Back to Back Tantalum Caps

I was recently looking at a design that had two Tantalum caps reversed biased to each other to make a bipolar cap. I know tantalums can work down to -1v and don't need a DC bias the way aluminum electrolytic caps do but this looked strange. What are the pitfalls to this method.

Thanks Paul C

The cost of tantalum caps perhaps ?

They *don't* need bias ? I rather thought they did to perform properly. In fact, under the right condirions, aluminium electrolytics work well for audio coupling under zero bias conditions. You need a big cap to ensure that the ac component of the signal is essentially miniscule but you might be interested to know that almost every single top-end modern audio mixing console uses unbiased aluminium electrolytics to couple signal - right at the very top-end too. I'm talking $100,000 + products here.

I do know an audio company that ( many years ago ) used back to back tantalums for coupling that biased their centre junction to ensure biasing polarity. Today that would be considered esoteric overkill and financial suicide for a large product.

I have used that technique with a pair of aluminium electros though, where the DC offset voltage at the source is subject to tolerances. Works nicely.

Graham

Makes it more nonlinear in many situations.

John

You might be better of putting a low Vf diode across each cap to keep the cap from ever conducting forward current.

Tam

Without knowing the DC bias, AC current and frequency, and the capacitance it's impossible to say how weird or maybe not weird this would behave. It's complex.

John

I'm just following Ian Sinclair's Passive Components for Circuit Design. He says that Alumunium Electrolytics require a DC polarizing voltage to maintain the insulating film and for Tantalum Caps he says "Tantalum electrolytics can be used without any DC bias and can accept a small reverse voltage, typically less than 1.0v." But he doesn't get into the frequency characteristics and I would guess biasing would improve the usable bandwidth.

Paul C

That's my intuition. I would pay for a bipolar cap with a data sheet to give me that warm fuzzy feeling of knowing what's going on.

Paul C

Yes, or clamp the signal to +/- 0.7 volts.

Paul C

"John Larkin" wrote in message news: snipped-for-privacy@4ax.com...

Wouldn't the cap acting as a diode be as bad or worse? He does not say what this is for; so, I am assuming he wants to put 60 Hz through it, which I have done at 24VRMS..

Tam

What I had done was put 2 caps back/back with diodes to get a NP 200uF capacitor for use with a reversible 2 phase electric motor in an antenna rotator. Worked fine. I know of people who have don this with speaker crossover networks, but as John points out, probably not a good idea.

Tam

It was in an AGC circuit. Basically a low pass filter with the caps in series to ground two 47uF caps and 1k resistor so a 4Hz 3dB. Not very demanding with respect to distortion or tolerances. I was just curious about how the limitations of the configuration.

Paul C

Any link ?

Oh well. That means almost every piece of current pro and consumer audio is built wrong then !

Alumiums are good for a few 100mV reverse volts typically. Perfect for split supply audio apps.

What 'frequency characterisitcs' ? Bias affects bandwidth ? Some very strange ideas here.

Maybe this Mr Sinclair is one of those audiophool gurus ?

Graham

I don't even want to know. If possible, I avoid tantalum caps because they short out when they fail. They can also burn a hole right through the PCB if you are unlucky.

When it comes to distortion, I think the best idea is to read the datasheet and determine how the capacitance changes with the voltage.

Section 4.8 is oversimplified. For the back-to-back config with diodes, small-signal AC (unbiased) series capacitance is indeed C/2, but large-signal (when the diodes are being useful) it approaches C.

Nonlinear as hell.

John

This may be interesting reading material:

"Nico Coesel" wrote

This document IS interesting, and it seems to be good technically. It will be useful if someone can get past the poor phrasing and reverse orders of many statements.

Perhaps the inserted reverse logic was intended as humor?

reversed

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what

A better method would be to put a bias resistor to a sufficiently large bias voltage connected to the junction of the 2 capacitors. That way, both are always operating correctly biased.

Regards Ian

Interesting. They call for the diode protection across each cap.

Paul C

The bias / bandwidth idea is one of my fool ideas. Ian Sinclair's book is recommended by Bob Pease in his Troubleshooting book, so it has been proofread by someone with some experience in the field.

Paul C

I'd be really smart, if I wasn't so stupid.