"clipping" transistors and a part question

[boog]

So I built the 741 (culturejam, I think) w/ MOSFET clippers last night; I've done quite a few clipping diode builds in the past and figured "hey! if you can toggle on/off and add saturation controls with diodes, you can do it with transistors!"   No, it doesn't seem like you can.  Can someone explain or direct me to why this doesn't work?  Or did I simply do a bad build?

To clarify, the build works with the toggle on, but the saturation (50k pot) is either on or off (fully counterclock it's off and immediately once you begin turning it's on and stays the same for the full turn), once I turn the toggle off it don't work at all.

AND; what's the mouser number for the vactrol for the Uglyface?  It's the only thing I can't find there.

[WLS]

I still kind of new at this and I am sure someone will tell me where I am wrong but here's my two cents.

The term clipping refers to cutting the top of the signal off. A diode does this by allowing voltage to flow through until it gets high enough to engage and stop the current. It does this until it drops back to a certain level or below.

A transistor does the oppsite. It passes signal instead of blocking at higher voltage levels.


Bill

[George Giblet]

I don't know what circuit you have built but if you connect the collector to the base you can use a transistor as a (single) diode no problem; one connection to C+B  the other to the E.

[boog]

There's 2 of them (2n7000) running in parallel with a 2m2 resister - here's the page http://www.forrestwhitesides.com/node/58; It works fine without the toggle and pot, I just don't understand why it doesn't react the way I anticipated.  Though I spose if I actually understood (entirely) why it worked in the first place I wouldn't have this problem

[smallbearelec]

what's the mouser number for the vactrol for the Uglyface?  It's the only thing I can't find there.

Mouser doesn't carry the Perkin-Elmer VTL5C3. I have it, SKU 2509 at smallbearelec.com

SD

[boog]

I'll keep that in mind for the future; for whatever my dad (who's a ham radio guy) has a couple of them.  So I bummed them off him

[JKowalski]

I still kind of new at this and I am sure someone will tell me where I am wrong but here's my two cents.

The term clipping refers to cutting the top of the signal off. A diode does this by allowing voltage to flow through until it gets high enough to engage and stop the current. It does this until it drops back to a certain level or below.

A transistor does the oppsite. It passes signal instead of blocking at higher voltage levels.

Bill

I'll be that person.

The term clipping refers to cutting the top of the signal off. A diode does this by allowing current to flow through when the voltage across it gets high enough to do so (typically 0.6V). It shunts the current from the signal to another location (typically ground) until the voltage across it drops back to a certain level or below, and the current can no longer flow to that seperate location. In this way, when it reaches that cutoff voltage, the voltage on the signal side is trying to go higher but the diode is sucking the current out that the signal needs to make that peak.

A transistor is a whole different thing. It's got a huge amount of complexity - but it can be summed up as "current gain". You put current into the base, and that makes a much larger current flow from the collector to the emitter.



When you connect the base to the collector to use as a diode - this is what you are doing.

A diode is a semiconductor with a P/N junction. Those are two silicon chunks  plastered to each other, with different amounts of added "impurities" in them which give them special properties (P is one, N is the other). A transistor is almost the same as a diode - EXCEPT, instead on a single P/N junction, it is a N/P/N junction. The n's are the collector and emitter, and the base is the P. (PNP trannies are the opposite). All you had to do to make a transistor was to tack on another N section. In fact, you can actually think of a transistor as two P/N junctions, or two diodes, coming out from the base towards the collector and emitter.

Now, when you attach the collector to the base - basically, you are completely bypassing the first N junction. What you have left is a P and a N junction - a diode.

I SPICED the transistor diode out of curiosity and took a peek at the VI curve. Looks exactly the same as a normal diode, (I used a 2N3904) but it has a slightly higher turn on voltage. Looks like 0.8-0.9V. If anyone wanted to know. (I did!)

[George Giblet]

> Now, when you attach the collector to the base - basically, you are completely bypassing the first N junction. What you have left is a P and a N junction - a diode.

Connecting B tand C doesn't actually bypass the junction  the transistor still manage to work as transistor (ie. Ic/Ib = gain) and the majority of the "diode" current passes through C.

[boog]

yup, that's the kinda answer i was looking for....not sure if i understand all of it, but i did ask the question.  Thanks!