# Getting the desired output voltage swing from the op amp output

I am designing a microphone preamplifier circuit. I have used op amp OPA 37 which amplifies the signal coming from the microphone to be used by the ADC. The usable voltage range of ADC is 0-2.5V. I am using a 9V power supply and two biasing resistors at the input to obtain the output voltage swing. The bias point is at 4.5V. I want to know what can be done at the output of the op amp to get the bias point at 1.25V so that it can be used by ADC. In the actual circuit microphone replaces signal generator.

• Why not bias your opamp at 1.25V instead and then remove C3? Your circuit doesn't give any DC gain (due to C1), so your output will be biased at 1.25V. Apr 17, 2015 at 20:13
• How close is the OPA37 to a rail-rail opamp? If it works rail-rail, the brhans has a good idea. Apr 17, 2015 at 20:15
• @brhans I tried your idea too. Using voltage divider and replacing the lower biasing resistor with 8.2 K should bias the output at 1.25V. I used a potentiometer and kept changing the resistance. At 10.9K, I am able to lower the bias point to 1.47V but after that the signal gets distorted really bad. Apr 17, 2015 at 20:24
• Turns out the OPA37 isn't even remotely rail-to-rail, so my idea won't work with that opamp. Apr 17, 2015 at 20:35

What about taking the output from C3 and putting that into a summing amplifier? http://hyperphysics.phy-astr.gsu.edu/hbase/electronic/opampvar5.html

In input is your audio and the other is 1.25V.

Since this is audio, you probably don't care about inversion of the signal.

You can put a voltage divider at the output of C3 (the ADC input). As long as the resistor values are high enough it should work fine, although it might put in more noise than you want.

As it is now, the ADC input does not seem to be biased (unless the ADC is self biased).

That op amp (OPA 37) is not suited to this application, and there is no reason to think that it will do what you want. If you look at the data sheet, its output voltage range (at +/- 15 volts supply) is only guaranteed to be +/- 12 volts with a fairly light load, and worse for small load resistance. That is, for a +/- 15 volt supply, the output is not guaranteed to get closer than 3 volts to negative supply.

If this number applies in your setup, you cannot produce an output of less than 3 volts.

Does the number apply? I don't know. The data sheet has no specs for a 9 volt (+/- 4.5 volt) supply, but I'd guess it does.

What you need is an op amp with "rail-to-rail output".

Additionally, you'll need to get rid of C3. This cap will produce audio signals which are centered at zero volts, and all the negative portions will be outside the ADC range.