LF351 rail splitter sanity check

[ElectricDruid]

Hi all,

I'm going a bit bonkers staring at this. Something obvious is eluding me. I've got a LF351 single op-amp on the breadboard set up as a voltage follower. I've connected my 9V bench supply to a pair of 10Ks and a 100uF and then connected the follower to the midpoint voltage.

I get 9.6V supply voltage across the resistor divider and across pins 7 and 4 of the op-amp. I get 4.8V on pin 3 of the op-amp (+ve input) but I get **1.4V** on pins 2 and 7 (-ve input / output)!

What am I doing wrong? I'm going to feel like a right dork when you tell me, but at this point I just want someone to put me out of my misery...

[ElectricDruid]

Ok, I've found it! And it was a "Doh!" moment.

Have a guess...

[iainpunk]

is it one of those weird opamps which have one or both inputs designed to be ground level biased while the output is at Vcc/2?

happened to me a few times when using random opamps from my IC stash

[ElectricDruid]

Yeah, you're on the right lines - it was the op-amp.

It turned out the LF351 op-amp from my tube of LF351 op-amps was actually an LF353 op-amp. The lettering was a bit worn and I must have stuck it in the wrong tube. Once I found a genuine '351 it sprang to life.

I thought about it some more, and figured that the circuit is simple, so there must be something wrong with the op-amp, so I started going through the tube to see if others in there worked better, and that's when I noticed.

There's a shadow from that blue wire across the exact bit you need to see, so you'd never have found it from the photo above.

[antonis]

There's a shadow from that blue wire across the exact bit you need to see..

I was ready to blame you for using defective grill for cooked-to-order devices.. 

[iainpunk]

so.... no sausage for me