Driving a tiny speaker via pwm, amplified with a BJT - in reverse?

Question

I'm playing around with a micro and a small buzzer (passive). I built up an simple emitter-follower buffer to reduce the load on my uC, but I accidentally installed the transistor backwards on the board. It worked great, although when I noticed my mistake and flipped it around (with the emitter side tied to gnd, and the load coming off the collector), the sound was greatly reduced, about the same as driving the buzzer directly from the gpio.

I'm trying to understand what's happening. The volume is much MUCH louder with the transistor 'flipped' (as in the diagram). Current from collector -> GND is 50mA, and I get the same reading from the speaker - > the emitter. I(b) is too small for me to read with my meter.

What did I just build?

^{simulate this circuit – Schematic created using CircuitLab}

Update:

After looking at the datasheet from the buzzer, and wiring it up as below, the volume output from the speaker is equally loud in either transistor orientation. Oddly enough, if I remove the flyback diode, then I lose a lot of volume in 'normal' orientation, but not in reverse.

I'll also note that I tried a few other tiny speakers, and didn't observe this behaviour. So, something about the particulars of the buzzers I was using.

^{simulate this circuit}

Answer 1

Okay, we've established it's an electromagnetic type transducer.

The resistor appears to be 270 ohms, not 2.7K.

You may be excessively overdriving it with the transistor working properly. An electromagnet transducer in that resistance range is probably expecting about 1.5V. It also should have a diode across it because of the inductance (one I happen to have measures 40.8\$\Omega\$ and 0.7mH), so try about 50 ohms in series and a 1N4148 across the pair.

For reference, here is a similar part.

Answer 2

The reversed CE gives full bandwidth at unity current gain.