I wish to use piezo transducer 273-073A to pick up and
amplify insect activity. What kind of simple circuit
would amplify enough for ear phones?
To understand piezo microphones, it's useful
to think of a piece of ceramic, that's going
to be flexed by an impinging sound wave. The
first problem is that the acoustic impedance
of ceramic is so much different than air that
it'll not interact very well, and only a weak
signal can be picked up that way. In fact,
using Google, it appears this transducer is
meant for use as a contact pickup, e.g. with
a guitar, piano, etc.
Second, in case you do want to work with the
piezo transducer anyway, to analyze its use,
you only have to realize it appears to be a
capacitor with its voltage signal in series.
Resistive loads on capacitive sensors make a
low-frequency rolloff, so to design your preamp
you need to know the piezo capacitance and your
desired low-frequency limit. For example, if
the capacitance is 150pF (I didn't find the RS
part's spec), and you wish it to work down to
50Hz, then the "load" will have to be above
22 meg-ohms. The answer is to use a JFET-type
preamp and keep the necessary dc-bias resistor
at 22M or higher. You can lookup JFET.
You'll want lots of gain, of course, but we can
tell you the noise level will be determined by
a parameter we call voltage noise, e_n, which
is lowest for JFETs with a large die area.
There are JFET opamps that are pretty good, but
discrete JFETs can do much better. If you look,
you'll find lots written about the subject.