GOD. Mike pls halp? Mosfet booster frustration

Pens · Post by **Pens** » Sat Sep 19, 2009 6:21 pm

Okay, so I'm getting back into trying to make pedals again. Looking at my old schems and shit, and getting inspired with new designs in my head.

So, to start it all off, I wanted to make the classic Mosfet booster to get me back in the game.

I built it all, but I can't get the damn thing biased for shit.

The voltage at A never drops below 8v. I made a few subs in the circuit, as follows:

R2: 100k trimpot for biasing
R3: 1M since this just sets impedance according to Jack's article
R4: 4.7k since I didn't have 2.7k and figured I could just fix the bias with the trimpot.

Yeah, I can't get point A anywhere close to 5v. Help?

Post by **Mike** » Sat Sep 19, 2009 8:53 pm

Hmm. Weird. What transistor are you using and are you 100% on the pinout. FETs vary a whole heap

Pens · Post by **Pens** » Sat Sep 19, 2009 9:55 pm

BS170 and yeah I will sheepishly admit when I first put it in I had it backwards, I had looked at a different mosfet pinout before, but it's right now based on the datasheet.

I'm going to try shorting around the 1M and see if that changes anything.

Post by **Mike** » Sat Sep 19, 2009 10:01 pm

Try removing the protection diode also, sometimes if you use a diode with the wrong threshold it will stop the FET from working.

Pens · Post by **Pens** » Sat Sep 19, 2009 10:14 pm

Mike wrote:Try removing the protection diode also, sometimes if you use a diode with the wrong threshold it will stop the FET from working.

I was just thinking this very thing after staring at the schem for a bit longer. I heard you could sub in an LED there? Yes/No?

Post by **Mike** » Sat Sep 19, 2009 10:16 pm

Just drop it out, it's only for Static Protection so for prototyping you're OK.

Red LEDs do work fine, I use 1n4148s

Pens · Post by **Pens** » Sat Sep 19, 2009 10:37 pm

FUUUUUUUUUUUUUUUUUUUUUUUUUUUUUU

Okay, both removing the diode nor shorting the 1M did anything to get the A point above 2v. I made a mistake earlier, when I said 8v I had it wrong. It goes between 1.5v and 2v at A, and that's with from 4.5v to 9.1v on the gate pin.

So. Goddamn. Frustrated.

Post by **Mike** » Sat Sep 19, 2009 10:46 pm

You're trying to get 4.5V at A?

Hmm. Have you a 100K you can parallelise with your R2 100K to make it more friendly?

What's Vr looking like?

Pens · Post by **Pens** » Sun Sep 20, 2009 12:46 am

According to the article by Jack Orman on this:

"The dc voltage measured from point A on the schematic to ground should be 4.5v to 5.5v with the circuit idling and no input. The value of R2 should be tweaked until the point A voltage is correct. If two or more of these circuits are mounted in the same enclosure, only one R1/R2 network is necessary and the R3 from each stage can be connected to the Vr point."

Vr is going between 4.5v and 9v. A always hovers around 2v in these ranges.

I have some more BS170s coming in the mail. I will try swapping in a new one when I get them. I'm putting this aside for now so I don't make my head asplode from frustration.

Bill Oakley · Post by **Bill Oakley** » Mon Sep 28, 2009 2:51 am

Make sure you are looking at the datasheet for the same manufacturer of your BS170's. The pinouts vary on these.

Pens · Post by **Pens** » Mon Sep 28, 2009 1:39 pm

I'll have to check into this. I'm not even sure what manu these are, I ordered them years ago. I have a new batch coming in the mail soon, though. Thanks for the heads up!