Vref to IC(s)

[Phend]

Hello:
Bread boarded the power section of the Micro V.

With 9.5 volt power supply,
Vref becomes 4.82 volts
Or to lessen it with a diode between Q2e and C8 Vref becomes 4.37 volts

Is it better to have Vref at a little over or under 4.5 volts ?
Or is it insignificant ?




 

[Kevin Mitchell]

The range control makes the small differences in the reference negligible - you can dial out the small difference (if it's even noticeable). Even so, it's all relative to the supply anyhow.

Plus, zenner.

[R.G.]

It's insignificant. There is nothing magic about 4.5V. 4.5V appears in most pedals because it's in the middle of a nominal "9V" battery supply. The point is to give a reference that's as near the middle as possible.

[ElectricDruid]

The point is to give a reference that's as near the middle as possible.

+1 agree, but even that assumes that whatever device we're talking about behaves symmetrically around the mid-point voltage. Quite often that's not the case. There are plenty of chips that will go closer to one rail than the other, so in those cases you could actually maximise headroom by pushing the bias slightly *away* from 4.5V on a 9V supply.

But this is all small-beer stuff, generally. You're gaining fractions of a volt or a volt if you're lucky. If headroom is that big an issue, you need to look at a bigger supply, not the exact details of where the bias voltage is.

[Rob Strand]

+1 agree, but even that assumes that whatever device we're talking about behaves symmetrically around the mid-point voltage. Quite often that's not the case. There are plenty of chips that will go closer to one rail than the other, so in those cases, you could actually maximise headroom by pushing the bias slightly *away* from 4.5V on a 9V supply.

Quite often making bottom divider resistor one E12 step larger than the top is close to optimum.

Another trick for onboard preamps running off battery is to tweak the bias point for symmetrical clipping when the battery is run down a bit.   That way it doesn't clip excessively as the battery dies.

[Matthew Sanford]

The point is to give a reference that's as near the middle as possible.

+1 agree, but even that assumes that whatever device we're talking about behaves symmetrically around the mid-point voltage. Quite often that's not the case. There are plenty of chips that will go closer to one rail than the other, so in those cases you could actually maximise headroom by pushing the bias slightly *away* from 4.5V on a 9V supply.

But this is all small-beer stuff, generally. You're gaining fractions of a volt or a volt if you're lucky. If headroom is that big an issue, you need to look at a bigger supply, not the exact details of where the bias voltage is.

Tom, to simplify, and if expecting large signals needing it, would one seek Vref at half the output voltage rather than supply? I.e, on 9v supply, TL07x being ~2-9v would have Vref at 5.5v, LM358 being 0-~7v would have 3.5v?

[PRR]

would one seek Vref at....

Wind it up until it comes out bent. (If you never do that, then it does not matter.)

With sound a little bent, vary the bias. It will sound a little different. Again it may not matter.)

[ElectricDruid]

Tom, to simplify, and if expecting large signals needing it, would one seek Vref at half the output voltage rather than supply? I.e, on 9v supply, TL07x being ~2-9v would have Vref at 5.5v, LM358 being 0-~7v would have 3.5v?

Yes, that's it. You'd need the bias point to be in the centre of the output range, whatever that is for the chip you're using.

Like PRR says, suck it and see!

[R.G.]

Yep - to a first approximation, half the supply. Next closer approximation, half the output range; third approximation, half the ...linear... output range, as some circuits get fuzzily non-linear before actually clipping.

Quickie notes to Matthew: TL07x family in particular starts limiting at 1.5 to 2V away from both ends of the power supply, so on a 9v supply the output range is about 2 to 7V, so 4.5V is still in the middle. Most dual supply opamps have similar limits, usually symmetric from the ends of the most positive to most negative power supply. Nominal single-supply opamps like the LM358, as you mention, include the negative supply but not the positive supply, so middle of the actual output range is what to shoot for; LM324s are similar.

Reading datasheets is always fun; TL07x only get withing 1.5-2V of either power supply, but don't invert their phase if the ...input... gets outside the specified common mode range. Some earlier opamps do. Get the + input outside the specified common mode range and the output bangs to the opposite output rail, near as it can get. So in some odd circumstances you have to watch that your Vref doesn't hold the inputs outside a reduced common mode range. Usually this can only happen if you have Vref and the chip supply coming from different power supplies, but an unusually big capacitor (for instance) might hold up a Vref when the power supply is reduced. Some very few older opamps can be damaged if the inputs are pulled too far apart by a reference and an output, so they were used with back to back diodes between the input pins.