#preamplifier circuit

So it's been a while but I thought I'd analyze another schematic similar to the last transmitter bug.  @JohnS_AZ also sent this one to us and it is a two stage FM transmitter bug - supposedly more powerful and capable.  It works with a 9v battery and claims to have a range of up to 1km in the open.

So as in the first transmitter we looked at, the microphone from where our mic level input comes from is an electret microphone.  The preamplifier circuit is quite similar to the one we've seen before:

The main differences you'll note are in the values of the resistor from the power rail to the mic and from the power rail to the collector electrode of the transistor.  I'm not sure of the significance, but perhaps because FM transmitter 2 has three amplification stages as opposed to two, the electret microphone is provided with less power than FM transmitter 1.

You'll note that the output of the transistor preamplifier stage still goes through a 100nf capacitor as before.  The one new thing we see is a 100nf coupling capacitor connecting the power rail to ground.  This must be some sort of dc filtering capacitor that helps to get rid of any additional noise.

And to reiterate some of what we learned from analyzing the last preamplifier circuit, the initial 22nf capacitor filters out DC currents and allows AC currents (our audio signal) through.

Then, the 22k resistor and 1M resistor do what is known as biasing the base electrode of the transistor.  Remember that it is both the signal and strength of the signal going into the base electrode that determines to what degree the transistor amplifies that secondary current that becomes the output of that stage.

This handy gif was what helped me realize that the two currents were separate but that the one going to the base electrode determined/allowed the amplified current to proceed.

Next up! The adjustable/tunable tank circuit and amplification 1 stage!

So @makerdino and @brainwagon as well as @whixr have been working on creating, analyzing, and optimizing a simple common-emitter preamp circuit that Dino made for his Hackaweek project. 

The above schematic is quite similar to the one that Dino used and was partially analyzed in my electret preamplifier circuit post.  Whisker took the circuit and added a few bits. 

If we look at this generally, there is a capacitor from the microphone to the circuit, and a capacitor from the circuit to the output.  These serve to filter out any dc current and only allow ac current, or the audio signal, through.  These are known as coupling capacitors as highlighted below:

Since starting the writing of this, @brainwagon has discovered that because the microphones we use are not specifically electret microphones, the first resistor (R1 next to C1) is unnecessary as that resistor was helping meter power to specifically electret microphones.  Highlighted below!

Adjusting that resistor's value does nothing for gain or function.  So we will in the future, take that resistor out, and even C1, so that the audio input goes through C2 to the amplifying transistor. **EDIT** It worked! The removal of R1 and C1 shown above led to no discernible difference in function or gain!

Now, the middle R1 serves to provide the power to the transistor that will be considered the amplified current.  R2 contributes to how the audio signal will affect the base of the transistor. 

In the electret preamplifier circuit, I thought this particular resistor (R2) prevented the power from influencing the audio signal and vice versa, but I believe that somehow the power is added onto the signal to help it affect the base of the transistor.  I don't know the exact mechanics, but if its role were only to prevent the power rail from affecting the audio signal, then there wouldn't be an electric connection at all.  Right?

Anyway so Whisker originally used a wire instead of R3, but that resulted in an ear-splitting squeal, so using the below tip, he found that a resistor worked nicely to bring the voltage down (to an appropriate but not severely diminished level) before the second amplification stage.

Tip: Using a potentiometer (or variable resistor), you can tune to see which specific resistance value works best for the circuit.  Then you can take out the potentiometer and put the "hardcoded" resistor in.

So now - the audio signal that has been slightly attenuated, again gets dc current filtered out.  And then encounters the same R1-R2-3N3904 amplification portion. 

Then finally, it encounters the last coupling capacitor for a last minute filter of any dc badness.  Then onward and forward to the output!  We put the output of the preamp into our mixer which fed the computer.

We will have a demonstration of this on youtube but what I can tell you is that we were able to amplify a Cascade long ribbon mic, Nady-rsm4 ribbon mic, Shure 55SH, and PG48 mic with such clean results that it pretty much sounds like our professional preamp that cost 500 bucks. 

Try it for yourself, let us know how it worked for you?

Whee!

(PS We called it a cascade preamp because of the two stages of amplification in series, but also because it works for our cascade microphone - punpunpun!)