LM741 connections with signal generator  - Page 1
Forum section: electronics / beginners

author: Gazmon
author_rank: Regular Contributor
timestamp: March 23, 2019, 06:42:53 pm
content: HiI have made a basic LM741 op amp circuit to test with my oscilloscope , I will supply my 741 at pins 7 and 4 with 9volt dc -+ supply’sPMy question I want to use a sine wave from signal generator as my input signal at pins 2 and 3Will this be ok? What settings would be advised as do not want to damage my sig-genThanksGazmon

author: Cliff Matthews
author_rank: Unknown
timestamp: March 23, 2019, 11:11:33 pm
content: Post the circuit diagram, we don't use crystal balls..

author: Audioguru
author_rank: Super Contributor
timestamp: March 24, 2019, 02:59:53 am
content: 1) The 741 opamp design is 53 years old and was designed to use a 30V supply. Some do not work with a 9V supply. They have trouble at frequencies above 11kHz.2) You NEVER feed both inputs of an opamp with the same signal.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 06:00:20 am
content: Apologies it’s 15 v DC - and+And aware that you don’t use the same signal for both inputsl, really just hoping for advice about using my signal generator at the inverting ang non-inverting inputs pleaseI have attached the circuit

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 24, 2019, 06:20:53 am
content: Don't be deterred from experimenting. You won't damage your function generator as long as you keep to the schematic you posted and don't try to feed 30 volts into the FG!You could put a small capacitor in series with the input to protect the FG from any DC backflow kind of errors. And there are circuits which feed the same signal to both inputs of an op-amp or comparator, at least some of the time. Experiment! Connect your oscilloscope to the output of the op amp (the "zero volts" level between +15 and -15 is your "ground" for scope and signal generator) and feed various signals into the inputs and see what happens at the output, using two scope channels, one for the input signal and the other for the output signal. Have fun, your chances of letting the magic smoke out of your signal generator are very small.

author: iMo
author_rank: Super Contributor
timestamp: March 24, 2019, 07:09:52 am
content: This is how it works.The 10Meg is the internal impedance of your oscilloscope probe (10x setting).Use at your own risk.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 09:43:31 am
content: Quote from: alsetalokin4017 on March 24, 2019, 06:20:53 amYou mean a capacitor in series connected to the positive line of my FG or both pos and neg leads?, something like 100uf?

author: iMo
author_rank: Super Contributor
timestamp: March 24, 2019, 09:54:03 am
content: @Gazmon: I would highly recommend you to download the LTspice simulator. It is free and pretty easy to work with.You have to learn to work with it when you are interested in electronics. It helps you simulate your schematics and your measurements before you do in real HW.

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2019, 10:07:08 am
content: Since the LM741 is powered from a bipolar power supply, the input and output capacitors are unnecessary.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 10:30:26 am
content: What’s easiest way to get a 15v negative power supply or can anyone provide a schematic I can build easilyThanks for the help

author: iMo
author_rank: Super Contributor
timestamp: March 24, 2019, 11:18:21 am
content: Better start a different topic on -15V provided you have searched for it and have not find a suitable schematics..

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2019, 02:54:01 pm
content: Quote from: Gazmon on March 24, 2019, 10:30:26 amMy advice is don't. If the signal is AC, keep the capacitors, bias the non-inverting input at half the supply and connect R1 to 0V, via a capacitor.What's the lowest frequency? You might need to increase the capacitor vales. I've changed them to polarised, just in case you want to use electrolytic.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 04:42:48 pm
content: Quote from: imo on March 24, 2019, 11:18:21 amSorry for the basic questions here , just wanted to check a few things ,From you schematicVop+ Connection on the Lm741 what’s feeding in here? , the non inverting - connection on the Lm741 is connected to ground via R1, so there’s no 15v negative dc - needed here ?

author: iMo
author_rank: Super Contributor
timestamp: March 24, 2019, 05:13:21 pm
content: In my first schematics you need two voltage sources, Vop+ and Vop-. The GND (ground) is in the "middle" of both sources.The input and output is referenced against the GND.There is the version from Zero999 - where you are using only a single voltage (Vop+ .. GND) while you create "a virtual GND" through the two resistors.You may take two 9V batteries and play with the 741. See below.In a schematics, when you have two independent pieces of wire labeled with the same "name", like the couple of short wires labeled "Vop+", it means the wires are connected together. People use this in order to not mess with a lot of long wires in the schematics.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 06:33:29 pm
content: Can I substitute the LM741 for an LM358

