Common emitter amplifier question - Page 1
Forum section: electronics / beginners

author: Stevie broon
author_rank: Newbie
timestamp: May 20, 2014, 07:32:55 pm
content: Hi, never used a forum b4 so apologies if I've posted in the wrong place.My question is.With regards to the common emitter single transistor amplifier, I understand the theory of its operation and biasing etc but I am confused when I see it used with a single rail supply.With Split rail I can easily imagine the output wave swinging near enough rail to rail around 0v and the speaker diaphragm pulling in and pushing out, but I've seen a lot of schematics and tutorials using a single supply with the output biased to half the supply voltage (good so far). But then I see that the other side of the speaker has been referenced to gnd.Does that mean the speaker diaphragm is only ever pushed outward then vibrated whilst in the outward position, therefore loosing half of the physical travel of the cone due to the other side of the speaker not being referenced to half the supply??Scratching my head at this one!Any answers would be much appreciated

author: retrolefty
author_rank: Super Contributor
timestamp: May 20, 2014, 07:36:05 pm
content: If I'm recalling correctly are not most single-ended audio amps capacitor coupled to the loudspeaker, thus eliminating any DC bias from the speaker voice coil?

author: Stevie broon
author_rank: Newbie
timestamp: May 20, 2014, 07:44:02 pm
content: Yea, there was a capacitor before the speaker.Still don't understand how the the voice coil could go inward past the rest position if one side was connected to 0v and the output from the transistor is always a positive figure between 0 and supply voltage.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 20, 2014, 07:46:17 pm
content: The output is normally coupled with either a transformer or capacitor, as retrolefty posted. The speaker coil will rest at its neutral position until the audio current moves it in and out.cheersRob

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 20, 2014, 07:47:24 pm
content: Quote from: Stevie broon on May 20, 2014, 07:44:02 pmThe capacitor blocks the DC relative to the source. Only the capacitor will have the quiescent voltage across it.cheersRob

author: Stevie broon
author_rank: Newbie
timestamp: May 20, 2014, 07:56:12 pm
content: Thanks for the quick replies guys.I must buy a scope to help me visualise things better !

author: mstevens
author_rank: Regular Contributor
timestamp: May 20, 2014, 07:56:54 pm
content: This image should explain it.https://www.google.com/search?q=ce+amplifier+design+biasing&rls=com.microsoft:en-US:IE-SearchBox&source=lnms&tbm=isch&sa=X&ei=67J7U5ntIpCkyAT2wYLQBA&ved=0CAYQ_AUoAQ&biw=1632&bih=865#facrc=_&imgdii=_&imgrc=ur5rpjYh3AE6OM%253A%3BkiOAJWbLfKkJZM%3Bhttp%253A%252F%252Fwww.phy.ntnu.edu.tw%252Fntnujava%252Findex.php%253Faction%253Ddlattach%253Btopic%253D2116.0%253Battach%253D3951%3Bhttp%253A%252F%252Fwww.phy.ntnu.edu.tw%252Fntnujava%252Findex.php%253Ftopic%253D2116.0%3B504%3B553Q is the quiescent point. Once you understand that figure you will understand. Figure is explained in almost every microelectronic text such as Neaman, Sendra & Smith, et al.

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 20, 2014, 08:01:23 pm
content: Maybe I can explain it better.At no signal, the voltage at the emitter due to DC current through the emitter resistor charges the capacitor to the same voltage (to ground) on the amp side, but no DC current can flow through the capacitor, so the speaker side of the capacitor is grounded through the speaker. No current flows throught the speaker and the coil is neutral.Along comes a signal that causes the emitter voltage to swingmorepositive. Electrons rush out of the amp side of the capacitor and this attracts electrons to the speaker side of the cap. Electrons flow through the speaker from ground to the cap. The cone moves (pick a direction) say, inward.Along comes the opposite polarity part of the signal and makes the emitter golesspositive to ground.Electrons rush back back into the amp side of the capacitor and that repels electrons out of the speaker side of the capacitor. They flow the opposite way through the speaker coil and the cone moves the other way.When the audio signal stops and the amp goes quiescent again, the capacitor charges to whatever the emitter to ground voltage is, and stops passing current. The speaker doesn't "know" what the voltage on the other side of that capacitor is...the speake AC current stops flowing and the cone returns to neutral position.cheersRob

author: Stevie broon
author_rank: Newbie
timestamp: May 20, 2014, 08:15:49 pm
content: Ahh, that makes more sense now. Never realised the cap had such an effect on the output. Thought it was there purely to stop the dc component from the bias circuit going to the output.Cheers rob

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 20, 2014, 08:24:01 pm
content: Well, in effect what I described is simply the details ofhowthe cap. blocks the DC component and passes AC, just a step by step breakdown of the same thing.cheersRob

author: Mr. Coffee
author_rank: Regular Contributor
timestamp: May 20, 2014, 08:28:06 pm
content: And yes, you are correct in that without that capacitor (or a transformer) the speaker cone would move in and out relative to an off-neutral position, because of constant DC current flow. The speaker coil would then just be in DC parallel with the emitter resistor. With the capacitor, it rests center (no DC) and is only in parallel with the emitter resistor for AC voltage.