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  • Author Author: jw0752
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Playing with an OP 191 Op Amp

jw0752

One of the really great things about the E-14 Web Site is how much one can learn just by reading and paying attention to some of the threads. On Jan 9th posted information on the OP191 and OP192 Op Amps which he likes to use in his designs. Being curious to play with them for myself I marked down the number and ordered some of the OP 191 for my shop. The OP191s that I received were in the SOIC8 package so it was necessary to mount it on an adapter board so that I could conduct some simple experiments on the bread boards. Here is the OP191 after I mounted it on the adapter and turned it into a DIP8.


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I next built a simple circuit with a gain of 10 and used my wave form generator and my oscilloscope to look at the limits of the chip and how well it performed. Here is the simple circuit.


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I used a +/- 10 volt supply for the chip and I ran the input level down until I could no longer get good readings on my scope. I was very impressed with how well the OP191 worked. Actually it doesn't take that much to impress me or to cover my needs. I could have taken 's word for it but what would have been the fun in that? The Op Amp did a very good job reproducing sine, square, and ramp inputs.


Here are the readouts of the test equipment during one of the experiments on the Op Amp.


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Thanks again to MK for the excellent recommendation and all the fun I had tonight playing with the OP191. It will be fun to use it in my next build that requires this level of Rail to Rail Op Amp. If you want to read MK's original post it is in this thread


Power Supply for Home lab?

by


John.

  • in reply to jc2048

    Hi Jon,

    I will try to do some bread boarding tonight. Thank you for continuing this exploration. This is what I find to be really fun and valuable, just following ones curiosity step by step.

    John 

  • 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.

  • 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.

  • in reply to jc2048

    Hi Jon,

    Thank you very much for your two comments. This may not be your specialty but you are still miles ahead of me. I did notice that the sine waves changed to ramps and your reply taught me that this is caused by the slew rate being too slow to keep up with the frequency. I agree that these little tests are not fair to the manufacturer but they are simply an attempt by me to better familiarize myself. When I use this Op Amp it is likely to be in a power supply or some other low frequency application. No that I know what the output should be I will do a little more experimentation when I get the chance to see if I can locate the cause of the quicker drop off in my measurements.

    John

  • in reply to jw0752

    Your roll-off is much earlier than the datasheet would suggest (Figure 15). It's a bit difficult to read, but at a closed-loop gain = -10 their graph shows the actual gain falling by half (-6dB down) at something like 400kHz.

     

    Here's what the simulation output looks like with linear gain and frequency scales. This is with 500mV pk/pk in. This shows the gain being half at 500kHz.

     

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    Wonder why your measurements are turning out different?