author: Zero999
author_rank: Super Contributor
timestamp: March 24, 2019, 07:48:17 pm
content: Quote from: Gazmon on March 24, 2019, 06:33:29 pmYes and no. If you've already designed the PCB, then no, because the LM358 has a totally different pin-out and it's a dual, rather than single op-amp.If you're just breadboarding or are designing a new PCB, then yes. The only thing you need to be aware of is the LM358 has a class B output stage, which can cause cross-over distortion and is more evident at higher frequencies and gains. This can be eliminated by adding a resistor from the output to the negative power rail, to ensure only one output transistor is conducting, thereby causing it to operate in class A.https://electronics.stackexchange.com/questions/341843/adding-a-resistor-to-reduce-crossover-distortion-in-an-lm324-lm358Neither the LM741 nor LM358 are great for frequencies above the the mid-audio band. The TL072 and NE5532 are common, faster op-amps which are much better and commonly available. Try a range of different op-amps and note what happens, as the frequency is increased.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 24, 2019, 08:08:27 pm
content: CheersAll Excellent info

author: Gazmon
author_rank: Regular Contributor
timestamp: March 26, 2019, 07:09:35 pm
content: Can I use 2 x 9v battery’s connected togeather to get a positive/negative voltage of 18v to power the LM741?Will 18v be ok ?

author: basinstreetdesign
author_rank: Frequent Contributor
timestamp: March 26, 2019, 08:23:03 pm
content: Quote from: Gazmon on March 26, 2019, 07:09:35 pm741's are supposed to be good to run on +/- 22V or up to 44V total between the V+ and V- power pins, however you split the supplies.So using two 9V batteries for each of V+ and V- is fine. This allows for higher amplitude signals through the op-amp, up to about 30V P-P at output.Also OK to use two 9V batteries to get +/- 9V and this allows signals of about 14V P-P.

author: Zero999
author_rank: Super Contributor
timestamp: March 26, 2019, 08:33:05 pm
content: Quote from: Gazmon on March 26, 2019, 07:09:35 pmYes.

author: rstofer
author_rank: Super Contributor
timestamp: March 26, 2019, 09:17:26 pm
content: Do understand that the voltage swing might be as low as +-10V given +-15V supply. In other words, depending on the load, the OA can only get to 5V from the rails. So, you might have a situation where your output swing can only be +-4V.See "Output Voltage Swing" on page 5http://www.ti.com/lit/ds/symlink/lm741.pdfw2aew did a great video on op amps and what happens when the signal gets to the limit of swing:Around 9:00, he shows on a scope how the output clamps.The "Input Voltage Range" (also on page 5) is also limited to 3V away from the rails so +-6V if you are using a pair of 9V batteries.

author: alsetalokin4017
author_rank: Super Contributor
timestamp: March 27, 2019, 06:31:43 pm
content: Note that R2 and R1 in these schematics form a voltage divider for the inverting input to the op-amp. It is fun to substitute other "Voltage dividers" for these two resistors -- like a potentiometer, with the wiper going to the inverting input, or a pair of Light-Dependent Resistors instead of the fixed resistors.Also there are a bunch of op amps that have the same pinout. It is fun to set up a circuit on the breadboard and then pop in different opamps, or even same part number parts from different manufacturers, and see how differently they behave.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 09:08:56 am
content: Quote from: Zero999 on March 26, 2019, 08:33:05 pmI need to take the battery ground connection to the ground connection (0v) on my breadboard setup yes

author: rstofer
author_rank: Super Contributor
timestamp: March 31, 2019, 02:57:42 pm
content: Yes, ground is as you shown it. Now, connect the signal generator ground to this point.Use the voltage follower circuit you provided in Reply 3 with, say, 10k resistors. Keep the gain to 1 until round two of your experiments.Keep your signal generator output low at about 1V peak to peak until round two. I would put a 10k resistor in series with the signal to protect the op amp. If my generator didn't swing around 0V I might have to put a capacitor in series with the signal. Anything around 1 ufd up to 100 ufd will probably work at audio frequencies.Remember, you can't get within 5V of the rails so your maximum signal swing is +-4V. It's also worth noting, your batteries are a little low as the minimum supply voltage is +-10V for all variants.Here is the datasheet:http://www.ti.com/lit/ds/symlink/lm741.pdfEvery number is important. Note that there are 3 devices: LM741, LM741A and LM741C and they have different capabilities in terms of output voltage swing. Your device should be marked but if it isn't, I would take the worst case of the 3 devices.At some point, you may want to build up a dual 15V power supply. I didn't need it but I just built:https://www.jameco.com/z/JE215-Dual-Output-Adjustable-Linear-Regulated-Power-Supply-Kit_20626.html

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 03:12:33 pm
content: HiI have attached my scope reading as they appear with my sigen input, why is my amplitude not correct to my outputted sigen signalJust outputting a triangle waveform and cycling through the othersI’m connected as an non inverting I think , positive sigen connected to the -

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 03:53:06 pm
content: People in electronics communicate with help of schematics diagrams.Show us your schematics and describe how do you perform the measurement.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 03:59:34 pm
content: Quote from: imo on March 31, 2019, 03:53:06 pmThe schematic is posted in the first few posts !!

