Open Collector config ULN2003

Question

Are open collector configurations like those found in ULN2003 always in saturation (when Sufficient Base voltage is applied)? Can they never go into the active region?

Answer 1

ULN2003 is simply a darlington array, each channel consisting of two transistors and a few resistors in a darlington configuration.

Surely they can go into the linear/active region. Just look up the datasheet and you'll find the following schematic:

There is also a graph showing exactly what goes on in the active region.

Although, the ULN2003 cannot be considered a representative of all open collector devices. There are many with TTL, CMOS or similar type of inputs that can only be on or off.

Answer 2

The manufacturer's intended application is saturated switch, with input pins driven by digital logic. All its data sheet specs for ULN2003 point to switch-type service.
But every pin brought to the external world invites alternate use. Can the ULN2003 operate in analog domain?
Of course - but this domain is at the users risk. You'll have to characterize much of its "linear" operating envelope yourself and expect little supplier support for such service (unless you're willing to buy very large quantities).
Although the data sheet shows an internal schematic with base resistor of 2.7k, 7.2k, 3k, these resistors likely have poor tolerance from one device to another.

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For example, a data sheet suggests that a substrate diode should not be used to carry current. If collector-emitter voltage should go negative, causing this diode to conduct, the manufacturer can claim violation of service. No reason is given for this particular operating-envelope caution.

There may be other operating-envelope cautions that can cause problems that never occur in the conventional switching service that the manufacturer just doesn't mention.