Transistor matters
Since this circuit will operate mostly open-loop, its linearity must be excellent. Let's check out the operating conditions of the first transistor, which is used to steer the DAC output current out while keeping its input pin at a known voltage. Simplified schematic :
Now, the bias current will need to be superior to the DAC max output current ; we might choose 15 mA for instance, so the operating current range will be about 10 to 20 mA. Since the main source of error is the base current which gets out of the signal path, we should chose a transistor with :
- high and constant hFe at in the operating current range.
- since it will operate at constant voltage (cascoded), fT is of no importance. However we need a transistor to cascode it, which will ned up with the same requirements anyway... so, high fT (low parasitic capacitance) ; this isn't very critical as most small signal transistors have fT in the >100 MHz range.
- low voltage and power because...
- SMD is preferred.
hFe(Ic) for Philips BC849
As we can see, this is very much out of the linear range of the BC849-850-549-550 family. The base current starts introducing temperature and amplitude-dependant nonlinearities in these before 10 mA.
Conclusion : these 100 mA transistors should only be used at 1mA.
For this we need a transistor with more current handling capability, like the BC817-40 or 337-40 family. The curves are for -55, 25 and 150°C from bottom to top.
...which is available in a matched pair which could be useful.
Conclusion : for 10 mA bias we need a 500 mA transistor. (this is also valid for the input diff pair of an amplifier, for instance, but the currents are a lot smaller usually).
The curves also depend on the manufacturers. For instance the Motorola curves are quite different from the Philips ones !