LM2901: Comparator LM2901 in VCO Circuit

Hi Anayat,

can you show a schematic?

Kai

Hello Anayat,

Yes. The LM2901 can be used to make a VCO, but depending on your needs, you may be better off using a device that was specifically designed as a VCO.

What are your needs? How far does the frequency have to vary? Over what range?

One of the most common mistakes is the over-assumption on how much the frequency can vary. You may be able to get a decade or two out of a simple oscillator if you accept some non-linearity, but more that becomes complicated.

To generate a true symmetrical triangle wave, a linear generator such as an op-amp will be needed. A simple relaxation oscillator using a comparator will have an exponential sawtooth (more like shark-tooth) ramp. YOu may need to "buffer" the signal on the timing cap to get the triangle wave.

There are several devices that can be put into VCO service, such as the LMC555, CD4046 and LM331.

The LM555, LMC555 and TLC555 (all bascially the same) have many resources on the web. A simple search for "555 VCO"  will bring up many resources.

The CD4046B VCO section is one of my favorites. A cap and two resistors makes a fairly simple oscillator (do not need to use the PLL section...just the VCO section).

The LM331 will be the most linear - but also a little more difficult to set-up.

Please find the attached schematic

i want to replace LM224 with LM2901. My requirement is   0-5Vp-p/4.5Khz frequency.

please 

Hello Anayat,

The LM224 is a quad op-amp, and the LM2901 is a open-collector comparator. They are not compatible. The LM224 is acting as a linear integrator, which will require a linear push-pull output.

You can try to make a comparator into a terrible op-amp..or...you can use an actual op-amp (recommended).

Is there a particular reason you wish to replace the LM224?  If you need something "newer", consider the next generation LM2902LV. I'm sure the amplifier folks will chime in with their latest, updated devices.

I want simplify the circuit. Don't want to use the existing circuit. I want to use LM2901 as triangular waveform generator. I generate square waveform using LM2901 but I am not getting triangular waveform.

Hello Anayat,

A "simple" LM2901 generator will not generate a symmetrical triangle wave of which the frequency can be varied with a control voltage (duty cycle possibly, but not frequency).

Please see the "relaxation Oscillator" cookbook circuit as a basis:

 Relaxation oscillator circuit

Note that the saw tooth is slightly "bent" due to the R-C timing.

The LM224 circuit uses an integrator to charge the cap with a constant current, which results in a linear waveform. The comparator inverts the integrator input current when the triangle "tip" threshold is reached.

You were asking for a VCO (Voltage Controlled Oscillator)  with a triangle wave output, which complicates things since something has to vary to achieve the "VCO" function. To reduce the time period, you have to either vary the transition "tip" thresholds, or, vary the rate at which the cap charges.

The simple comparator solutions charge the cap at a constant rate (with a resistor). So the frequency is fixed. Varying the transition thresholds would also proportionally vary  the amplitude of the resulting triangle wave.

So what is left is to vary the amount of current charging the cap and fixing the transition thresholds. This gives a constant triangle amplitude and varies the speed. But it requires an integrator and a comparator. To make things even more fun, the adjustable charging current into the integrator must also be reversed between the positive and negative slopes - so that means switching symmetrical + and - voltage controlled current sources.

Now we are getting into design territory that is beyond the scope of these forums. If you do a search on "Voltage Controlled Triangle Wave" you will get  quite a few hits.

But note that most of the circuits that appear are fixed-frequency, or, require pots to adjust the frequency. The circuits you are interested have at least four active devices - so it is not a simple circuit.

Can I use LM224 only to generate the required triangular wave of 0-5vp-p / fixed frequency? (any frequency from 1.5KHz to 5KHz) Vcc=10V

Anayat

sorry for the delay, Paul will get back to you on this tomorrow.

Chuck

Hi Anayat,

I would do it this way:

anayat_triangle.TSC

To decrease the frequency, increase R1.

Kai

Hi,

Thanks for reply. Could you please send me the calculations involved in your design?

Please check and comments on the attached schematic. It's working fine generates triangular wave of 4.15Khz/5.2Vp-p.

Please check the schematic and your expert opinion is required. i have tested the circuit on breadboard with LM224 and its working good. Its generate a triangular wave with 4.15Khz and Vp-p is 5.2V.