# Bjt as aswitch

i want to know why at some point the low output voltage for a bjt biased between 0 and 5
on the base
and voltage 5 at the drain
ranges between 0.225v and 75mv so which of them is the low voltage pz answer me now i have a project to deliver

## Answers and Replies

BJT? BASE? DRAIN?

A BJT has three terminals, EMITTER, BASE, COLLECTOR,
There are two varieties of BJT:

NPN and PNP which represent differeing doping polarity used in making the devices sections.

A BJT transistor driven into fully saturated conduction
will usually have a relatively small voltage drop between
its COLLECTOR and EMITTER; the value of that voltage
is called "V_CE_SAT" for "saturated Voltage between
COLLECTOR and EMITTER" and it is often around
0.35 Volts.

When there is little BASE current the transistor will present
a very low conductance (i.e. a high resistance) between
COLLECTOR and EMITTER, and thus the voltage drop
across the COLLECTOR and EMITTER will be large, almost
as large as if the transistor was taken out of the circuit
and a large -- e.g. 50 million ohm -- resistor is placed
between where the collector and emitter terminals were.

And usually one doesn't speak of "low" voltage with
a single BJT since usually a logic circuit involves
at least one BJT and one diode and at least some
resistors or other BJT components.

However comparing magnitudes, 0.075V is LOWER than
0.225V; I suspect 0.225 could be V_CE_SAT, I have
no idea what 0.075V represents....