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 04:03:50 pm
content: Show us your schematics with the exact wiring and how is your generator and the oscilloscope wired.Do not expect the forum members will do for you.They will certainly try to help you, but you have to provide the exact inputs.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 04:11:26 pm
content: Point taken sorryR1 is 2kR2 is 10kMy input signal is 9v pos/neg, appreciate this is slightly low , but it’s a startSigen connections are positive to pin 3 lm741Sigen connection negative to pin 2 lm741Appreciate the info request , and apologiesOscilloscope is connected ground and to the output pin of LM741

author: rstofer
author_rank: Super Contributor
timestamp: March 31, 2019, 04:23:14 pm
content: Quote from: Gazmon on March 31, 2019, 03:12:33 pmI'm pretty sure you want negative feedback and you absolutely don't want a differential input. You do not connect your generator between pins 2 and 3.Signal goes in on +, generator ground connects to scope ground and battery ground. Negative feedback like you posted in Reply 3. R1 as shown in Reply 3 also connects to that common ground point.Your gain is awfully high considering how little swing you can have with your limited voltages. For a first cut, I would drop the gain to 1 and see that I got the same voltage swing out as in.You would be building a non-inverting amplifier:https://www.electronics-notes.com/articles/analogue_circuits/operational-amplifier-op-amp/non-inverting-amplifier.php

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 04:32:36 pm
content: The Gain of your opamp is set toA = 1 + R2/R1 = 1+ 10k/2k = 6With 9Vpp (??) input signal from your generator you overdrive the opapm.With +/-9V batteries the max opamp's output you may get could be 14Vpp (+/-7V) max for example.With A = 6 the max opmap input (your generator output) could be max 2.2Vpp (peak to peak) therefore.Also mind your generator's DC level must be 0V (your input signal must be centered around 0V) otherwise 2.2Vpp will not fit into 14Vpp output range.Better set the generator's output to 500mVpp at the beginning and watch the output.Also the generator shall be connected between + input and GND (GND is the middle of the +/-9V batteries).PS: in case it is your first experiment with 741 always use some protection resistors (1k-10k in series with input and output) at the opamp's input and output, or use coupling capacitors (as in my schematics above) in order not to smoke your 741.

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 04:38:42 pm
content: Ok I will adjust my connectionsAnd again apologies for the lack of info above at firstWill post my results , Thanks again everyone

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 05:02:40 pm
content: For example:R3 and R4 are protection resistors (1k-10k).The input is 5kHz square, -0.5V to +0.5V, 1Vpp.Output is 6Vpp (-3V to +3V).PS: the second picture is 3kHz 2.2Vpp sine (-1.1V to +1.1V) input.The 3rd picture is the same 2.2Vpp sine signal but DC shifted to +0.5V (centered around +0.5V). You may see the output does not fit into the opamp's output range anymore (with +/-9V batteries).The output is clipped at +8V (the max swing).

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 05:17:15 pm
content: Ok I will add a 1k- 10k to the output connection in series then and at the input

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 05:34:49 pm
content: I've updated above the examples and pictures - with that settings you have to see the same on your o'scope screen..

author: Gazmon
author_rank: Regular Contributor
timestamp: March 31, 2019, 09:45:21 pm
content: I’m guessing that from my experiments based in the above my output readings will also vary based on my protection resistors fittedIs this correct?I am getting a sine wave now 👍

author: iMo
author_rank: Super Contributor
timestamp: March 31, 2019, 10:11:28 pm
content: You should not see any visible difference with or without the protection resistors.

author: rstofer
author_rank: Super Contributor
timestamp: March 31, 2019, 10:50:09 pm
content: Quote from: Gazmon on March 31, 2019, 09:45:21 pmNo, see Rule 1 here:No current flow, no voltage drop. Now, in reality there is a tiny current flow (see datasheet 'Bias Current') that could be as high as 1.5 uA. This would drop 1.5V if the input resistor was 1 Mohm but at 1k, it drops 0.0015V and that's worst case.Watch Dave explain op amps, it will help.

author: Gazmon
author_rank: Regular Contributor
timestamp: April 01, 2019, 08:36:51 pm
content: The video is spot on thank youIf I want to check some measurements with my multimeter based my setup and the info in previous posts, I can do this between 0v and the output pin ?

author: rstofer
author_rank: Super Contributor
timestamp: April 02, 2019, 01:36:29 am
content: In general, that ground between the two batteries is the reference voltage for ALL measurements. There is simply no other point that works. Ground is ground for everything: Scope, signal generator, DMM, everything.