Help with circuit from "Learning the Art of Electronics" - Page 1
Forum section: electronics / beginners

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 04:48:42 am
content: Hi- I'm getting ready to wire up a circuit from "Learning the Art of Electronics." The text leaves out some details of the actual wiring. I'm hoping that an experienced person could take a look at my circuit to make sure it's correct. It seems to be working correctly in the simulator, but I'm unsure about a few things.Attached are 2 pages from the text that explain the circuit, and a screenshot of my proposed circuit in the simulator. Thanks in advance!

author: floobydust
author_rank: Super Contributor
timestamp: July 07, 2017, 05:38:09 am
content: It looks good but in the EE lab tons of (CA3046) transistor array IC's blown upand students having a hard time and smoke too.The traps to look out for:Make sure your signal generator is not earth-grounded (is floating), because your oscilloscope and probably DC power supply are earth-grounded. These three instruments will short things out due to the common earth-ground path which makes this really good learning experience on something important like a diff amp, kind of a hellish, no fun.You will need to know how things are grounded, or measure with an ohm-meter to know your equipment.If you do not have a dual-tracking supply and say first turn on +15V, then -15V or using a couple 9V batteries clipped on one at a time for power it's not ideal - a diff amp really likes both power supply rails to come up at the same time.If you are using the array IC, they are fragile. It would be good to first test the two transistors beforehand with a multimeter.The array IC's have some hidden internal diodes that can cause things to not work as expected or make smoke. It's the "substrate diodes" I am mentioning.SeeCA3018, CA3046 app note.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: July 07, 2017, 05:45:58 am
content: The circuit appears to be correct, but why would you draw it like that?Note that it won't work quite right, if you use a traditionalground referencedsignal generator for the differential mode source ("1kHz").They're usually floating (or can be floated), but have capacitance to ground. It's better to avoid the common mode altogether by using two independent, ground referenced sources that have a phase relationship. You can use a signal generator that has two outputs, one which can be set to an inversion or 180 degree phase shift of the first ... but I'm not sure offhand how many have this feature. Not many I think, as most are single output.Tim

author: KE5FX
author_rank: Super Contributor
timestamp: July 07, 2017, 07:18:21 am
content: Why the heck would they call for a monolithic diff-pair IC for a tutorial circuit? It'll work fine with any two discrete transistors you have lying around. For added 'education' you can observe what happens as you heat one transistor relative to the other.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 03:31:24 pm
content: Quote from: floobydust on July 07, 2017, 05:38:09 amThanks so much. One of those pitfalls would probably have messed me up.I had planned to use a transformer to get around the floating issue, as the book describes later.Regarding turning on the power rails simultaneously, I have a Dr. Meter HY3005F-3. Presumably I can just push the main on button. Do you think this will be okay?Thanks again.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 03:35:20 pm
content: Quote from: T3sl4co1l on July 07, 2017, 05:45:58 amThanks for the suggestion. I actually have a 2-channel function generator with independent phase control (Rigol DG1022A). Would the attached circuit get around all the floating issues? Would there be a problem attaching the scope across the 10K resistor, from a grounding perspective?

author: alm
author_rank: Super Contributor
timestamp: July 07, 2017, 03:53:43 pm
content: I seem to recall the AoE emphasizing that common circuits like differential pairs should always be drawn a certain way to make the easy to recognize. The way you are drawing it is definitely not that way. Took me a couple of seconds to figure out what the hell your first circuit was doing. Negative rails should be at the bottom, not in the middle. Power supply would usually be on the left. Ideally you use symbols for ground and power rails instead of drawing all the wires. Not sure if your simulator supports the latter.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 04:02:30 pm
content: Quote from: alm on July 07, 2017, 03:53:43 pmApologies for the drawing. I wired it "longhand" so I could actually watch the current flow. I should've cleaned it up before posting it on the forum.

author: floobydust
author_rank: Super Contributor
timestamp: July 07, 2017, 07:11:29 pm
content: Your linear power supply HY3005F-3 you can float if the green binding post (earth-ground) is unconnected.The Rigol ARB, I believe has a switching power supply- so floating it probably unsafe if you don't use an isolation transformer to power it. Is this the transformer you mean?I am talking about Y-capacitor leakage currents in the Rigol's internal power supply causing stray voltage.Another possibility is to use a small audio signal transformer to isolate the output of the ARB.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 08:11:42 pm
content: Quote from: floobydust on July 07, 2017, 07:11:29 pmThanks flooby. Attached is the transformer setup the book recommends. It uses a 6.3 volt transformer we have from another project.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 08:47:03 pm
content: Attached is the completed circuit with the transformer in place. Hopefully this drawing is better!

author: alm
author_rank: Super Contributor
timestamp: July 07, 2017, 09:04:29 pm
content: That is indeed a much better schematic! If you use a transformer to isolate the function generator like that, and you make sure your power supply is not grounded, then it should work. Connect two of the power supply outputs in series (positive of channel 1 to negative of channel 2, or the other way around), but not to the green earth terminal. Then connect the ground of your circuit to one of those middle terminals.If the second function generator channel is indeed connected to ground, then you would still have to be careful where you clip your scope probe. To measure across the 10k resistor you could set the scope to CH1 - CH2, then disconnect the ground leads from both probes, and connect channel 1 to the top of the resistor (in the schematic) and channel 2 to the bottom.

author: John0922
author_rank: Contributor
timestamp: July 07, 2017, 09:29:11 pm
content: Quote from: alm on July 07, 2017, 09:04:29 pmInteresting, I didn't know the scope could be set up that way. Thanks!