741 0pamp ic amplification problem - Page 1
Forum section: electronics / general

author: shibas
author_rank: Newbie
timestamp: February 19, 2013, 10:38:35 am
content: Iam using a vibration analyzer or sensor.I need to amplify the above sensor's output signal.For this purpose i used opamp ic 741 in the non inverting configuration.But when i gave the input to the amplifier from a function generator the amplifier gave a gain of about 50 and 68 initially.When i changed the input frequency from the function generation to 1 khz amplifier gave a gain of 100 even though the amplifier circuit was originally designed to give a gain of about 101.But for my application,when i give the input to the amplifier from the output of the vibration analyzer the gain of the amplifier circuit changes to a huge value of about 450.What may be the reason for this?At first i used AD620 ic for the amplification instead of the op-amp ic 741.The behaviour of the gain at that time was also similar to now.Somebody please help and reply fast.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 19, 2013, 10:45:56 am
content: Hi shibas,You cant replace AD620 with a 741!!!a 741 is a verry old opamp with low specs.Bad bandwith enz.Let us see your schematic, then mayby i can help you.Kind Regarts,Blackdog

author: amspire
author_rank: Super Contributor
timestamp: February 19, 2013, 12:55:58 pm
content: Quote from: shibas on February 19, 2013, 10:38:35 amBit hard to answer without a circuit and some details. A Vibration analyzer can be getting an input for anything from a few Hz to tens of kHz.If it was a low frequency vibration input and you were getting a gain of 420, it is something about the circuit. Perhaps you have a capacitor in series with the feedback resistor that is too big.It is all guesswork without some details.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2013, 01:31:48 pm
content: Quote from: blackdog on February 19, 2013, 10:45:56 amits within the OP BW. 741 takes ±18V supply, doesnt do output reversal, robust, diehard and everlasting.

author: blackdog
author_rank: Frequent Contributor
timestamp: February 19, 2013, 02:47:01 pm
content: Com on Mechatrommer,You cant compare a AD620 and a ua741.Look @ the offset/drift/bandwith enz.And... AD620 is a diff amplifierI hope it was a joke of youKind regarts,Blackdog

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 19, 2013, 03:11:55 pm
content: Quote from: blackdog on February 19, 2013, 02:47:01 pmi agree about we cant compare them, but i'm not jokingsince OP didnt provide enough info, i assume 741 specs are acceptable for him. whats wrong and why the saying that 741 is "old"? i agree its old but that doesnt mean its rotten or obsolete. its still usable, right?

author: shibas
author_rank: Newbie
timestamp: February 21, 2013, 09:05:34 am
content: I am posting the circuit diagrams of both the amplifiers:-1.circuit of AD 620 instrumentation amplifier,designed for getting a gain of about 106.2.circuit of non inverting amplifier using op amp ic 741,designed for getting a gain of about 101.The gain equations of both the amplifiers are written inside a box near both the circuit diagrams iam attaching with this post.I tried both the above amplifiers for amplifying the output of a vibration sensor or analyzer.But when i gave the input from the function generator both the circuits amplified well.For the non inverting amplifier using 741 ic,when i gave the input from the function generator at first i was getting a gain of about 50 and 68 when the input signal frequency was about 9.7 KHz.When i changed the input signal frequency to about 1 KHz i got the gain of about 100.But the original gain designed for this amplifier circuit was 101.When i gave the sensor output signal to the input of the non inverting amplifier it gave a gain of about 450.When i used AD620 amplifier circuit,I get a gain value of about 100 when i was giving input signal from the function generator.When i gave the output signal from the vibration sensor as the input to the AD620 amplifier it also gave about a gain of about 450.Why is the gain becoming very high when i give the sensor output as the input to the amplifier?Which amplifier should i use for my need(for amplifying the sensor output)?Should i make any modifications to my circuit.If then please suggest those modifications clearly.Somebody please help me and reply fast.

author: ignator
author_rank: Regular Contributor
timestamp: February 21, 2013, 10:02:38 am
content: To answer your question about :Why is the gain becoming very high when i give the sensor output as the input to the amplifier?What is the vibration sensor? My experience these are charge devices, and require very specialized amplifiers. You could have a Pezio device. But coulomb sensors are another type.Using a 741 opamp for a gain of 100, is not a trivial design. What is the gain bandwidth product (go look that up and see if you are exceeding it).Also vibration testing is not normally done beyond 2Khz, mainly because of the humongous speaker coil system that drives a huge mass. That assumes you have such a piece of test equipment.

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 21, 2013, 10:59:59 am
content: Quoteso it seems your problem is not the opamp, they both behaving the same when feed by the sensor. whic you are not showing.

author: CarlG
author_rank: Regular Contributor
timestamp: February 21, 2013, 11:16:37 am
content: You don't say how you measure the gain. I think your problem is that you're loading the sensor when you measure the input signal, making it weaker than it gets when you remove the meter (or whatever you got) to measure the output. This gives you the illusion of higher gain than using the signal generator, which is able to handle the loading by the meter.

author: fcb
author_rank: Super Contributor
timestamp: February 21, 2013, 11:22:18 am
content: The correct answer is use the circuit that works and meets your performance criteria..1. The 741 has a GPBW of about 1MHz on a good day, therefore at 10KHz, the MAX gain will be 100, and that's not a good way to design stuff (running upto the GPBW) and it's why your seeing a lower gain at fIN. So if you've got your heart set on the 741, you need to cascade a couple of stages with lower gains. That's how they used to do it in turntable's at all BBC radio stations...2. How you landed on the AD620 is anyone's guess. But that should do the job OK, and the 470R should give you the correct gain. Why are you getting a gain of 450? Simple, you've either implemented the circuit wrong or your measurements/assumptions are wrong.Fast enough?

author: Mechatrommer
author_rank: Super Contributor
timestamp: February 21, 2013, 12:59:24 pm
content: Quote from: fcb on February 21, 2013, 11:22:18 amwhy?

author: fcb
author_rank: Super Contributor
timestamp: February 21, 2013, 02:13:47 pm
content: Bandwidth is defined by the -3dB point, so you if you have a GPBW of 1MHz (i.e. 741), you will not get a gain of 100 @ 10MHz, probably more like 70.GPBW is determined by the internal design of the opamp, and will be prone to production variation (bias currents, hfe, etc..), temperature variation (bias currents vary hugely with temp, so will capacitance) and quite possibly Vcc.

author: ignator
author_rank: Regular Contributor
timestamp: February 21, 2013, 11:11:49 pm
content: Quote from: shibas on February 19, 2013, 10:38:35 amI neglected in my post this morning to ask what the transfer function of the vibration sensor is.That is, g's (acceleration) as a mechanical input, and what is the electrical output? e.g. volts, amps, charge. Do they need any excitation?The accelerometers we use for vibration testing of avionics (black boxes), use charge amplifiers. These were rack mounted modular equipments that were operated by technicians, so I have no idea what was inside the electronics.Sorry I can't be more help.