A transistor in a common Emitter configuration is used as a switch, and it is in the ON position. The output voltage Vo is :
Equal to VBE
Equal to Vo
Equal to Vi
When the transistor is in saturation region with switch in ON position , IC is very large and VO is almost zero.
In the following common emitter configuration, an NPN transistor with current gain b=100 is used. The output voltage of the amplifier will be