NPN Audio Amp Question

[rexxar]

Could someone explain how this amplifier circuit works? It uses just one 2n2222 transistor, and I've attached a schematic.

What I think is going on is when the transistor is turned on, it pulls ground high, and when it releases, it causes a pulse on the speaker? Am I close? I know the output is only going to be half of the waveform, but is it going to be positive or negative?

An extra question; if I wanted to amplify the entire waveform, could I just stick a PNP in parallel with the NPN, or does it need its own biasing resistors?

[rexxar]

Hm. It looks like I drew the schematic wrong. I uploaded a correct one.

Looking at it this way, it's clear that the transistor switches on and pulls the speaker low, so it pulses when the transistor is on. That would mean the output is effectively negative?

[Hideki]

It may be an attempt at making a class A amplifier.

The idea is that you bias the transistor to sit in the linear region when there's no input signal. That way there is always some collector current flowing, which will go up and down as the input signal changes. However, connecting the speaker that way is a bad idea, since it will only drive it in one direction and the constant DC current will heat up the coil. When you only have a single supply rail, the speaker is typically connected through a capacitor and the other side of the speaker is connected to ground.

You can add a PNP in SERIES to make it class B (or with some improved biasing, AB).

Time to do some more reading?

[fcb]

As stated before it's a class A amplifier.  Theory is you bias the transistor to a point where your where you still have current flowing through the transistor on peak negative input.  In practice there are many other ways to build an amplifier better.

Build it and see.

[tszaboo]

Please no. On the first I was laughing, on the second crying. In a Class A you dont run current constantly on a speaker, as it supposed to work with alternating current. With DC you kill it.
It is fine for class D or self driving buzzers.
If you want 1 transistor amplifiers, then search for the Zen or the DoZ amp.

[Zero999]

A small current will not harm the speaker, as long as the power dissipation isn't excessive, for example a quiecent current of 150mA through a 0.5W 8Ohm speaker will dissipate just 180mW which is safe. It's not the correct way of doing things but it will work.

[fcb]

Please no. On the first I was laughing, on the second crying. In a Class A you dont run current constantly on a speaker, as it supposed to work with alternating current. With DC you kill it.
It is fine for class D or self driving buzzers.
If you want 1 transistor amplifiers, then search for the Zen or the DoZ amp.

1. Class A refers to the amplifier stage, not the speaker coupling method.
2. As Hero999 has calculated, you won't kill the speaker. It will just be limit the travel and will probably have some interesting distortions due to the mechanical asymmetry.
3. The Zen and DeathOfZen both have more than one transistor, and they use coupling caps.

You could build a single transistor amplifier that sounds good and gives a reasonable power output if you used a transformer.