# Op-amp non-inverting amplifier with clamping?

I am designing a simple electret microphone amplifier for use with Arduino. My current design is shown below:

I was wondering if there was a way to combine the diode clamp (D1 in the schematic) with the non-inverting amplifier to make the clamping voltage 0V instead of around -.6V. If possible I would like to just use the op-amp in the design and not add in the second half of the MCP602.

Thanks!

• Addition / comment re what has been said: In your original circuit there is no DC path to IC1A non inverting input so DC point is undefined as OPAmp is unable to formally drive the pin DC wise. – Russell McMahon Mar 12 '15 at 22:49

The diode in the picture isn't a clamp rather it glues the negative portion of the signal to about -0.7 volts whilst the positive part of the signal rises freely. In effect it tries to self bias the average output after the capacitor to the p-p voltage divided by 2.

Setting the "clamp" voltage to be a tad higher can be done by using a forward biased diode generating +0.7 volts and connecting the circuit diode to this instead of 0 volt.

I'm not really sure this is what you may actually require - if you are trying to digitize a microphone signal to mess with the audio in detail then you'll need an amplifier that just centres the signal to mid-band on the ADC. Alternatively, if you are wanting to just analyse the magnitide of the audio signal in order to trigger an event (without trying to analyse detailed audio signals) then this circuit can be fed thru a loss pass filter to get a dc signal that increases or decreases with audio level.

This doesn't directly answer your question, but I have a few suggestions to improve the design which should make it unnecessary for you to use the clamping diode at all.

simulate this circuit – Schematic created using CircuitLab

1. Move the 10k/10k resistor-divider to your opamp's non-inverting input to give your input signal a DC bias point of half-supply. The reason for this is with your original arrangement, your AC-coupled signal from your microphone will result in negative voltages applied to the opamp's input - not a great idea when using a single supply.
2. Add a capacitor in series with R4 in the gain-adjustment negative feedback part of the circuit. What this does is ensure that your circuit only has any gain >1 for AC, and only a gain of 1 for DC. Without this feature, the DC bias we applied in step 1 would be amplified along with the signal - quickly saturating the output for any gain much more than 1.

Now what you have is an output which is normally biased at 2.5V (half of your supply) with the amplified AC signal from your microphone superimposed onto that.
I haven't chosen specific values for the capacitors in this circuit - you may need to make some adjustments depending on the frequency response you're aiming at.