Fet matching

Papa_lazerous · December 20, 2006, 04:28:40 PM

Going to build the fet matching circuit from the gallery as I need to do some matching for my phase 90. the layout http://aronnelson.com/gallery/album75/Fet_Matcher_1 specifies a TL071 (or indeed any single op-amp. I go a bunch of TL072's laying around I figure it will be ok to use one of them as long as I get the pinout correct so that I am only using one of the 2 op-amps in the IC.

Is that right? If so I'm set to go without having to go buy parts

Papa_lazerous · December 20, 2006, 06:24:49 PM

Any takers....

Ardric · December 20, 2006, 06:33:17 PM

Right. You could connect the unused + input to the voltage reference, the same node as the + terminal on C1. The output connects to the - input. Now it'll idle happily.

But look! Its now buffering the voltage reference for free! So I just used that buffered reference for the test fixture. I socketed the drain resistor R2 so I could test with different values. I added a 79L09 voltage regulator, 22 ohm current limiting resistor, 470uF filter capacitor, and LED with 1k5 resistor. The power cable from the board ends in a 4-pin PC molex connector, where I steal +12VDC from my desktop PC. Finally I added four binding posts to the corners of the perf board, labelled +9VDC, +4.5VDC, GND and TEST. Now it lives beside my breadboard as a power supply for goofing around with.

Papa_lazerous · December 20, 2006, 06:37:39 PM

Any chance of re-winding on that and talking slowly.... LOL

So I cant just not connect the other stage of the op-amp I have to connect the un-used parts too, to make it idel. Any chance you could explain that a bit I looke at the layout and what you said and it doesnt make total sense sorry

Ardric · December 20, 2006, 06:51:58 PM

The layout isn't right; you're changing the pinout of the chip. You're going to need a new layout. I built mine on perf board by looking at the schematic, so the best I could offer there is a bad photo and encouragement to do the same. I've never used vero board.

Papa_lazerous · December 20, 2006, 07:02:08 PM

a bad photo would help, I will probably use breadboard anyway.

a schematic would be better be really great. As I will be using TL072 it confuses me as I am only learning about op-amps and dont want to mess it up

mdh · December 20, 2006, 07:16:13 PM

The schematic is on http://geofex.com. Click on "Tech Tips" in the left menubar, and look for "JFET matching fixture" on the resulting page. I'm not providing a direct link, because GEOFEX uses frames, and learning to navigate it is one of the rites of passage in our community

Just breadboard it according to that schematic, and you should be fine.

Papa_lazerous · December 20, 2006, 07:20:55 PM

Thanks for telling me where the original schem is mdh,

Dont worry I know my way around geo surprised I havent seen that schem already. you are missing my point a little. that is I dont have any single op-amps and cant get any locally so I am going to build using a dual op-amp. I was more worried about hat I did with the rest of the IC that wasnt being used. the + - ins and the output. can I just leave them un connecte or do I have to do something to keep them happy?

Ardric · December 20, 2006, 07:24:13 PM

Here's a rough drawing of what I did. Note that it doesn't include the opamp IC power connections.

Papa_lazerous · December 20, 2006, 07:29:15 PM

oh I see what you did there, you buffered one into the other, and the two outs are for the DMM. I like it cheers

News:

Fet matching