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High-side vs. low-side NPN transistor

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permal:
Hi,

In another recent thread I learned that NPN transistors must be placed on the low-side of the load to fully open, as also demonstrated in this tutorial.

Before I redo my design I'd just like to have a behavior I'm seeing in my (non-compliant) circuit explained, see the attached image.

My question is why can I only measure 2.7V at the output to ground using a DMM? I'm thinking that there is such a low current that the voltage drop, Vce, should be negligible and thus read near 5V? Is this due to the transistor not yet having saturated because the emitter is offset from ground through the DMM?

Thanks in advance.

spec:
Hi permal,

That circuit can never output a voltage higher than 4V4: there will always be the NPN transistor's 0.6V VBE drop from the 5V line.

But in order to find out what is going on with your circuit we would need to know what is driving CTRL.

permal:
CTRL is driven by an output of an MCP23017 (I2C IO expander).

T3sl4co1l:
For a 3.1V input, emitter voltage rising above 3.1V would imply not only that the base-emitter junction is reverse biased, but also that leakage current is flowing in the same direction, i.e., from emitter to base.  This can only happen if something is supplying current into the emitter!

The C-E path won't do this, because it's not actually a path, the B-C junction has leakage to the base.  In this region the transistor only acts like two reverse biased diodes smooshed together.

You can still get 5V output for some base voltages -- but you need to apply more than 5V to do it.  This is impractical in most circuits, as spec is referring to.

If the output is switched frequently (as is the case for switching supplies), a bootstrap supply can generate this extra voltage.  A number of LT converter chips come to mind, which internally use a bipolar high-side switch with an external bootstrap capacitor that supplies the transistor while it's on.  Example: https://www.analog.com/media/en/technical-documentation/data-sheets/lt1616fs.pdf  This is probably much more effort and complication than you need so don't worry about it. :)

Tim

spec:

--- Quote from: permal on January 12, 2019, 03:33:31 pm ---CTRL is driven by an output of an MCP23017 (I2C IO expander).

--- End quote ---
What supply lines are the MCP23017 using?

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