Confounding transistor bias configuration

[gregallenwarner]

I've been looking around at discrete transistors with built-in biasing resistors, and I came across this part:

http://www.mouser.com/ds/2/302/PIMN31-117357.pdf

These are dual NPN transistors with built-in biasing resistors. However, the configuration is one I've never seen before.
I've attached a screenshot of the internals taken from that datasheet.

This part has R1 (1kohm) in between the base of the transistor and the external pin, but then it has R2 (10kohm) directly connecting the base to the transistor's emitter.

I've never seen the base connected to the emitter through a resistor before in any biasing configuration I've ever come across. There's usually a separate emitter resistor, and a separate voltage divider on the base. Can someone explain what the purpose of this part is please?

[ohmineer]

You can find this configuration in many devices, look for pre-biased transistors (http://rohmfs.rohm.com/en/products/databook/datasheet/discrete/transistor/digital/dtc144e.pdf). The pin-to-base resistor is the base current limiting resistor and the base-emitter resistor acts as pull down.

AFAIK, these kind of transistors are also called digital transistors and designed to work as switches/gates.
I don't think you would use those for an amplifier. As instance, they can be utilized to turn on and off an LED directly from a microcontroller pin with only the current limiting resistor in the collector.

[gregallenwarner]

Ah, I see. I hear the phrase "biasing" and I automatically think amplifier circuits. It makes much more sense now that I understand this thing is just a digital switch.

Thanks.

[David Hess]

In switching and linear applications the base-emitter shunt resistor provides drive to turn the transistor off when the drive is only configured to turn it on.  Monolithic Darlington transistors usually have that resistor built in on the output transistor because there is no access to its base.  The base-emitter shunt resistor is also important for sinking Icb at high temperatures and in power devices.

[SeanB]

A lot of consumer electronics use these as switches or to interface to actual switches, like in VCR players where they were used to read mechanism switches and to drive things like LED's and low power motors or solenoids. Simple and low leakage along with only having a single package to mount.