operational amplifier practices  - Page 1
Forum section: electronics / beginners

author: arcaderix
author_rank: Newbie
timestamp: September 20, 2023, 07:20:39 pm
content: which one is better to do operational amplifier lab practice, is it better to use operational amplifier with mosfet or operational amplifier with jfet or operational amplifier with internal bjt?Which one is the best and which one does not fail so much when testing them?

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:09:35 pm
content: The choice of op amp depends on your circuit.For DC and audio applications, here are some trade-offs for BJT vs JFET:BJT: lower voltage noise, higher current noise and input currents. Therefore, better with lower source impedance (and feedback resistors).JFET: higher voltage noise, lower current noise and input currents. Therefore, better with higher source impedance (and feedback resistors).With modern devices, high speed and low offset voltage drift can be obtained with either active device: you must read the datasheet for individual op amps.MOSFETstypicallyhave even higher voltage noise and even lower input current.If you destroy any of these devices during testing, that is probably not the fault of the device (unless it is a counterfeit part masquerading as a real part).

author: MikeK
author_rank: Super Contributor
timestamp: September 20, 2023, 08:23:33 pm
content: Use both. Use the LM358 and learn of its limitations and what it's good for, what conditions. And do the same for the TL082.

author: Zero999
author_rank: Super Contributor
timestamp: September 20, 2023, 08:24:45 pm
content: The attached photographs show a TL082 (J-FET input) and an LM358 (BJT input) op-amp.The TL082 has a higher input impedance, lower voltage noise, is much faster and lacks the crossover distortion of the LM358, but it uses more power, requires at least 7V to work properly, doesn't work with its inputs at the negative rail and its output won't go within 2V of either supply rail. Its inputs will usually work up to the positive rail, but this isn't guaranteed.The LM358 uses less power, will work down to 3V, its inputs work down to the negative rail and its output can go within 50mV of the negative rail, but it's slow and has horrible crossover distortion.It depends on the application. The LM358 is better for low power, DC applications, whilst the TL082 is the better choice for something like an audio amplifier, which requires low distortion.

author: TimFox
author_rank: Super Contributor
timestamp: September 20, 2023, 08:29:19 pm
content: For serious audio applications, the NE5534 BJT input amplifier is much better than the LM358 unless you really need low supply voltage (and correspondingly low signal voltages).It does pull more power than the TL082 or TL072 JFET devices, but it can drive a 600 ohm load.This is why they have datasheets.The crossover distortion mentioned by Zero999 is a function of the output stage, but your comparison is relevant to the input stage of the amplifier.

author: magic
author_rank: Super Contributor
timestamp: September 21, 2023, 05:54:09 am
content: Quote from: Zero999 on September 20, 2023, 08:24:45 pmTo be exact, it shows a Chinese TL082 knockoff with a dodgy TI-like logo, which in my (and others') experience is invariably an LM358 inside.The other one is a Chinese LM358 with equally fraudulent TI logo, but at least it's almost surely a 358 as promised.Almost because I recall somebody who ordered opamps from AliExpress and got LM393 comparators marked as opamps

author: analityk
author_rank: Regular Contributor
timestamp: September 21, 2023, 11:42:42 pm
content: As long as you need it for own skills improving you should buy cheapest.Since you need performance there is rabbit hole with many many different models.Even that LT1028 is still lowest noises one (voltage noise), OPA189 have input voltage offset in 0.3uV range, and there is at least one opamp (I forgot model but digikey will help you find it) with femtoampere input bias current. Of course each ot them have some drawbacks and there is many between them in case of properties and price. LT1028 is not cheap.Even all of this you can build hybrid stage's with two or more opamp, where overall performance is better than two separate stages (low noise and low voltage offset).