Amplifier not working - Page 1
Forum section: electronics / beginners

author: ThijssjihT
author_rank: Newbie
timestamp: March 10, 2015, 03:00:46 pm
content: We have a lot of microphones and other analog sensors, for use with an educational interface. For compatability reasons we can't use this interface anymore, so we upgraded to a newer, compatible version of this interface, wich doesn't have microphone input. I want to connect these microphones to the analog inputs of the newer interface anyway, instead of buying a lot of €70,- sound sensors.The microphone has an output of between -7,5mV and +7,5mV. The imput impedance of the old interface is 10k.The new interface has analog input from 0 to 5 V. So I need to offset and amplify this signal.I just designed, simulted and build the attached circuit.Between 5V and ground is a voltage devider. Between its center and ground is 7,5mV (calculated and measured). It's added to the microphones output, and then divided by two because of the 2 10k resistors, so there is between 0 to 7,5mV on the non-inverting input of the opamp.The opamp's gain is 1+10k/15=667.67.The max output of the opamp is 7,5mV * gain = 5.0 VThis circuit might clip a little in the extreme upper and extreme lower regions, but that's no problem. It's used for measuring waveforms, so if it clips, the students should produce sound with less amplitude.Everything sould work in theory, and works in the simulation. But in practice this circuit doens't work, and outputs a constant 3,53V. Is this because the opamp and the very low input voltages? I tried different opamps. a ua741cp, lf351n and lf356n.

author: Seekonk
author_rank: Super Contributor
timestamp: March 10, 2015, 03:26:36 pm
content: 15 ohm..... Asking for trouble when you try to amplify AC and DC at those levels. Why don't you just let the DC float on the feedback with a capacitor to ground and just offset the input to 2,5V. Then you don't have to worry about op amp offset. Someone else will be nice enough to draw you a picture.

author: ThijssjihT
author_rank: Newbie
timestamp: March 10, 2015, 03:46:48 pm
content: Thank you for your reply, but I don't understand it very well.QuoteWhich resistor? I use 2x15 ohm. Both?QuoteI can't change the levels. I have -7,5mV to +7,5mV input and I need 0-5V output. I can only change the circuit, if I know how.QuoteThe 5V dc? or the 7,5mV DCQuotefloat? Sorry, my English is not very good, and I don't know this word in this context. I'll google it.Quotea capacitor in a lineair amplifier?Quoteor should I let 2,5V float on the feedback?QuoteI hope so. Please?

author: ThijssjihT
author_rank: Newbie
timestamp: March 10, 2015, 04:23:11 pm
content: I think I tackled most problems you indicated for me. I have a new attached file.Instead of a non-inverting amplifier, I use now an inverting amplifier with offset on the non-inverting input. Still not on the feedback pin, as you indicated. The 10k resembles the interface. Please give me your notes. I will build and test this circuit tomorrow.

author: jlmoon
author_rank: Unknown
timestamp: March 10, 2015, 06:28:08 pm
content: Question, how do you intend on handling that DC bias on the output pin of your op amp?No DC blocking cap on input or output?What about supplies? If you're going to use dual supplies +5,-5 or whatever then no DC caps needed on input / output of said amp.Is this op-amp a single supply device, will it work at 5 volts?Just some basic OP-Amp functional questions

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 10, 2015, 07:30:43 pm
content: You are making it too difficult for yourself, just power the microphone and use a cap to block the DC from entering the amp, something like this:Or make it even easier by using a prebuilt:http://www.adafruit.com/product/1713Of course you can make this yourself too, have a look at the datasheet:http://www.adafruit.com/datasheets/MAX9814.pdfThe MAX9814 looks like a nice solution.

author: Seekonk
author_rank: Super Contributor
timestamp: March 10, 2015, 08:00:28 pm
content: OR the non inverted version where C1 is much larger and grounded. The input is now the + input with 100K resistors instead of 10K.

author: ThijssjihT
author_rank: Newbie
timestamp: March 10, 2015, 09:15:13 pm
content: Thank you for your help. I understand this schematic, but I can't design them myself yet. I'm just beginning electronics. I will study this, and learn from it. I'll try to adjust it to suit my needs.To answer the questions: I have only 5V and ground and analog in on the interface. I can change opamp if needed. A little clipping with too loud noises is no problem. Signal between 2 and 4V is also doing the job. There is calibration possible in the software.I'll try the non-inverted one, but if I can't design it myself, I'll try the inverted, and invert it again. I can do that

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2015, 08:59:49 am
content: Quote from: PA0PBZ on March 10, 2015, 07:30:43 pmThe values on that circuit are wrong.The RC time constant for R2 and C1 is too short so the cut-off frequency is under 1.6kHz.The input impedance is far too low.The LM358 doesn't have enough gain or slew rate for the higher end of the audio band. It's also noisy compared to more modern op-amps. I don't know why people use the LM358 for audio amplifiers. It was never designed for audio.Use non-inverting configuration. That way the input impedance can be much higher.I'd normally recommend the NE5532 for audio but it probably won't work from 9V, certainly not much below. The TL072 should work but may struggle when the battery voltage falls below 7V. The newer TLE2072 would be perfect though.If you want better go for the OPA134/OPA2134 but it's more expensive.

author: ThijssjihT
author_rank: Newbie
timestamp: March 11, 2015, 11:03:36 am
content: Quote from: Hero999 on March 11, 2015, 08:59:49 amI don't need quality. I have students who I want to see the difference in waveforms with different sounds (aaaaa, eeeee, iiiiiii, oooooo). In the case an LM358 won't do the trick, thank you for your suggestions.Quote from: PA0PBZ on March 10, 2015, 07:30:43 pmI can't get it to work.Quote from: PA0PBZ on March 10, 2015, 07:30:43 pmIndeed, but I did think of some new approach.If I make a 2x10k voltage divider between +5V and 0V output of the interface, and take that as the reference point of my microphone, I will have a +2,5V and -2,5V in reference to my mic, and I won't need an offset. I can do with a very, very easy non-inverted amplifier.I'll try it, and let you know. Please give me your thoughts.

author: Zero999
author_rank: Super Contributor
timestamp: March 11, 2015, 11:21:32 am
content: Like I said, that circuit won't work.What sort of microphone are you using? We assumed electret.Does it look like this?If quality isn't critical try the MC601 or TS272 which are fairly cheap.

author: PA0PBZ
author_rank: Super Contributor
timestamp: March 11, 2015, 12:15:23 pm
content: Quote from: Hero999 on March 11, 2015, 11:21:32 amSorry, I just posted it as an example how to couple the electret to the amp, I didn't have a long look at the components, it was the first reasonable picture I could find.

author: Seekonk
author_rank: Super Contributor
timestamp: March 11, 2015, 01:17:09 pm
content: Every circuit has advantages and issues. Non inverting is likely your best choice but it will have a low frequency cut off depending on the capacitor in the feedback loop.Inverting input is often used with ultrasonic transducers. As a bonus gain peaks up as the transducer reaches resonance. This can often eliminate the need for filtering further on in simple applications. If wanting a flat response, inverting should be avoided. Inverting inputs generally require a source impedance at least ten times lower than the input feedback resistor.