how to know if a common emitter amplifier can drive an 8 ohm audio speaker

Thread Starter


Joined Oct 1, 2020
My question is basically just what the title says, there's part of a question which asks if the common emitter amplifier from previous parts of the question can drive an 8 ohm audio speaker but tbh I don't even know how I could tell if it could or not, could anyone tell me what I should be looking for or am I just being stupid and not realising it.


Joined Oct 2, 2009
You need to examine the actual circuit configuration.
You are then looking for driver output impedance to be as low as possible. This is not easily attained with common emitter configuration unless you use a step down transformer.

You can try common collector configuration.


Joined Jul 10, 2017
You need to determine whether the collector current or power dissipation would exceed the recommended limits for the transistor with a load as low as 8 Ohms.


Joined Mar 14, 2008
Also depends upon the power you want to the speaker.
A emitter-follower amp is generally not practical for more than a watt.

Audioguru again

Joined Oct 21, 2019
For audio, a single transistor must be operated in class-A for it to swing the speaker cone back and forth.
To make 1W in an 8 ohm speaker then the RMS current must be 0.354A which is 0.5A peak. 1W is not loud.
Since a speaker cannot have DC in it then the transistor must have its own collector resistor and use a capacitor to couple the signal to the speaker. The current and heating in the transistor and in its collector resistor must be massive, even when it is not playing.
Or you can make it produce low current, low heating and a very low output power in a speaker.

Amplifiers use a push-pull pair of emitter-follower (common collector) transistors operating in class-AB then their current and amount of heating is very low when not playing.