CD4049 and Opamp swap. Can it be done?

[ragtime8922]

I've done very little work with the CD4049UBE but I had a few circuit designs that I would like to use just one chip for instead of a bunch of opamps. Here are my questions: (I know that multiple opamps on a single chip exist but I still have the following questions.)

1.)Can a opamp stage be replaced with a syage from the hex inverter?

2.)Can an active filter be made with a CD4049 stage?

3.)Due to the fact that 4049's invert and 4050's do not why is the 4049 more popular with effects? Couldn't a circuit be simplified if you didn't hace to re-invert the phase at the end of the circuit?

[R.G.]

1.)Can a opamp stage be replaced with a syage from the hex inverter?

Maybe. It depends on the circuit.

Hex inverters have no noninverting input, so criteria one is that the opamp circuit have the noninverting input tied to ground or a bias voltage only, no signal going to it, and no LFO or other movement on the noninverting input.

Criterion two is that the opamp circuit be used as an inverting linear amplifier or as a switching buffer. The hex inverter is actually a  better inverting buffer, as this is what it's designed to do and the switching buffer is a modest misuse of the opamp.

Criterion three: the opamp circuit require only quite low gains, usually less than four. This is because the hex inverter has a much lower open loop gain than the opamps, perhaps 20-30db (x10 to x30), versus the opamp's 60 to 100db (x1000 to x100k). This criterion has two results. The hex inverter circuit will have neither the maximum potential gain of the opamp nor the gain accuracy. Gain accuracy is that quality that makes the gain of the opamp circuit be accurately Rf/Ri. The actual expression for the gain includes a term relying on the open loop gain to reduce some imperfections to nearly zero. The low forward gain of the hex inverter will not reduce these "error" terms to nearly zero, so the closed loop gain only approaches Rf/Ri, but is noticeably lower to accurate measurement.

Criterion four: the opamp circuit must not rely on the high frequency rolloff of the opamp for its special sound. The hex inverter has a much higher frequency response at open loop than the opamp because of the opamp's feedback compensation.

Criterion five: the opamp circuit must not require even modest DC accuracy. The hex inverter simply does not have any mentionable DC accuracy compared to the opamp.

What's left? Circuits with low, inverting AC gain and no particular accuracy requirements. Fortunately, there are a lot of those.

2.)Can an active filter be made with a CD4049 stage?

Yes. Some active filters need only a gain-of-one inverting or non-inverting buffer. Hex inverters can do this. The non-inverting buffer is made from two hex inverters strapped for unity (voltage) gain in series. This is particularly true for simulating inductors in gyrator circuits. Some other actives only need gains of one, and these work fine. Twin T filters and bridged T filters as well as Wien Bridge filters are adaptable to the capabilities of hex inverters.

High gain requirements in the filters will mean that the hex inverters will not work as expected. There is a whole class of filters that rely on semi-infinite gains in the opamps. These will not work.

3.)Due to the fact that 4049's invert and 4050's do not why is the 4049 more popular with effects? Couldn't a circuit be simplified if you didn't hace to re-invert the phase at the end of the circuit?

Maybe. However, the 4050 is a noninverting buffer with voltage gain, not unity gain. Each section of the 4050 is actually two inverters in series inside the package. That means that you cannot lower the voltage gain with feedback, because feedback around the whole mess is postive, and it will latch up. You cannot get at the point inside the chip where the two series inverters are joined to put negative feedback around each inverter separately. So it only works if you want the full, noninverting gain. DC accuracy is even worse than with a single inverter as well, so things get out of hand.

That's not to say it can't be used if you want a high noninverting gain and are willing to give up any feedback control or DC operating point control. Just that it's hard.

[ragtime8922]

2.)Can an active filter be made with a CD4049 stage?

Yes. Some active filters need only a gain-of-one inverting or non-inverting buffer. Hex inverters can do this. The non-inverting buffer is made from two hex inverters strapped for unity (voltage) gain in series. This is particularly true for simulating inductors in gyrator circuits. Some other actives only need gains of one, and these work fine. Twin T filters and bridged T filters as well as Wien Bridge filters are adaptable to the capabilities of hex inverters.

High gain requirements in the filters will mean that the hex inverters will not work as expected. There is a whole class of filters that rely on semi-infinite gains in the opamps. These will not work.

I am working on a parametric EQ design and I wanted to see if I could use the 4049. Is there a schematic of a 4049 filter somewhere? I understand your aboce description for the most part but I'm not sure how to tie the outputs together to complete the filter. Also, due to the gain issue, I may need an opamp somewhere in the circuit anyway.

The Mr. EQ project at the ROG site uses the 4049 for 7dB cut/boost I think. I wanted to adapt it for parametric use but I'm guessing that it would produce less than the 7dB which is low to begin with.

After understanding your hex filter completely would it be possible to combine the 4 filters (lo, lo-mid, hi-mid, and high) in to a summing amplifier for audio? If not I'll probably just parallel them in to an opamp like a lot of other circuits.

[gez]

As RG mentioned, gain is fairly low in inverters so they're not always a suitable candidate for some filters.  

You can up gain by either running the chip from a lower supply voltage (specifically 5V or lower) or by connecting three inverters in series and treating them as a single 'unit'.  With the latter, I've had excellent results but current consumption goes through the roof at 9V.  Current consumption drops dramatically when the chip is run at lower supply voltages but output impedance increases, which can be an issue as a RC low pass can be formed with the input capacitance of any following stage (especially if it's another inverter) resulting in a muffled sound (only found this to be a real problem from 3V suppies, or thereabouts).

I've had very good results when running inverters at 5V and buffering the outputs using FETs run off the higher 9V supply (to avoid clipping).  Headroom might seem an issue, but a lot of op-amps don't have rail to rail swing, as inverts do, so there's not much in it.

[puretube]

free information sucks...