How common base amplifier actually amplifies the signal?

Question

I think I understand how common emitter amplifier works but common base is confusing me. Can somebody explain?

Plus how can I figure out resistors and capacitors values for the common base? Thanks

Answer 1

In common base, the input current and output current are almost identical, but small input voltages result in large output voltages, hence voltage gain.

Easiest way to bias a CB stage is to ground the base, run the emitter from a negative supply through a resistor. Eg, a -15 volt supply through a 15k resistor will bias the emitter at -1ma or a bit less, actually 14.4/15 or 0.96ma.

Run the collector to a + voltage via a resistor. The emitter current sets the collector current. In this example, +15v and a 7.5k resistor will bias the collector at about 7.5 volts.

Or you can bias it like you do CE, just remove the bypass cap on the emitter and feed the signal in there. Bypass the base to ground.