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What could be a near rail-to-rail opamp with very low voltage for this use? LM386 seems to be out of limits for my needs.

How do I choose it from datasheets? What values are important (of course limits & power) but what else? Is there something more important than power inputs & current limits?

schematic

simulate this circuit – Schematic created using CircuitLab

on this schematic I did not calculate component values yet.

I want to make a very light and portable mic amplifier on 2 batteries. It should consume very low power and have medium/high amplification.

What could be a near rail to rail amp with very low voltage for this use?

I tried from this datasheet to guess that the LT1494 would fit this need. Is this correct? (If not, why?) LT1494 datasheet

For audio, will 100nF be ok for C1 & C2?

mic will be an electret connected device may be another amplifier or why not a speaker

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  • \$\begingroup\$ What's your mic? \$\endgroup\$ – Reinderien Oct 18 '18 at 21:41
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    \$\begingroup\$ The LM386 is not an opamp. Do you intend to connect a speaker to this thing, or does it connect to some other device? \$\endgroup\$ – JRE Oct 18 '18 at 21:44
  • \$\begingroup\$ What kind of microphone are you using? A dynamic microphone, or an electret microphone? \$\endgroup\$ – JRE Oct 18 '18 at 21:45
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    \$\begingroup\$ OP490 will run at +/-1.2V and can go to the negative rail, but the datasheet doesn't seem to describe the positive output limit when run on very low voltage supplies. Also quite pricey. analog.com/media/en/technical-documentation/data-sheets/… \$\endgroup\$ – Russell Borogove Oct 18 '18 at 22:03
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LM10C is one of my favourite op-amps and will run from 1.1 Vdc to 40 Vdc. The output will swing within 15 mV of the supply rails.

It's not particularly fast but should be entirely adequate for your needs.

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  • \$\begingroup\$ thanks all of you. I think I will try from LM10C & calculate components needed around, soon. \$\endgroup\$ – francois P Oct 19 '18 at 18:06
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IMO, your best bet is to do a parametric search on a decent distributor (Digikey has a pretty good setup). Note that you will want to select rail to rail as a parameter, and then the power supply voltage range- read those carefully as they can be split up into single ended and dual supply types, and you can miss a good candidate such as NCS2001, NCV2001. One important thing to note is that these opamps tend to have low gain-bandwidth products, so make sure the part you select has the chops for your microphone type (you didn't have that info on the schematic). In the worst case you can divide up the gain stages (helps with oscillation problems too).

Most won't have a problem running with split supplies or single supplies, your choice, but you might also add an output coupling capacitor in case you connect something that has DC coupling on the input, which would drain your battery quickly.

The LT1494 would likely not work well due to its extremely low gain bandwidth product of 2.4KHz.

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    \$\begingroup\$ Regarding C1 and C2, it really depends on the drive capability, microphone impedance, etc. \$\endgroup\$ – isdi Oct 18 '18 at 21:53
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You can run this circuit on only a single battery, if you wish. The output voltage will not be rail-to-rail, if you use only bipolar devices (even CMOS/MOS devices can only get "close" to rails and even then only if the ZLOAD is a light load. 1 Kohm is not a light load.)

schematic

simulate this circuit – Schematic created using CircuitLab

On 2.4 volts, assuming each gain stage is biased to VDD/2, you will get 1.2/ ( q/K*T) = 1.2 / 0.026 = 50X gain (34dB) per discrete bipolar transistor

To run on a single battery, adjust values of R3 and R4, to move Vcollector closer to VDD, and have Q2 operating with a higher Vemitter, so you can get more output swing.

A better approach for a single-battery is to drive the Vout from the Q1 collector (DC-blocked, of course). The load capacitance will strongly affect the bandwidth.

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Here is a better "opamp"

schematic

simulate this circuit

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