I am trying to use an LM358 opamp to amplify an electret mic to something that I can read with my attiny85's ADC. (What can I say? I like DIP-8's 🙂
I feel that I need a gain of about 100 to get a decent signal.
I'd like to use a single-supply configuration, with a voltage divider to provide a virtual ground.
My first design used one half of the LM358 as a basic inverting amplifier with gain 100, and the second half to provide a virtual ground as in:
http://www.swarthmore.edu/NatSci/echeeve1/Ref/SingleSupply/SingleSupply.html
Now, the LM358 has a gain-bandwidth product of 1 MHz or so, which means that my 100 gain will start to be affected at around 10 kHz.
What I'd like to do is to use both halves of the LM358, each with a gain of say 10 and a coupling capacitor between them.
The page I list above indicates that the virtual ground current of an inverting amplifier is zero, which suggests that I should be able to use a resistor-divider for both amps.
Does that sound right? Or am I missing something?
Best Answer
If the resistors making up the voltage divider are of low enough value, and the supply rails are sufficiently low impedance, yes, a single resistor divider can be used to provide virtual ground to multiple op amps.
In order to stiffen up the ground a bit, a capacitor may be added between the supply and ground rails at the resistive divider, and a second capacitor between the virtual ground point and either of the rails.
simulate this circuit – Schematic created using CircuitLab
Alternatively, a dedicated virtual ground IC such as the Texas Instruments precision rail splitter TLE2426 comes in handy for the purpose.