4.0 AMP Power Transistors - Page 1
Forum section: electronics / beginners

author: vidarr
author_rank: Frequent Contributor
timestamp: November 01, 2018, 06:19:29 pm
content: From a salvage piece I have a circuit board with a BD675 & BD676 (Medium Power NPN Darlington Bipolar Power Transistor) together in a (sub?)circuit. I looked up the datasheets and one transistor has a diode one way and the other the other way in reference to the collector and emitter.Right now, I am trying to design/build a small power amp circuit to provide a bass speaker to another stereo amp chip circuit (based on the TDA1517). The goal is to build a very simple, 2.1 speaker system so that I can understand all the different components and how they go together.The diode allows the flow one way, so there are two going in opposite directions. Is this to create a stereo circuit? Or is this something else, like more power. I do not understand what the two different diodes are doing together in the circuit (inside the chips, in the same circuit, etc). I do not want to copy the circuit I have either. I want to unsolder this and build from scratch. So, if I am creating a single, mono amp for the bass, do I need both transistors? I have more questions about this, but for now this is good.Thank You!

author: Benta
author_rank: Super Contributor
timestamp: November 01, 2018, 06:36:59 pm
content: The diodes are unimportant and only relevant in inductive switching applications. The transistors will be fine for your amplifier.

author: wraper
author_rank: Unknown
timestamp: November 01, 2018, 06:52:42 pm
content: Amplifiers often use such diodes in addition to transistors for output protection.QuoteIf you ask such question, IMHO it's way too early to for you to design amplifier. It would much better to take some circuit with a good explanation of how it works and what component does what. Not waste a ton of peoples and your own time by 'designing' something without a clue what you are doing and asking for suggestions, which will wildly differ. And then you would choose what suggestion to implement without actual understanding why you choose it to begin with. And worst of all, result will be inferior to just taking off the shelf circuit.

author: vidarr
author_rank: Frequent Contributor
timestamp: November 01, 2018, 07:58:14 pm
content: Quote from: wraper on November 01, 2018, 06:52:42 pmWell, this is my first amp from a power transistor. I know that multiple transistors can be put together to make an amp. I was just wondering if I should, since I have them already. My other amp is a complete circuit. It is with the transistors doubled to make two channels. If I put two of these PTs together, will it be overkill for integrating with the other, two channel chip. Your opinion is hardly humble; you need a girlfriend or something. Relax.

author: brybot
author_rank: Contributor
timestamp: November 01, 2018, 08:14:56 pm
content: Quote from: vidarr on November 01, 2018, 06:19:29 pmThe BD675 is NPN and the BD676 in PNP. They are both in the darlington configuration which provides extra gain.Quote from: vidarr on November 01, 2018, 06:19:29 pmThat's a great way to learn, but you may find that starting small and working your way up will work better. You'll learn a lot that way.Quote from: vidarr on November 01, 2018, 06:19:29 pmWhat you're describing sounds like a class B amplifier. The NPN and PNP are configured as the high and low sides of the amplifer.You'll need both transistors to create a class B amplifier. However, you can create an amp with a single transistor. It's known as a class A amplifier and is an easy place to start. You may want to read up on the types of amplifiers. For example:https://www.elprocus.com/classification-of-amplifiers-classes-its-applications/

author: wraper
author_rank: Unknown
timestamp: November 01, 2018, 08:31:40 pm
content: Quote from: brybot on November 01, 2018, 08:14:56 pmSingle transistor amplifier works in class A but it's not a definition of class A amplifier as such. Secondly, it certainly won't work as amplifier for subwoofer. It's basically useless by any reasonable standard for power amplifier.

author: wraper
author_rank: Unknown
timestamp: November 01, 2018, 08:36:52 pm
content: Quote from: vidarr on November 01, 2018, 07:58:14 pmNope, what is hardly humble are your ambitions. What you are intending to do is like writing poems without learning the language first. You first should learn how amplifier works (at least to minimum extent), then design your own. Not the other way around. Assembling already existing circuit is a nice way to learn. In any case, slapping together what was suggested on the forum won't suddenly become your own design anyway. Actually it will be just repeating some off the shelf circuit in awkward way, without a proper explanation. There will be zero innovation in it.