Playing with an OP 191 Op Amp

RE: Playing with an OP 191 Op Amp

jc2048 — Sun, 24 Jan 2016 12:25:18 GMT

I thought I'd try a proper filter next. Here is a traditional Sallen-Key second-order low pass filter.

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Gain in the passband is +10 (20dB). That's defined by resistors R1 and R2. The 3dB point, with these component values, is at 2.5kHz (where the 'a' cursor is). The 3dB point is where the response has fallen to 0.707 times the gain in the passband and is the point that is normally used for stating the bandwidth. However, if you look at the response you'll see that a second-order roll off is quite modest and at 20kHz the gain is still 0dB [ie a gain of 1, where the output would be the same as the input]. If you filtered an audio signal with this [use an input capacitor if you do] it would diminish frequencies above 2.5kHz, not completely eliminate them - effectively it's the treble control of an amplifier turned down to progressively reduce the higher tones.

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Here's more on the filter configuration if you want to look at the theory:
https://en.wikipedia.org/wiki/Sallen%E2%80%93Key_topology

If you decide to try it on your breadboard, experiment with changing the frequency response. You can either change the resistors (R3 and R4) or the capacitors (C1 and C2). Probably best to keep the Rs and Cs identical in value, so if you change R3 to 22k, change R4 to that as well. Perhaps try doubling the resistor values and halving the resistor values and see what effect it has on the 3dB frquency. Do the same with the capacitors if you have some suitable values. You'll quickly see that it's possible to adjust the response without going back to first principles and doing all the calculations from scratch.

I've shown the decoupling capacitors on the +V and the -V. For my simulation it doesn't make any difference to anything - the voltage source providing the 12V is perfect and has no noise or ripple on it - but for a real world application they need to be there, so it's best that I set a good example. For anyone who's not used to this kind of thing, the 100nF caps need to go right next to the part (on a pcb you'd place them within a few milimetres (tenths of an inch)). On a real board you'd also have some bulk decoupling where the power enters - 10uF or so would do.

RE: Playing with an OP 191 Op Amp

jc2048 — Sun, 24 Jan 2016 11:59:42 GMT

Ah, the website is back and working.

I'm having great fun with this simulation stuff. Another circuit you might want to try is an "ideal diode". You often used to see this circuit on old op-amp datasheets - particularly the wonderful ones that Nat Semi put out with dozens of circuits on them - but I've never had a use for it in real life.

Here's the circuit

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here are the waveforms for a 5kHz sine wave going in

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Although it is far superior to the 1N4148 by itself, it's not quite perfect. The small step on the output as the input cuts 0V on the way up is where the opamp output has to suddenly move from close to the -12V rail to the 0.6V it needs to forward bias the diode and start lifting the output. That change is limited by the slew rate of the amplifier output.

This shows that point in more detail (green is the input, red is the output)

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The datasheet gives the slew rate as 7V/uS typical (rising), so that suggests 1.8uS for the -12V to 0.6v transition plus a bit of settling time. The simulation is about 3uS, so it looks like the model is safely worst-case and you'd see something faster than that if you measured it (which you should be able to do easily if you look at the input and output on a scope and place them on top of each other). It's encouraging that the TI model seems to be this detailed.

RE: Playing with an OP 191 Op Amp

DAB — Fri, 22 Jan 2016 20:18:27 GMT

Very good Blog John.

I have not played with an opamp like this since my early years, so it is nice to see someone go back and just check the basics.

Well done.

DAB

RE: Playing with an OP 191 Op Amp

jc2048 — Fri, 22 Jan 2016 12:14:53 GMT

Very nice part.

TI have a spice model for it, so I thought I'd try your experiment virtually on my computer.

Here's the circuit (inverting amplifier, gain of 10) with +12V and -12V supplies

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And here are the waveforms, the input is 100mV peak and the output is 988.55mV peak so it works! A gain of 9.8855, so almost 10.

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And here's the frequency response. The roll-off is what you'd expect from the decline in the open-loop gain at higher frequencies, so it looks at first glance like the modelling is quite good and simulation would have some use for experimenting quickly with circuit ideas and configurations.

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RE: Playing with an OP 191 Op Amp

michaelkellett — Fri, 22 Jan 2016 07:57:54 GMT

I'm glad it's working well - I've just finished the design of a board for a client with 9 OPA192s on it - the bare boards should be here on Tuesday so I'll post some pictures (and a some bits of circuit) once I've built and got power on them.