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 that help themselves."

"If you think you need floating point to solve the problem then you don't understand the problem. If you really do need floating point then you have a problem you do not understand."

 

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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