Author Topic: Zener Bias an op amp  (Read 1883 times)

Jneely88

Zener Bias an op amp
« on: December 28, 2015, 12:42:28 PM »
Hi


New here. Just curious of the drawbacks of zener reference for an op amp? It seems that this can yield more current than a divider if needed. Also it can be exact.

Wonder if the zener will add hiss? Noise?

Sorry if this has been covered I searched first but nothing really gave me the exact answer with my keywords.

Part count is the same give or take. Ran it on sim but doesn't give life experience there.

Thanks
JEff



Keppy

Re: Zener Bias an op amp
« Reply #1 on: December 28, 2015, 01:23:58 PM »
Zeners can be noisy. Opamps INPUTS don't take much current. 1/2 supply gives the most headroom, so it can be hard to find a Zener that's exactly where you want it, which varies depending on your power supply. Plus, a resistor is a few cents cheaper.

There's no reason it can't work, but in low current designs a resistor divider is cheaper and better. More info here: http://www.geofex.com/circuits/Biasnet.htm
"Electrons go where I tell them to go." - wavley

Jneely88

Re: Zener Bias an op amp
« Reply #2 on: March 01, 2016, 11:18:15 AM »
Thanks.

I did realize noise might be a key issue.


Hatredman

Re: Zener Bias an op amp
« Reply #3 on: March 02, 2016, 04:25:57 PM »
Not only noise.

Most pedals work at 9V, so 1/2 supply is 4.5V.

Standard zener values are 4.3 and 4.7, which can be used, but you'll get less headroom and a slightly assimetric operation. It works, but a voltage divider with resistors works better and is cheaper.

So it's not only a question of noise. If you use zeners, your circuit will be worse in ALL specs.
Kirk Hammet invented the Burst Box.

Jneely88

Re: Zener Bias an op amp
« Reply #4 on: March 03, 2016, 02:28:48 PM »
Doesn't the MXR phaser use one?

Hatredman

Re: Zener Bias an op amp
« Reply #5 on: March 03, 2016, 06:38:32 PM »
Doesn't the MXR phaser use one?

Yes, and it is a 5.1V one. Very unbalanced.

But still, the Phase 90 is a phaser, which should have an overall gain close to unity. It's not a gain pedal (OD,Dist,Fuzz), where noise AND headroom are problems. Besides, it's a design decision, maybe there was a reason for that and I'm nos seeing (LFO stability, maybe?).
Kirk Hammet invented the Burst Box.