Inverting Amp Distortion - Page 1
Forum section: electronics / projects

author: scott-williams-113
author_rank: Newbie
timestamp: September 07, 2017, 07:39:24 am
content: Hey everyone. Long time watcher of Dave, first time poster.I'm a final year EE student at uni and am just working on my thesis. Would be bit of a long story to discuss in full so I'll just get to the point.I'm fairly experienced with both discrete and operational amplifiers, so this is sort of confusing me at the moment:I have a +/-5V rail powering an LM324. I've used these HEAPS for DC circuitry (instrumentation stuff), but not much for AC.In this context I'm essentially inputting a 0.65Vpk-pk AC signal with 0.35V DC offset, which can be a variable frequency from 10Hz - 100KHz.I'm using the 4 Op-Amps on the chip as:1. Unity Gain Buffer then2. Summing Amplifier to remove DC offset (high pass filter 1Hz cutoff was giving me trouble after 50KHz) then3. Inverting Amplifier with a rheostat at Rf to vary the signal 0-6Vpk-pk, and finally4. Unity Gain BufferIn the pictures below I've attached what the input (yellow) vs the output (green) at each of these stages (in order) looks like, at around 10KHz.As you can see, there is increasing distortion at both the summing amplifier and inverting amplifier stages, which occurs at the midpoint of the signalThis distortion starts to creep into the output signal as stuff as low as 500Hz, and gets worse with increasing frequency.What's more to notice is that only the circuits with resistors (inverting and summing amp) seem to add to the distortion.So my question is, what could potentially be causing this? For my next PCB this will be present on, I'm planning to use a $10 OPA4743 (rail to rail, 7Mhz bandwidth) - so if the reason is simply the fact that LM324's are cheap and somewhat crap at these frequencies, then (maybe) this problem will go away. If it's something you have seen before and has an easy fix, please let me know. It's not due to breadboard capacitance I'm fairly certain as this was occurring on my old PCB (using the lm324).Struggling pretty hard to find help on this, so any advice would be appreciated!Thank you

author: tombi
author_rank: Regular Contributor
timestamp: September 07, 2017, 07:50:26 am
content: Google opamp cross-over distortion.Tom

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2017, 07:52:23 am
content: The LM358/324 is a class B amplifier and what you're seeing is crossover distortion.https://youtu.be/VgodYtiD_F0Yes, it will go away with the OPA4743 because it has a class AB output stage.

author: scott-williams-113
author_rank: Newbie
timestamp: September 07, 2017, 07:55:26 am
content: Thanks so much! When I look up "distortion" trying to research all I get is results regarding hitting the rails and phase shifting, not this specific problem. Thanks.Do you think I can ditch the summing amp and use a proper high-pass filter (same as his in the video, but 1uF and 1MOhm for a 0.1Hz cutoff) if I use the OPA, also?

author: Zero999
author_rank: Super Contributor
timestamp: September 07, 2017, 10:23:46 am
content: The crossover distortion, definitely will get worse, with increasing frequency, as the open loop gain decreases, thus reducing negative feedback, which otherwise helps to reduce distortion. The LM324's slew rate and gain bandwidth product is probably also too low for what you're trying to do.The OPA4743 will have a fast enough slew rate for 100kHz.How much gain do you need? The OPA4743 as just under 40dB at 100kHz. You might need more. One option might be to add some gain to the input and output buffers, so the gain of the inverting stage can be reduced.Yes, you can probably simplify the circuit considerably. An AC coupling capacitor is normally used to remove the DC offset, no a summing amplifier.It may be better to use the potentiometer as an attenuator, rather than in place of RF.

author: StillTrying
author_rank: Super Contributor
timestamp: September 07, 2017, 12:02:08 pm
content: There are other threads on the LM324 distortion, a pull down resistor gives some improvement.https://www.eevblog.com/forum/beginners/what-causing-this-distortion-in-my-3-opamp-differential-amplifier-circuit/

author: David Hess
author_rank: Super Contributor
timestamp: September 07, 2017, 10:33:00 pm
content: The 324/358 suffers from an unusually large amount of crossover distortion because it has 3 uncompensated Vbe drops in its class-B output stage. The LT1014/LT1013 is a direct improved replacement with a class-AB output stage. Or you can use a resistor or current source tied from either supply to the output to force class-A operation.

author: Audioguru
author_rank: Super Contributor
timestamp: September 08, 2017, 03:25:55 am
content: The lousy old LM324 quad and LM358 dual opamps are cheap because they were designed and started to be sold a long time ago and because they have more than one opamp in a package. They have crossover distortion and a poor slew rate because they were designed to use very low supply current.They cannot produce frequencies higher than about 2kHz at full output level.Most modern opamps use a reasonably low supply current, have NO crossover distortion and work well up to at least 100kHz. They cost only a little more.

author: David Hess
author_rank: Super Contributor
timestamp: September 08, 2017, 12:09:31 pm
content: Quote from: Audioguru on September 08, 2017, 03:25:55 amThe 324/358 is cheap because it was designed with transconductance reduction (1) to reduce the size of its integrated Miller compensation capacitor making it about 1/6th the size of the capacitor built into the 741. The smaller area allows the operational amplifier to be replicated economically. The 1458 is a dual 741 type operational amplifier which also uses transconductance reduction for the same purpose and is almost as cheap as the 358.I do not know why they left 3 uncompensated Vbe drops in the 324/358 output stage. They could have compensated for at least 2 of them without increasing the quiescent current. Maybe it was to save area or cost through better yields rather than improve a characteristic which was irrelevant for its intended applications. I thought I remembered reading the datasheets for some old but improved 324/358 designs not long ago which just added a class-AB output stage; maybe I can find them again. (2)(1) Simplified 324/358, and 1458, schematics do not show it but complete ones do. See where one collector of each transistor in the differential pair goes to the base? That reduces the transconductance of the differential pair without lowering the emitter current so the compensation capacitance may be lower and most importantly, physically smaller. The 30pF capacitor which would normally be needed takes up a huge amount of space.(2) It was the Fairchild uA798 dual (from 1976!) and it has the same quiescent current rating as the 358 dual. I do not think Fairchild published the complete schematic but except for the common bias stage and gain stage, it looks exactly like the 358 except with a Vbe multiplier added to compensate the 3 Vbe drops in the output stage. So I do not think the class-AB output was left out of the 324/358 design to save quiescent current.