Trying to find "Middle ground" in understanding op amps - Page 1
Forum section: electronics / beginners

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 12, 2013, 03:36:52 pm
content: I'm a hobbyist newb and I've begun to move away from Arduino, blinky led's, transistors, and the like and I would like to begin understanding amplifiers. I'm finding that there are a lot of good tutorials but they tend to go from an ideal op amp and jump right to a completed circuit. I'd like to find something a little more practical that explains why each element is there.This kid is a little weird but I LOVED this video:http://youtu.be/6ok-ruHlopEI liked it because he started with the component all by itself. I'm really sorry that he didn't get a "Part 2" done yet because I was looking forward to him adding components one at a time.I was wondering if anyone has come across any really good component by component explanations of different parts of an amplifier circuit and the practical reasons each part is there. I have a schematic showing a low pass filter on a power input line for a benchtop LM386 amplifier. But... why is there a low pass filter on the power input line?? What if it's not there? What will happen?Stuff like that.Thanks!Mike

author: John Coloccia
author_rank: Super Contributor
timestamp: November 12, 2013, 03:43:16 pm
content: Can you post the schematic? Often times the rails have decoupling caps on them, and depending how the schematic is drawn it might look vaguely like a low pass filter, especially if there's a voltage divider in there.

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 12, 2013, 03:51:57 pm
content: Thanks for the reply. This question was mostly a general "are there good component by component tutorials on amp circuits" question.I have specific questions in this thread:https://www.eevblog.com/forum/beginners/trying-to-understand-a-couple-of-lm386-circuits/But the "low pass" example I gave was in this circuit:http://www.instructables.com/id/LM386-Audio-Amplifier/step3/Observe-Diagram/R1 = 10 ohmC1 = 220uf

author: Dongulus
author_rank: Regular Contributor
timestamp: November 12, 2013, 04:30:18 pm
content: Hi, I have some clarifications for you. First, you asked about op amps, but you then ask about the LM386, which is an audio amplifier, not really an op amp. The difference is that the gain of this LM386 is by default internally set while an op amp exclusively uses external components to set gain. Second, the filter at the output of the amp in the instructables circuit is a high pass filter, not a low pass.I'll leave the audio amplifier for the other thread and just address op amps. Op amps are a good place to start when learning about amplifiers because they are so versatile and prevalent in electronic design. There are loads of circuits that op amps are used for, but to start you should absolutely understand these basic circuits:-Inverting amplifier-Non-Inverting amplifier-Voltage follower-Voltage summerEach of these circuits require few components and behave reasonably close to "Ideal Op Amp Land". You should be able to understand these circuits by injecting tiny amounts of Ohm's Law and remembering the basic op amp assumptions. Once you master these circuits, you can start learning about some advanced non-idealities of op amps (frequency response, offset voltage, slew rate).

author: John Coloccia
author_rank: Super Contributor
timestamp: November 12, 2013, 04:47:41 pm
content: If you simply removed R1, that cap on power would be a standard decoupling cap. I have no idea what the purpose of the resistor is.I believe the LM386 references the output to midscale, so if your input power was 12V, the output would be 6V + whatever the signal is...and that's convenient for coupling to other audio stages. I guess it's intended that you don't have to bias the input. For the final output, however, you need to remove the DC offset. The final cap before the output is coupling cap and forms a high pass filter, as Dongulus said. It removes the DC from the signal and is typically selected to be large enough to pass through the entire audio band, though depending on what you're doing you'll sometimes set it to cut some of the low frequencies to remove the "woofiness" from the signal.

author: edavid
author_rank: Super Contributor
timestamp: November 12, 2013, 04:50:40 pm
content: Quote from: Szewczykm on November 12, 2013, 03:51:57 pmThat's a form of supply decoupling. The resistor makes the capacitor more effective. Without knowing how the amp was used, it's hard to say why the RC was added, or if it was a good idea.

author: Szewczykm
author_rank: Regular Contributor
timestamp: November 12, 2013, 04:56:26 pm
content: Quote from: edavid on November 12, 2013, 04:50:40 pmDoes that form have a name? Something I could look up and learn about?The amp is being used as a small MP3 player amplifier. Plugged into the headphone jack and a small speaker for output.

author: edavid
author_rank: Super Contributor
timestamp: November 12, 2013, 05:04:18 pm
content: Quote from: Szewczykm on November 12, 2013, 04:56:26 pmRC decoupling.QuoteThere may have been a problem with digital noise from the MP3 decoder getting into the audio output.

author: John Coloccia
author_rank: Super Contributor
timestamp: November 12, 2013, 05:10:19 pm
content: Yeah, it's not something you typically see with just a simple audio amp and of course it forces you to us a larger cap, but it does a good job of filtering a noisy supply.

author: zapta
author_rank: Super Contributor
timestamp: November 12, 2013, 05:29:14 pm
content: Quote from: Szewczykm on November 12, 2013, 03:36:52 pmGreat video. I like his humor and he explains it very well, including the 'don't question it' and 'pin 1, nobody is using it'.

author: StubbornGreek
author_rank: Frequent Contributor
timestamp: November 12, 2013, 10:59:10 pm
content: Here's a nice explanation (outstanding channel overall):