Op Amp with single dc supply - Page 1
Forum section: electronics / projects

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: December 26, 2016, 11:07:50 pm
content: I have a simple op amp circuit. (please see screenshot/image). Currently, I havean AC signal generator for the ac input.I have a single bench top power supply I made for the DC supply.Therefore, I ONLY see the amplifiedtop/positivepart of the input sign wave.What i want is the entire amplified waveform on the output. I have been told there is a way to step-up/offset the ac wavefore by vpp/2 thereby converting the AC to a sign DC wave (all above zero volts),and then on the output, I could use a cap to remove the DC offset.Can anyone tell me HOW to do this? Do you have any sample/similar schematics you could share or point me to?Thx in advance

author: Benta
author_rank: Super Contributor
timestamp: December 26, 2016, 11:29:07 pm
content: What you have is an 1.0 Vpp AC input (0.5 Vp), amplified x22, which would give you 22 Vpp at the output.However, an opamp will not go beyond the supply rails, so you'll need to look at requirements again.Apart from that, level shifting / AC-coupling can be done easily

author: ehilarioc
author_rank: Contributor
timestamp: December 26, 2016, 11:31:38 pm
content: Level shifting is easy, just use a potentiometer and connect the wiper to the non inverting pin and tune until you get the signal you want at the output. Then you can replace the pot with resistors if needed.

author: Benta
author_rank: Super Contributor
timestamp: December 26, 2016, 11:39:40 pm
content: Quote from: ehilarioc on December 26, 2016, 11:31:38 pmPlease look at the schematic again.

author: floobydust
author_rank: Super Contributor
timestamp: December 26, 2016, 11:52:01 pm
content: Your op-amp's output cannot go below 0V yet your input sine wave does. The sim shows that.I believe you need to bias up the op-amp to 1/2 of 12V (apply 6VDC at pin3) and also AC couple the input via capacitor to R1 (side effect is this gives a low frequency roll-off).Benta mentioned you are also overdriving the op-amp; it can't output 22V when powered by 12V

author: Zero999
author_rank: Super Contributor
timestamp: December 27, 2016, 12:02:05 am
content: Bias the op-amp so its output sits at half the supply rail and AC couple the input and outputs via capacitors.As mentioned above, an op-amp's output can't go beyond its power supply rails, so you need a higher voltage power supply to amplify a 1Vp-p signal with a gain of 22.

author: ehilarioc
author_rank: Contributor
timestamp: December 27, 2016, 12:05:38 am
content: It is true that the opamp output won't go higher than the supply rails, however the only thing mike needs to do is to lower the amplitude of his input signal a little, or increase his voltage supply.

author: KhronX
author_rank: Frequent Contributor
timestamp: December 27, 2016, 12:13:00 am
content: ... Or reduce the gain a bit, so the output signal "fits" within the confines of the supply rails(Minus whatever drop-out the opamp's output stage is limited by...)

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: December 28, 2016, 03:01:24 am
content: Thank you all. Indeed I was over-driving it with the 22k Rf.I reduced that. I am really just trying to get the concept (rather than any specific gain) at the moment.That said, I tried one of the suggestions above, yet I am still not getting desired results. (upper left of screen is what I tried to model in the sim)I was thinking I could somehow shift the ac input up, so that I could get an entire waveform on the output.On the attached, you will see clipping. I welcome feedback.Thank you

author: MK14
author_rank: Super Contributor
timestamp: December 28, 2016, 03:36:39 am
content: For starters, C3 and hence the 12V voltage source, does not look like you connected it up right. Compared to the diagram on the left.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: December 28, 2016, 03:15:12 pm
content: I see what you mean.I suppose the intent of the diagram was to simply have a DC voltage divider applying 6 volts to the non-inverting input, ...correct?So I will need to have the cap in parallel instead of series. Sound right?I will plan to rework the non-inverting bias when I get home this eve.

author: Zero999
author_rank: Super Contributor
timestamp: December 28, 2016, 03:22:33 pm
content: Quote from: mikeinfodoc on December 28, 2016, 03:15:12 pmYes, put C3 in parallel with V1, rather than in series.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: December 29, 2016, 04:15:16 am
content: Ok, I corrected the bias on the non-inverting input, and NOW I am getting a full wave on the output. (see attached)It also appears the gain is what I expected.A follow up question:Can anyone help clarify the need/purpose for caps: C1, C3 and C4? (also are there any recommended values for them?)

author: Zero999
author_rank: Super Contributor
timestamp: December 29, 2016, 10:44:18 am
content: Quote from: mikeinfodoc on December 29, 2016, 04:15:16 amC1 is an AC coupling capacitor. Its value depends on the minimum frequency of interest and the input impedance (RG on your schematic).C = 1/(2pi×FC×R)Where:C is the value of C in in FaradsR is the input resistanceFCis the lower cut-off frequencyIdeally C1 should be larger than the value given by the above formula, which returns the capacitance required to reduce the voltage of the signal to 70.7% of the original voltage.C3 is a general power supply decoupling capacitor. It smooths out any noise on the power. If the power supply voltage increases, it charges a little, absorbing the extra energy, if the voltage decreases, it discharges slightly. You may be able to omit this if the circuit is running from a battery and there are no high current or digital circuits running off the same power supply.C4 smooths the voltage on the potential divider formed by RG1 & RG2. The voltage on the potential divider is proportional to the supply voltage and is connected to the non-inverting input of the op-amp and is passed through to the output. Without C4, any fluctuations in the supply voltage will pass through the op-amp and appear at the output. Again if it's being run off a battery and there are no other high current or digital circuits, then you may be able to get away without it.

author: danadak
author_rank: Super Contributor
timestamp: December 29, 2016, 12:52:06 pm
content: Tghere are multiple ways of handling signals that have below ground swings,in a single supply system. Link shows some methods, and a calculator for asimple method using 2 Rs.https://www.dropbox.com/s/k1yn4m7heaixvmg/ADinput%20negative%20inputs.zip?dl=0Regards, Dana.

author: mikeinfodoc
author_rank: Regular Contributor
timestamp: December 29, 2016, 03:56:27 pm
content: Great, thank you all again.I took a circuit analysis class this past fall, and thought I would be "on my own" after that.However, I am finding this forum is unbelievably helpful.This was my first post, and I am already awestruck.