Darlington ease emitter bypass resistor
Although the Darlington pair circuit is often used in its basic format, it is often seen with a bypass resistor between the base and emitter connections of the final transistor.
The bypass resistor is included to aid the switch-off process. Without the resistor in place, there is no discharge path for any charge held in the capacitor formed by the base emitter junction. Including it enables the charge stored in this capacitor to dissipate and this aids a faster turn-off.
It is good design to include this resistor, but if speed is not an issue then the resistor can be omitted. However, unless cost and component count are key drivers within the circuit design, then it is wise to include this electroniccomponent to provent any unusual turn-off phenomena.
Determining the resistor value is not an exact science. Smaller resistors will give a faster turn-off, but if they are made too small then a large proportion of the drive current for the second transistor passes through the resistor and gain is lost.
If the value of the resistor is low and it robs current from the base of the second transistor, then current gain will be reduced and the equation for the overall gain of the Darlington will need to accommdate this.
Typical values might be a few hundred Ohms for a power Darlington transistor and a few thousand Ohms for a small current transistor.