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Transistor as switch doubt

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Junior Member level 2
Jun 23, 2015
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Suppose if i take an npn transistor and apply some voltage at the base the transistor will be ON and the emitter is grounded and the output at the collector is 0V. Similarly if i make the base cutoff the emitter is open circuited and the current is 0, the collector output is supply voltage. Now my doubt is when you say open circuited does not mean the physical connection from the emitter to ground is completely gone? In that case the emitter pin should be floating? The voltage at the collector is not supply voltage?

The emitter does not float, the collector does. And since the collector floats during OFF mode, its voltage is essentially the supply voltage.

Two things
a. Collector is floating does it mean physically disconnected?
b. if it is floating then where is the reference voltage and how it can be supply voltage if there is no reference?

It means electrical isolation and not physical. There's no real importance of physical isolation in terms of electricity. We always speak of electrical isolation.
The emitter is grounded, it is obvious that emitter is at reference voltage ie. 0V

yes now I understood thank you but maybe the term floating should not be used since in microcontrollers float means not connected.

The insulation qualities of open collector transistors are defined by leakage when off and the breakdown voltage Vce(max) and are rated for voltage much higher than Vcc. Whereas CMOS drivers in MCU's are only rated to not exceed the supply rail when in tri-state mode so Sch. diode. protection must be included. Open Drain MOSFETs are also rated by leakage and Vds max like open collector BJT's but tend to be lower ratings in general.

Most common use of open output has a pullup resistor selected to over come load leakage and EMI noise on lines.

yes now I understood thank you but maybe the term floating should not be used since in microcontrollers float means not connected.
Again you are speaking in terms of connected and not connected.
Floating only means electrical isolation ie. a high impedance path.
For that matter, even an unconnected pin in a microcontroller is connected through air. And during OFF state the collector and emitter are connected through depletion regions. The thing common is that both air and depleted region in silicon are insulators and do not conduct even if they are connected across voltages.

The term float is rightly used to differentiate from zero voltage state.

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