Opamp Oscillator Shootout

What’s this?

Data sheets are one thing. Real world performance is another. Graphics cards and CPUs have benchmark comparison sites, but we humble analog circuit people don’t have an equivalent.

I was recently in need of a vaguely sinusoidal oscillator with a simultaneous pulse/square output, of around 120 KHz. A huge number of op amps should be able to do this without a problem—including all of the quad opamp parts I had. Now, I quite like the pinout of op amps, and having an extra op amp to buffer the output of my simple oscillator circuit would be a definite advantage over a dual part. So, I made an inventory of the reasonably nonexpensive quad op amps I had in my shelves, and came up with:

• LM2902 in two versions (Texas Instruments and STMicroelectronics), 1MHz GBW bipolar op amp; output goes to negative rail
• TL074, JFET input, 3MHz GBW op amp.
• LF347, JFET input, 4MHz GBW op amp.
• TL974, Low noise high-speed, rail-to-rail output (not input) bipolar opamp, 18MHz GBW
• TLV2372, CMOS rail-to-rail input & output op amp, 3MHz GBW

Op amp-based oscillators are not high frequency; the phase shift across frequencies accumulate rather quickly; therefore, op amp oscillators are limited to hundreds of kilohertz even with very fast op amp parts. Current feedback op amps, which are faster than regular (voltage feedback) op amps are not suited for oscillator use. However, there are many valid applications for a low (0-500 KHz) frequency oscillator.

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The test setting

The test setting is a simple, two-opamp oscillator circuit with two output points, TRIANGLE and SQUARE

The actual circuit was built on a piece of simple, tri-pad copper clad perfboard. The negative rail was created by a charge-pump type DC/DC converter from the 9V positive rail input.

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The frequency is adjusted with two potentiometers (10K and 100K). The filter capacitors are all polyester film capacitors, whereas the charge pump uses tantalum and multilayer ceramic capacitors. The main power supply is itself bypassed by a .47µF ceramic capacitor, within close distance to both integrated circuits.

The test setup uses a Rigol DS1102 100MHz/1GSS oscilloscope with x10 500MHz probes from Tektronix.

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Test results

The yellow trace is the TRIANGLE output; the blue trace is the SQUARE output in all of the tested devices.

The LM2902 from Texas Instruments

The worst performer of them all. The LM2902 is very similar to the LM358, and is a basic, class-B output op amp that supports output down to the negative rail. However, it is unusually disappointing that the maximum acheivable frequency was less than 10 KHz.

The most likely cause of this is poor phase margin – as the frequency increases (as well as that of the harmonics), you eventually get to a phase shift that causes destructive interference. Therefore, a set maximum frequency will appear. If you decrease R or C above that, the frequency will again begin to drop.

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The LM2902 from STMicroelectronics

I’ll be damned! Just because a part has the same number, doesn’t mean it’s identical, when it’s from two different manufacturers. This is a very important caveat – especially for those old multi-sourced parts (the 555, the 741 etc.).

The STM part performs better than the TI part – but still with a maximum frequency an order of magnitude less than what the goal was. Also, note that the clipping artefacts on the “triangle” wave can’t be reduced with reduced gain in the circuit. It won’t slam against its output maxima, but the wave shape will still be clipped off.

All in all, no matter where you get the LM2902 – an old National part, the TI-branded part, or STMicroelectronics, it’s a piss-poor chip for oscillator purposes. As is its relatives (LM358, LMx24).

The TL074 from Thomson (STMicroelectronics)

A true classic. The TL074 is a nice, FET-input, reasonable-bandwidth op amp. Just don’t expect to get too close to the rails! Also, an aside. Just look at that chip photo! The 13th week of 1985, and the original Thomson Semiconductor logo, before the world turned mad, and Thomson turned into STMicroelectronics.

Now, we’re within striking distance of our target goal. In fact, I’d be happy with the TL074’s results – it’s cheap, and you can hit about 200KHz with it if you’re willing to accept a lot of distortion. But, this distortion can be filtered out with a few RC stages… Not bad! Not bad at all!

The LF347 from Texas Instruments

Very similar to the TL074, but the increased gain-bandwidth product reveals itself in an increased distortion-free frequency. And, it is, in fact, cheaper than the TL074—at least if you’re ordering new TI stock for both. However, it comes at the cost of increased harmonic distortion and noise density. This seems like a good all-rounder for the oscillator application, and earns a definite recommendation from here.

The TL974 from Texas Instruments

Impressive! An all-bipolar operational amplifier performs 40% better in maximum frequency than the FET-input op amps. The TL974 is a bit of a powerhouse though; a very high gain-bandwidth product that ranges from 12 to 28 MHz depending on supply voltage; very low noise, beating all of the FET-input amplifiers, and even a rail-to-rail output swing (though not rail-to-rail input!)

However, there does seem to be a bit of an amplitude issue with the triangular waveform. And the square wave slopes less crisply than the FET amplifiers. The latter isn’t anything that can’t be solved with the judicious use of a proper comparator, but the great frequency performance comes at a cost.

Editor’s note I actually managed to destroy a TL974 during testing. It seems to be less resistant to voltage transients as well as loading than the other opamps.

The TLV2372 from Texas Instruments

Well, well, well. We all thought that expensive, rail-to-rail input/output CMOS parts were here to save the world. And then, they perform miserably; worse than their gain-bandwidth product would indicate.

If you increase the frequency beyond the 25KHz mark in this test application, you end up with weird, non-linear distortion of the waveshape. Which indicates that it’s the phase margin that is lacking; the complementary transistors likely have much more internal capacitance than the FET-input and pure bipolar parts, causing phase shifts over >180° across the harmonic spectrum.

Therefore, it seems that CMOS parts are exclusively for audio-frequency applications, at least with this type of oscillator setup. Mind you – there is one huge advantage to the TLV2372: look at the output swing! And, the square waveform is free of the over/undershoot, the FET-input opamps suffer from.

However; these lackings of the “lesser parts” can be corrected for. The low frequency capability of this (comparatively expensive) CMOS part can’t, and therefore, it’s not the best in this application!

Conclusion

So which would you pick? Honestly, the LF347, although it misses the mark slightly on the frequency un-distorted, would probably be my pick. Both the triangle wave and square output are of high quality, and the frequency can be pushed more if you add a two- or three-pole low-pass filter (with the cutoff frequency at twice the oscillation frequency) on the triangle output. And that last reason is exactly why I’d prefer a quad part – don’t go loading the oscillator down with several filter stages. To use this oscillator output, you need to buffer it, and having two extra amplifiers just helps with that.

The LF347 is about 0.2 euros (for 1 piece) from most suppliers, and it’s an excellent op amp for this application. Doesn’t mean it’s great for other things – at low voltages, I’d use the TLV2372 any day; and if I needed precision and fast response, the TL974 would be my choice.

But today – just for today – the humble LF347 wins!