Two bipolar NPN transistors in pair

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Hello,

 

can someone please explain this circuit to me? I searched over the web and can`t find infos. Found lots of examples of NPN+PNP connected in pair, but never saw this configuration of two bipolar NPN together. IF it helps, the circuit in picture is from a digital input circuit.

 

Thanks

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Last Edited: Tue. Sep 12, 2017 - 09:07 AM
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'This forum helps those who help themselves.'

 

pragmatic  adjective dealing with things sensibly and realistically in a way that is based on practical rather than theoretical consideration.

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Thank you.

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This is a constant current sink, of which there are many versions:

http://www.pcbheaven.com/userpag...

http://www.talkingelectronics.co...

https://electronics.stackexchang...

 

The bottom NPN sets a constant voltage across the bottom resistor because its collector controls the base of the top transistor with negative feedback. This constant voltage has the value of VBE, ~0.7V, so the constant current obtained is ~0.7/RLower minus the base current of top transistor. You can remove the base current error by using a MOSFET as top transistor, and the bottom transistor can be replaced by a TL431 type shunt voltage regulator to obtain a precision constant current sink.

Note: TL431 wastes 2.5V, nowadays there are more modern versions with very low voltage, for example the ZXRE160 has a drop of only 0.6V, comparable to the NPN VBE, but highly stable and precise.

 

Note: sure, if you just want to drive LEDs, you don't need precision. But for instrumentation, you might need a stable current, for example to excite a sensor.

Last Edited: Tue. Sep 12, 2017 - 10:43 AM
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thanks for detailed explanation

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It makes it a lot easier if you embed the image, so that we can see it - like this:

 

 

Full instructions on how to do that here: http://www.avrfreaks.net/comment...

 

 

Klemko wrote:
the circuit in picture is from a digital input circuit.

 

The TLP291 is an Optocoupler (or Optoisolator):

 

Image result for tlp291

 

So, now that you know that your circuit is a constant-current driver - think about why that might be useful in "a digital input circuit" ... ?

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Thanks for guide link.
It is to enable different voltages on the input, since the diode in the optocoupler is a current driven component.

Last Edited: Wed. Sep 13, 2017 - 10:30 AM
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Indeed.

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El Tangas, nice description on how that circuit works.  +1