# Using Microphone preamplifier IC Max4468

I am designing microphone preamplifier using MAX4468 (datasheet), but the reference diagram shows output from microphone (microphone input) going to both inverting and non-inverting terminals of the device. (picture attached below). Non-inverting terminal should be connected to ground or both non-inverting and inverting terminal should be given microphone input.

The datasheet describes a point: The MAX4466/MAX4468 are decompensated for a minimum stable gain of +5V/V.

Does it means that opamp will provide inherent gain of 5, irrespective of feedback resistors.(Page 1 of datasheet)

• going to both inverting and non-inverting terminals of the device. Not true, for AC signals the IN+ is grounded bij the 0.01uF capacitor. You're confused by the fact that this circuit does not use a negative supply rail so it biases the input signal at a DC voltage above ground. Also, the top microphone connection is not for signal but for supplying a voltage to the microphone's build in amplifier FET. Commented Jan 31, 2017 at 18:31
• Thanks, I got it. There is only one output going to the inverting terminal of the op-amp other is power which is required for powering on the microphone. Commented Feb 1, 2017 at 15:52

Does it means that opamp will provide inherent gain of 5, irrespective of feedback resistors.(Page 1 of datasheet)

No, it means that you must not choose resistors that produce a gain of 5 or under or the output signal may go unstable.

Non-inverting terminal should be connected to ground or both non-inverting and inverting terminal should be given microphone input.

No, that's not how this circuit works. As shown it appears fine and will have a gain of about -10 (minus means inverting). The non-inverting pin connects to a mid-rail voltage to properly bias the op-amp when using a single supply.

• You have just told that stable gain will be produced when gain is <=5. But then in reference diagram, why they have chosen gain of -10. I got your second point thanks. Commented Feb 1, 2017 at 15:58
• No I haven't. I said "you must NOT choose resistors that produce a gain of 5 or under" Commented Feb 1, 2017 at 16:05
• Got it. Just wondering, choosing gain of 5 or less will make the op-amp unstable, is it a control system problem. Commented Feb 1, 2017 at 16:16
• Yes, if you view the opamp as a control system, the lack of sufficient phase margin is the issue. Commented Feb 1, 2017 at 17:21

200k/20k gives a gain of -10 with input from (Vin-) grounded and using non-inv input adds DC bias with the IC allowing Shutdown to save current.

I think it assumes a stereo plug with the 2 tips being the mic polarized signal and the outer ground as a shield only. Unlike mono plugs this uses all 3 connections for better noise immunity I think but it is not clear how to connect.

• perhaps something like above. but more likely a toy piezo element for integration together but maybe not using the same ground as this would shunt the -ve signal as the +ve side is already shunted by a cap.

instead you may use a mono 3.5mm plug with typical desktop mic..

simulate this circuit – Schematic created using CircuitLab

BEWARE that the purpose of this amplifier to make an integrated mic Amp with Enable/shutdown. If this is what you want, then simply follow the datasheet.

However if you are trying to use an existing electret mic, this was not intended for this but rather just the cheap piezo element.

It is not clear why you chose this IC and may be for the wrong reasons.

• In my previous design, I was using precision low noise amplifier. But I thought it will be good idea to use IC which is specifically built for microphone pre-amplifier. Is it a good idea to use IC made specifically used for microphone pre-amplifier. Commented Feb 1, 2017 at 16:12

Maxim is one company with ICs designed to reduce the trash induced by RF energy coupling onto signal pins of amplifiers.

Check the datasheet. A normal opamp is not designed specifically for use in the interference-laden environment of a cellphone.