analog section of a circuit. Two voltages as needed:

• Vref = 3 V for ADC
• Vref/2 = 1.5 V for signal conditioning circuits (with a load of about 40mA)

simulate this circuit – Schematic created using CircuitLab

The 3 V precision reference is already available, and I need an hint to generate a precision 1.5 V reference with a load of about 40 mA. In the schematic above, were are possible error sources? I can identify:

• divider resistors tolerances
• input current of OpAmp "+" pin
• OpAmp offset
• what else? I guess that BJT is in the high-gain loop of the OpAmp, so its finite gain and drifts should be automatically compensed. What do you think about?
• Is noise an important consideration? What's the bandwidth under consideration?
– jonk
Commented Jan 14, 2021 at 9:55
• How much does the 40 mA load current vary in value? Commented Jan 14, 2021 at 10:12
• Flicker (1/f) noise might be an important consideration; there are amplifiers that do not suffer from it. Commented Jan 14, 2021 at 10:33
• What is the Vcc? Do a calculation on the maximum current through Q1. How much power will Q1 then dissipate? Is that below its maximum? Will a heatsink be needed? Is the opamp's DC offset relevant? You're using 1% resistors, OK but you talk about precision, how much precision are we talking about? 1% (since you use 1% resistors). How accurate does your 1.5 V need to be? Is trimming allowed or needed? Precision doesn't mean anything unless you give numbers. Have a look at the datasheet of a TL431 and see how they specify precision. Commented Jan 14, 2021 at 10:53
• If it's quite constant then why bother with the BJT - use a potential divider from Vcc to set the approximate voltage and let the op-amp output tweak that to be exactly 1.5 volts. An op-amp is good for +/- 5 mA without much hassle. Commented Jan 14, 2021 at 11:00

Your $$\I_{CE}\$$ is 40mA and $$\V_{CE}\$$ 1.5V. The BJT at approximately those conditions (50mA, 1V) has an $$\h_{FE} \ge 60\$$; for less current you'll have more gain (to a point). The op-amp needs to try and feed a small base current that will not saturate the BJT - based on figure 16 this will be less than 3mA. Such a low current should not see any adverse effect from the output impedance of the op-amp itself, though I'm not an expert in this area.