•The resistance of R1 can be set such that the overall min/mid/max values of the circuit are: 5.00 kΩ/5.56 kΩ/6.00 kΩ.

•Given a Vbe of 590 mV for Q1, this produces a current range of 92.4 µA (at R1=5.0 kΩ) through to 110 µA (at R1 = 0 Ω). These calculations assume that the PWM is at its maximum entered value.

• Worst-case range: 98.0 µA … 105 µA. (This is with all resistors at either their high or low values, whichever is worse for the particular calculation.)

•To judge the linearity, compare the average, which is 101 µA, with the value determined by the potentiometer at its midrange position, which is 99.8 µA.

•The adjustment range, relative to the pot's midrange setting, is -7.4% ... 11.1%.

•Optimal value for R4 is around 17 kΩ. [Note 1 ]

[1] The value of R4 is generally not critical: if it is too high the desired current will not be produced since Q2 could be starved of base current. Ideally the value of R4 should be such that the current through it (as determined by Vcc-2*Vbe) will match the design current. This ensures the two transistors have similar base currents, which ensures that the current set by the R1—R2—R3 combination is very close to the calculated value.

[2] The meter's resistance need not be determined, however if it is, the voltage across the meter can be checked against the headroom available as determined by Vcc-2×Vbe. If this resistance is not known, just use the default 100 Ω.

[3] The default is set to 240/256. This adjusts all calculated currents so that the circuit will attain full scale at this PWM setting.