2n3904 pre amplifier improvement

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This is actually a good circuit to learn about transistors. Everybody that took an electronics class built some version of this circuit back in the day. I don't know what they teach now in electronics classes. But it's well worth building, measuring, changing resistor values and remeasuring, etc.

I can't tell you how many times I had that light bulb go off in my head when I was fiddling around. Those lessons last a lifetime. I remember when I was 6, I substituted a "C" battery for a "D" battery in a simple switched light circuit. I expected the light bulb to be dimmer, but it wasn't - and I instantly knew why. From that point on, I was never confused about voltage and current.

I am using breadboard. His mother will kill me if I show him soldering iron.

I was soldering when I was 10. I realize that now you would probably be jailed for allowing a 10 year old to use a soldering iron.

Times sure do change. My buddy almost had his grandson taken away from him when he brought him to the hospital with a sprained finger. Cops showed up and everything, gave him the business. The kid fell playing baseball, for crying out loud!
 
By the way I am not a old folks or anything like that.
I am 15 year old and hopefully be 16 this year on my birthday.
I soldered my first circuit at age of 11 . My granny taught me to do it . Using a old school soldering iron(like a arrow head of copper allow) on gass stove in our kitchen while she was cooking.

Sadly my most of classmate's don't know about anything of electronic . They can't at least identify components.
 
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If you want to use only one transistor
there are some better ways than that.

Can I know a circuit to do it?

Here you have a circuit showing how I would do it with one 2N3904.
I have not tested it, but I think it would work well.

The voltages may not be like 1.50V and 5.00V exact.
But wil be something like it.

The teoretical voltage gain is R2/R1. (=4.7)
With a load at the output it may be more like 4.0.

I think this circuit will do fairly well,
but of course must be tested before we know for sure.
 

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PRR

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> can be made better.

Better for WHAT??

It does not say so, but this is *only* for electret microphone. R1 10K to V+ can do damage to any other source, remove it. The maximum input is 20mV, mike level, but too easy to overload with ANY other source. And the input impedance is unnecessarily low, maybe 1K. And the biasing is not optimum.

We add R4 to set the gain and raise the input impedance. Gain will be a bit less than R3/R4. Base bias resistor should be near (R3+R4)*hFE, or like 1.2Meg-3.6Meg. But odd-Meg resistors are rare, lower is more stable, use 1Meg. This form of bias is self-adjusting over a wide range of supply (2V to 40V or more) and tolerates a good range of hFE; so works with 90% of the NPNs in your parts-box.

Gain is about 4.4. With 9V supply it can take signals to over 0.5V cleanly. Input impedance is around 100K, a nice high value which will not load-down or upset sources.
 

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