Transistor Replacements for this? - Page 1
Forum section: electronics / projects

author: skyler
author_rank: Regular Contributor
timestamp: December 23, 2013, 02:33:51 pm
content: I am wanting to build an optical AM receiver for modulated light communications. I found a circuit herehttp://modulatedlight.com/Modulated_Light_DX/modlightrx.html, but it seems pretty outdated and none of the transistors were in stock on digikey. (I was surprised the photodiode was!)So, what transistors do I need to replace the ones shown on the circuit -2N5457, BC179, BC109, AND MPF102( i would like them to be all digikey, shipping can get expensive, especially for a 15yr old experimenter)I would like to keep it discrete- I like to at least try to understand everything, but if an op-amp is 100% required, I guess I can do that

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2013, 02:55:42 pm
content: MPF102 is hard to find because it's a JFET, not because the particular part number is obsolete. It's actually one of the more common ones. You'll have an easier time finding SMD JFETs - they have loads of MMBF4416, which should work fine there. They also have J113 in a TO-92 through-hole package, which is not as similar but should still work. Likewise... MMBF5457. Or eBay has piles of both parts relatively cheapBC179 is a pretty pedestrian PNP transistor. Try 2N3906, 2N4403, BC557, etc. Same for the BC109 - 2N3904, 2N4401, BC547...Note that the MPF102 could be replaced by an NPN transistor easily, unless I am missing something there. As it's just a follower, you may indeed be able to directly replace without changing anything else! But the 2N5457 must be a FET for high impedance (you could use a FET-input op amp instead, but a single JFET really isn't that expensive and I see no reason bothering to change the circuit)(Damn, I edited this about three times correcting errors. Maybe I should slow down??)

author: dannyf
author_rank: Super Contributor
timestamp: December 23, 2013, 02:56:28 pm
content: There doesn't seem to have any thing special about those transistors. Unless you are blinking at some ungodly speed, garden variety transistors will work.

author: NiHaoMike
author_rank: Super Contributor
timestamp: December 23, 2013, 03:10:11 pm
content: Nowadays, you can use opamps. Even some good ones are quite cheap.

author: dannyf
author_rank: Super Contributor
timestamp: December 23, 2013, 03:18:35 pm
content: QuoteNot in this case. You will likely need to use positive feedback to increase the input impedance, if a bjt is to be used in place of the jfet.

author: c4757p
author_rank: Super Contributor
timestamp: December 23, 2013, 03:20:42 pm
content: Are you sure? The MPF102 is the one at the output. It is being fed by an emitter follower with an output impedance on the order of 200 ohms.

author: dannyf
author_rank: Super Contributor
timestamp: December 23, 2013, 04:28:08 pm
content: You are right. I was thinking about the jfet input stage. My fault.

author: Alana
author_rank: Frequent Contributor
timestamp: December 23, 2013, 05:38:20 pm
content: But what about this wierd connection from gate of first transistor to the last? Its as if photodiode took reference voltage from gate of last transistor, or maybe schematic is incomplete.

author: dannyf
author_rank: Super Contributor
timestamp: December 23, 2013, 05:45:01 pm
content: That's by design.

author: skyler
author_rank: Regular Contributor
timestamp: December 23, 2013, 06:55:24 pm
content: Quote from: NiHaoMike on December 23, 2013, 03:10:11 pmI would if I knew a good circuit, it just seems like every circuit requires something different that one website dosen't haveI wish I could design circuits, I want to build something, but one component is obsolete, and I can't find anything. Then, someone tells me to just throw an operational amplifier in there, and I don't even know where to start.

author: underwurlde
author_rank: Newbie
timestamp: December 23, 2013, 08:18:02 pm
content: Start with a transimpedance amplifier maybe? Should be quite simple if using an opamp for this.A

author: calexanian
author_rank: Super Contributor
timestamp: December 23, 2013, 08:34:56 pm
content: I would try to find a little more modern of a circuit. A single dual op amp would do everything needed there. however the parts there are all available from Amazon. Nothing particularly rare. I do see Digikey is out of stock on those fets, but that is typical. Elsewhere online they are available. I think you could find an easier circuit. Look up the radio shack engineers series notebooks. I recall a very simple circuit in one of those made with very simple parts.

author: skyler
author_rank: Regular Contributor
timestamp: December 23, 2013, 10:20:36 pm
content: I have another question - what should the value of Rf be?

author: skyler
author_rank: Regular Contributor
timestamp: December 23, 2013, 10:22:08 pm
content: Quote from: calexanian on December 23, 2013, 08:34:56 pmOk. Thanks, Jameco electronics has everything for this circuit so I might try it anyways

author: calexanian
author_rank: Super Contributor
timestamp: December 23, 2013, 10:26:30 pm
content: Good old Jameco