SAP15P Datasheet, PDF (3/6 Pages) Sanken electric – Darlington transistors with built-in temperature compensation diodes for audio amplifier applications

2. External Variable Resistor
Total forward voltage (at IF =2.5mA) of the diodes is designed to be equal or less than that of total VBE (at IC
= 40mA) of the transistor, thus the idling current is required to be adjusted at 40mA with an additional
external variable resistor.
The relations are shown as below:
Total VF of Diode Total VBE of Transistor + Total VRE of Emitter Resistor
∆V=0 to 500mV
The VBE of the transistor is dependent to the hFE, and the VBE is lower with higher hFE and vice versa. The
hFE for both the PNP and the NPN varies between 5k and 20k; thus the VBE is the lowest with the
combination of maximum hFE (20k) each and it is the highest with the combination of minimum hFE (5k)
each.
Presuming the voltage difference between the VF of the diodes and the VBE of the transistors (including the
total voltage drops of the two emitter resistors) as ∆V.
Minimum VBE – Maximum VF variations of the diodes = 0
Maximum VBE – Minimum VF variations of the diodes = 500mV
The current flowing over the diodes and the VR is adjusted at 2.5mA; therefore
500mV 2.5mA = 200Ω
Consequently, the applicable VR value is to be 0 to 200Ω
IC
40mA
VBE Min.
VBE Max.
(P and N: hFE Max.) (P and N: hFE Min.)
VBE
Di VF
TR VBE
Variations
Variations
∆VF = 500mV