Line In Amplifer to Piezo Help

Question

I am trying to build a very simple amplifier type device to power a 4ohm 150 watt piezo tweeter. I am getting my signal from an mp3 player and then want to use a voltage source, 9-20 volts, to amplify the signal to as much as possible and as simply as possible. The goal is 120db. I have talked to a few people and all give me different ways of doing it. The simplest way I was told is using a basic transistor amp circuit and it would work.

Can I use this very simple circuit and just replace the small transistor with a larger mosfet and apply my 9-20 volts? Will anything back feed into my mp3 player and destroy the output?

I don't really care about quality sound since it will just be constant tones coming out. I just need them to be amplified as much as possible and very simple.
Thanks

Answer 1

No, that won't work. The piezo is basically just an oversized capacitor and for a common emitter to work effectively it needs a certain amount of dc collector current . The piezo will totally block the dc.

I'd be tempted to use an audio output transformer or some kind of push pull amplifier.

Answer 2

A single transistor with a current gain of say 300 can be used to either impedance from input (10k?) to output (300) or amplify voltage but not both the product of change in V/R gain depends on the current gain of the transistor.

120dB gain means 6 decades or 1 million.

So you do the math, and look at existing solutions instead of trying to design a "square wheel".

If the transistor in your circuit had a current gain of 500 it would offer a 1/R gain of 100k/300=333 and a voltage gain in the range of 30 dB depending on device. close but no cigar

Answer 3

Your application is very similar to the outside siren used on intruder/burglar alarm systems, in the UK its called a bell-box. In these a piezo horn is driven by a step up transformer to give about 90V pk-pk. They can easily achieve 110dB at 3m, so with a larger piezo horn you should be able to meet your target of 120dB. In a siren the piezo horn and step-up transformer have to be designed to work together to get the highest sound output without damaging the piezo, which are easily damaged if overdriven. The transformer is usually driven by a darlington transistor such as a TIP121.

Although I have shown a transformer with separate primary and secondary windings in practice it is an auto-transformer with a common primary winding.

^{simulate this circuit – Schematic created using CircuitLab}