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  • KingBeauregard commented on lukas_tek's instructable Pre-amp to electret mic!7 months ago
    Pre-amp to electret mic!

    So I've been fiddling with the featured circuit more, to see if I could control the circuit characteristics more precisely. In particular, I wanted a circuit that "idles" with a V(ce) of 4.5V (since I'm powering it with a 9V battery) and I want 20 mA going across R3. I also want the output from the microphone to center on 4.5V. Using datasheets, Kirchoff's Laws, arithmetic, and a little trial and error, I've come up with these recommendations:R1 - 18k (this could vary depending on your electret microphone)R2 - 68kR3 - 220 ohmThis makes for a satisfyingly sensitive microphone, and the electrical characteristics are about where I want them to be. I could probably rejigger this so that it runs on less current, but then I'd also have to figure out new R2 and R3 values (since, ...see more »So I've been fiddling with the featured circuit more, to see if I could control the circuit characteristics more precisely. In particular, I wanted a circuit that "idles" with a V(ce) of 4.5V (since I'm powering it with a 9V battery) and I want 20 mA going across R3. I also want the output from the microphone to center on 4.5V. Using datasheets, Kirchoff's Laws, arithmetic, and a little trial and error, I've come up with these recommendations:R1 - 18k (this could vary depending on your electret microphone)R2 - 68kR3 - 220 ohmThis makes for a satisfyingly sensitive microphone, and the electrical characteristics are about where I want them to be. I could probably rejigger this so that it runs on less current, but then I'd also have to figure out new R2 and R3 values (since, for what I understand to be best performance (which you supposedly get when Vce is half of Vcc), the current is going to determine R3 and therefore R2).

    For R2 and R3, values of 470k and 1k seem to work pretty well too.

    ... you know, a smarter person than me wouldn't have spent so much time trying to work with data sheets and algebra and so forth. It's two simple steps:1) Once you've decided on the current you want at Q, that tells you what R3 needs to be: it's Vcc divided by current, divided by 2 (because the intention is for the drop across R3 to be half of Vcc, and the other half will be across the transistor).2) How to figure out what R2 needs to be to keep the transistor at half of Vcc? Just hook up a potentiometer, and adjust it until the voltage across the transistor is indeed half of Vcc. Then measure the resistance on the potentiometer.When I just did this, I ended up with 72k; pretty close to what I got via much more complicated means earlier. Except using the potentiometer it took me le...see more »... you know, a smarter person than me wouldn't have spent so much time trying to work with data sheets and algebra and so forth. It's two simple steps:1) Once you've decided on the current you want at Q, that tells you what R3 needs to be: it's Vcc divided by current, divided by 2 (because the intention is for the drop across R3 to be half of Vcc, and the other half will be across the transistor).2) How to figure out what R2 needs to be to keep the transistor at half of Vcc? Just hook up a potentiometer, and adjust it until the voltage across the transistor is indeed half of Vcc. Then measure the resistance on the potentiometer.When I just did this, I ended up with 72k; pretty close to what I got via much more complicated means earlier. Except using the potentiometer it took me less than five seconds.

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  • KingBeauregard commented on lukas_tek's instructable Pre-amp to electret mic!7 months ago
    Pre-amp to electret mic!

    This is pretty darn brilliant. As near as I can tell, the circuit finds its own point of stability, by self-correcting until the current through the base opens up the transistor exactly the right amount to be stable. (In other words, where V(ce) = V(b) + I(2)R(2), with V(b) as 0.7V.)This circuit inspired me to finally, FINALLY build a common-emitter amplifier that seems to work to my satisfaction (see image). The electret microphone that it's connected to "idles" at about 4.5 V, and so does the transistor. It picks up sound well, oh and the transistor doesn't overheat and then go "poof" like with some of my previous attempts. :-) I don't know that this circuit is any better than the one lukas_tek gave us; mine's more complicated and you may find it doesn't wo...see more »This is pretty darn brilliant. As near as I can tell, the circuit finds its own point of stability, by self-correcting until the current through the base opens up the transistor exactly the right amount to be stable. (In other words, where V(ce) = V(b) + I(2)R(2), with V(b) as 0.7V.)This circuit inspired me to finally, FINALLY build a common-emitter amplifier that seems to work to my satisfaction (see image). The electret microphone that it's connected to "idles" at about 4.5 V, and so does the transistor. It picks up sound well, oh and the transistor doesn't overheat and then go "poof" like with some of my previous attempts. :-) I don't know that this circuit is any better than the one lukas_tek gave us; mine's more complicated and you may find it doesn't work as well (YMMV). Nevertheless, it's an example of a standard common-emitter amplifier that works and is made of parts with pretty conventional values.

    This is pretty darn brilliant. As near as I can tell, the circuit finds its own point of stability, by self-correcting until the current through the base opens up the transistor exactly the right amount to be stable. (In other words, where V(ce) = V(b) + I(2)R(2), with V(b) as 0.7V.)This circuit inspired me to finally, FINALLY build a common-emitter amplifier that seems to work to my satisfaction (see image). The electret microphone that it's connected to "idles" at about 4.5 V, and so does the transistor. It picks up sound well, oh and the transistor doesn't overheat and then go "poof" like with some of my previous attempts. :-) I don't know that this circuit is any better than the one lukas_tek gave us; mine's more complicated and you may find it doesn't wo...see more »This is pretty darn brilliant. As near as I can tell, the circuit finds its own point of stability, by self-correcting until the current through the base opens up the transistor exactly the right amount to be stable. (In other words, where V(ce) = V(b) + I(2)R(2), with V(b) as 0.7V.)This circuit inspired me to finally, FINALLY build a common-emitter amplifier that seems to work to my satisfaction (see image). The electret microphone that it's connected to "idles" at about 4.5 V, and so does the transistor. It picks up sound well, oh and the transistor doesn't overheat and then go "poof" like with some of my previous attempts. :-) I don't know that this circuit is any better than the one lukas_tek gave us; mine's more complicated and you may find it doesn't work as well (YMMV). Nevertheless, it's an example of a standard common-emitter amplifier that works and is made of parts with pretty conventional values.

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