Non-inverting amplifier  - Page 1
Forum section: electronics / beginners

author: sy
author_rank: Regular Contributor
timestamp: May 07, 2023, 09:13:09 am
content: I built a non-inverting amplifier from an LM358P but don't quite get the results I am expecting.I have a 150mVpp sine wave input applied to the positive terminal with 350mVdc offset to avoid clipping (I am using single supply).R2=10KOhm, R1=1KOhm, so I should be expecting a gain by a factor of 11 on the output end (which more or less looks accurate from the graphs).Still new to using the oscilloscope but as far as I gather, I should be reading 300mVpp on Channel 1 but I am getting 340mVpp?And the vertical offset is -698mV which doesn't make sense because my supply rail only goes from 0V to 15V.Also, I can't measure my output for some reason as it just comes up with "***" for the readings.Any suggestions? I am using Siglent SDS2104xP.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 07, 2023, 10:32:12 am
content: I suspect that the output impedance of the waveform generator in the scope is 50 Ohms and the settings apply when the output is terminated in 50 Ohms. That is not the case in your setup. The amplifier input presents a high impedance so the actual voltages for amplitude and offset into your amplifier are twice what you thought you set. Either halve the settings or add a 50 Ohm resistor (51 is nearest preferred value) between input and 0 Volts on the breadboard.

author: sy
author_rank: Regular Contributor
timestamp: May 08, 2023, 07:16:08 am
content: Quote from: wasedadoc on May 07, 2023, 10:32:12 amI have checked my wave generator and you are indeed correct! -- the output load says 50Ohms (and there is also an option for High Z).Quote from: wasedadoc on May 07, 2023, 10:32:12 amSo does this mean that the waveform generator will only output the correct waveform when applied to a 50Ohm load? But my coaxial cable is also rated at 50Ohms alreadyEdit: apologies for the sideways image

author: wasedadoc
author_rank: Super Contributor
timestamp: May 08, 2023, 07:21:32 am
content: 50 Ohm coax does not of itself act as a 50 Ohm load.

author: sy
author_rank: Regular Contributor
timestamp: May 08, 2023, 07:30:03 am
content: Ah I see. So all I need to do at the positive input terminal is throw in a 50Ohm resistor to ground?I was just alsoreadingthat if connected to a high impedance load -- like an operational amplifier -- that the function generated would have twice the voltage.In that case, could I just switch over to the "High Z" option instead of throwing in a 50Ohm to ground at the positive input terminal?

author: wasedadoc
author_rank: Super Contributor
timestamp: May 08, 2023, 08:48:35 am
content: Yes the High-Z option is acceptable at the low frequencies such as you are using. At higher frequencies, say several tens of MHz, it could give issues.

author: newbrain
author_rank: Super Contributor
timestamp: May 08, 2023, 10:56:02 am
content: Quote from: sy on May 07, 2023, 09:13:09 amTo answer this, that was not addressed by wasedadoc:The offset you see on the scope channel setting is the amount you have have moved the 0 V level of the trace on the screen.The 0 V level normally sits in the center of the screen.As the waveform has a positive offset, to be able for it to be shown the trace needs to be moveddownwards, hence the negative offset.

author: wasedadoc
author_rank: Super Contributor
timestamp: May 08, 2023, 11:02:54 am
content: Quote from: newbrain on May 08, 2023, 10:56:02 amI did wonder about that but dismissed that reason because of the negative sign. If I was designing a scope the readout would be the voltage of the central horizontal axis, ie would have the opposite sign to what Siglent appear to have implemented.