Difference between TL072 & TL082 in a comparator circuit? - Page 1
Forum section: electronics / beginners

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 02, 2025, 05:21:50 am
content: Hi,I have a eurorack (+/-12V) module PCB I bought, I sourced the parts and soldered it up, the creator has sold a thousand PCBs and hasn't seen my problem.https://www.eevblog.com/forum/beginners/please-help-me-understand-voltage-in-this-circuit/It uses a TL072 or a TL082 as a comparator (I used a Tayda TL072) to take an incoming square wave clock and convert it to a short pulse to feed into four 4042 clock inputs. My build has been a nightmare, the clock pulse was entering the TL072 and not exiting.I changed the capacitor from 100p to 470p and that worked but it's still very temperamental with other clock sources, basically anything under +5v.So the reason for this post is there something in the TL082 design that makes it better suited to this application, I have an ST Micro TL082 here that I could switch to, or is it maybe just I need to order a branded TL072?Please note, I didn't design this circuit, I just bought the PCB, I got responses on the other thread to say that using an opamp for a comparator isn't a great idea here, that's great but I just want to get the existing PCB working reliably.Thanks.Richard

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2025, 08:00:40 am
content: As far as I'm aware the TL082 is the same as the TL072. The latter might have slightly lower noise, but that shouldn't make much difference to a simple comparator circuit. I think it's more likely you have a fake TL072 and a branded part will work.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 02, 2025, 08:11:14 am
content: There is no big difference between the TL072 and TL082. The TL082 is the cheaper version and may have higher noise, but that is about it with the difference. There is a good chance that modern TL082 chip will also meet the TL072 specs.The difference between batches and manufacturers could well be larger.As another complication there is the new TL072H, which is a quite different OP-amp with CMOS input.One could consider a real comparator (LM392) in combination with a pull up resistor for the output. Of cause there is still the issue with the 2nd OP-amp.A possible problem can be the slew rate for the output - this is slow even for the slow 4000 series logic. There may be also differences in the 4042 chips how they react with a slow rise time.

author: iMo
author_rank: Super Contributor
timestamp: July 02, 2025, 09:19:12 am
content: Starting several threads on the same topic is not good..How is your opamp TL072 powered?

author: iMo
author_rank: Super Contributor
timestamp: July 02, 2025, 09:48:17 am
content: Here is the LTspice sim.Your output pulse at "out" is aprox 21us wide with your 100pF coupling, so it could be your TL072 has got issues with processing that (it is slow). Use 1nF or a higher value..PS: the output pulse is 2ms long with 10nF..Also I would add a hysteresis there..

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2025, 10:06:49 am
content: How is the TL072 powered?Assuming it's only got a +12V and the negative rail is 0V, then that's no good.Refer to the circuit from your other thread.The inverting input is biased at 1.1V (Search for potential divider formula, for how to calculate this) and the non inverting input is tied to 0V. This is outside the common mode range. You might have been lucky with one op-amp and it worked, but it didn't with a different one.A comparator such as the LM393, as mentioned above, will work, but bear in mind that the non-inverting input will go below 0V on negative edges, which is again outside the common mode range of the LM393, so will need a Shcottky clamping diode, to limit it.

author: iMo
author_rank: Super Contributor
timestamp: July 02, 2025, 10:16:27 am
content: When using 4042 cmos there - find there two cmos inverters, first with capacitive coupling and 1Meg feedback between its inp/out, the second one afterwards in order to make nice edges. You may add a diode or a zener at the input of the first inverter to limit your 30Hz 8Vpp input.The R1 is not needed with Schmitt trigger inverters..An example below, adjust the zener's voltage based on your cmos logic levels..

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 02, 2025, 10:24:21 am
content: The TL072 is powered by +12V and -12V. INV B is 1V.

author: Zero999
author_rank: Super Contributor
timestamp: July 02, 2025, 12:03:57 pm
content: Quote from: rthorntn on July 02, 2025, 10:24:21 amWhat a strange circuit.Why use the TL072 and not the LM393 for the comparator functions? I could understand if it used the quad TL074, but not three TL072s.And why use 5 TL072s which gives 10 op-amps and only use 7 of them?Definitely replace the TL072 used for comparator functions with the LM393 and connect the output of each to +12V, via a 10k resistor.

author: rthorntn
author_rank: Frequent Contributor
timestamp: July 03, 2025, 09:33:36 pm
content: Thanks all.It's working - I just replaced the Tayda TL072 with an ST Micro TL082 from Mouser that I had laying around, it's like night and day, it clocks as low as 1.3V now

author: Zero999
author_rank: Super Contributor
timestamp: July 03, 2025, 09:45:23 pm
content: Quote from: rthorntn on July 03, 2025, 09:33:36 pmIt was probably the LM358, with TL072 printed on the case. It's is much lower than the TL072 and has a lower input impedance.