# Dual-Rail for amplifier

#### tjones91

Joined May 6, 2020
19
Hello, I have a question about a circuit that i got from here. It is supposed to be a crude long-range electret microphone listening device. I know from past knowledge that an analog signal such as sound must be reconstructed using an Amplifier that is capable of driving from both (positive) and (negative) voltages to mimic the analog voltage of sound. In the circuit which i will post below, they use a 555 timer to generate the negative voltage for the lm358.

My questions are,
1) isn't there an easier way to create a negative voltage for the Op-amp?
2) The page says that that the microphone is connected in series with a 10k resistor. Is this considered superimposing an AC waveform on a DC bias?
3) To my best of knowledge this circuit looks very bare minimum, does anyone know the legitimacy of this circuit and will it work ( even if very crude)?
P.S I tried uploading the schematic but for some reason the way the file is, it wont let me. I even tried saving the image as a PNG( but it is on the link above)
Sorry and Thanks

#### ronsimpson

Joined Oct 7, 2019
811
The LM358 can run as low a 3v supply but it struggles.

I might run it from a 9V battery. Using two 22k resistors, make a 1/2 supply voltage.

#### tjones91

Joined May 6, 2020
19

#### MrChips

Joined Oct 2, 2009
21,634
1) Is there an easier way to create a negative supply? Yes. You shift the COMMON upwards. This way your 0V rail becomes negative with respect to COMMON.

2) The purpose of the 10kΩ resistor in series with the microphone is to supply current to the amplifier in the microphone.

3) I have not examined the circuit in question in detail. You do not need to generate a negative voltage. You can create a microphone amplifier with a single supply voltage. DC voltages at the output of the amplifier can be removed using a simple high pass filter, i.e. a capacitor at the output. Simply use two resistors to bias the COMMON of your amplifier half-way between the power supply rails.

You cannot get much simpler than these:

#### KeithWalker

Joined Jul 10, 2017
1,137
You don't need a separate negative rail for a LM358 amplifier. All you have to do is bias the input to about half the supply voltage.
Regards,
Keith

#### schmitt trigger

Joined Jul 12, 2010
310
But in the case you really want to generate a negative supply, the common ICL7660 is specifically designed for this purpose.
Ultra simple to use, and only two external caps are required.

#### OBW0549

Joined Mar 2, 2015
3,503
Minor nit: the circuit on the right is incomplete: there needs to be a resistor from pin 3 of U1A to GND to provide a DC path for the op amp's input bias current.

Likewise, the circuit in the second diagram (the one that's been redlined, below) also needs a DC path from pin 3 of U1A to some voltage divider set at Vcc/2:

#### Audioguru again

Joined Oct 21, 2019
1,733
The horrible circuit you found will not work:
1) The + input of the first opamp is missing an important resistor to ground for its input bias current.
2) A capacitor to ground is missing in the feedback circuit to prevent the opamp from amplifying the DC bias voltage.
3) The lousy old LM358 is never used for audio because it is too noisy, it produces crossover distortion and has a poor high frequency slew rate above a few KHz. I would use an OPA134 single audio opamp.

As shown above, opamps do not need a negative supply if the + input is biased at half the supply voltage and input, output and feedback to ground coupling capacitors are used. Then the output of the opamp can swing up and down to produce the Audio signal.

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#### andrewmm

Joined Feb 25, 2011
526
Re the ICL7660 ,
if you use it, be aware, its built in charge pump runs right in the middle of the audio band,
it has a very noisy output, which is hard practically to filter for an audio amplifier.

Re original question,
you are right , an audio signal goes up and down above a common voltage.
traditionally, that common voltage is zero, and as such, the power supplies to any amp that is DC coupled , needs to have a negative and positive power supply with ground as the common,

BUT, as others have said, the choice of ground as the common is arbitrary,
there is no reason why the common can't be say 2v5, and the power supplies at 0v and +5v.

In recent decades, this has become the norm, primarily driven by the need for audio systems like MP3 players and phones to have a single voltage supply.

Take as an example, one of many this
https://www.electronics-lab.com/project/low-noise-mini-electret-microphone-preamplifier/

shows an electret micorphone amplifier , running at a single voltgage.
In this case, the input is AC coupled, and the amplifier generates its own mid rail common voltage

hope this helps you