Yet another "Please evaluate my circuit design" post - Page 1
Forum section: electronics / projects

author: Maximus
author_rank: Contributor
timestamp: May 12, 2015, 09:26:15 am
content: So this circuit is my first big step into big boy analog electronics design. I have had to learn a lot of what I know on my own so I am in the position of not knowing what I don't know. Which is where you come in.I would be eternally grateful if you could give me a helpful critique of this circuit I designed and the schematic's layout. Hopefully you guys can catch any circuit breaking oversights before I spend money on a PCB.It is an exponential converter aka exponential amplifier which implements the function Vout = 2^(-Vin+5V). Im not sure what information you need in order to evaluate it so instead of posting a few pages of calculations just ask for anything you need. I have attached the schematic and a graph of how it is expected to work in a perfect world.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: May 12, 2015, 10:56:05 am
content: Seems reasonable, at a glance. Readable schematic, nothing glaringly wrong (the "0>Vin>-10" looks weird, but it's self-consistent anyway).What's the data for a temperature sweep?V+ needs to be tightly regulated, of course, in particular the points going to the bias resistors.Can you actually buy CA3146? Or do you have some stashed away?There are still a few monolithic transistor pairs floating around, these days. Beware that there are "matched transistor pairs", very cheap -- but they consist oftwo dies in a single package, cut from the same wafer. Electrically they're close, but thermally, nuh uh. The thermal resistance between two dies in a SOT-23-6 package is similar to the thermal resistance to ambient; meaning, they're only slightly better than two loose transistors on the PCB. On the upside, you don't need much power (the C-E voltages are equal, and the currents are small), but any small difference in temperature will still manifest as shifting parameters, especially insidious being the time dependency (ms to s time constants of the die to package and package to PCB, neither of which will quite track to the thermistor, either).Don't suppose you have a thermal model in that there simulation? Could be an interesting exercise, assuming the simulation lets you plug in a live temperature for the transistors.What application did you have in mind? Analog computing? Synth? Just for kicks? Any limits for accuracy, or just "as good as it does"?Tim

author: Maximus
author_rank: Contributor
timestamp: May 12, 2015, 06:50:24 pm
content: It is for a synth, but I am not sure how accurate it will need to be, right now im hoping for >1% but we will see what happens. I could redesign the bias circuit with a voltage reference instead of the positive rail, but i will also check how much effect noise on the supply has on the output and see if it will be necessary.The model does have some sort of thermal model but I dont have a good understanding of how it works so I am reluctant to put much value on its results.The temperature compensation resistor is tiny so I hope that it will react quickly. I plan on epoxying it to the bottom of the IC which will be socketed so there will be clearance for the resistor.I do have a bunch of matched transistor ICs including the CA3146, some new some salvaged from old scopes.Thanks for your opinion! Im glad to hear that it at least looks like it should work. I will order the parts by the end of the week so I can make a prototype.

author: eetech00
author_rank: Regular Contributor
timestamp: May 13, 2015, 01:58:56 am
content: Quote from: Maximus on May 12, 2015, 09:26:15 amHiI'm not an audio circuit expert, but I see two things to consider.1. Bypass caps (0.1uf) at each IC +/- power supply pin.2. DC blocking caps between different audio opamp stages to Isolate AC from DC where necessary.Sadly, I can't tell you where these need to go, but I think at least at the input and output of the circuit.

author: PeterFW
author_rank: Frequent Contributor
timestamp: May 13, 2015, 02:14:43 am
content: Quote from: eetech00 on May 13, 2015, 01:58:56 amIf my crystall ball is still within spec calibrated this should be a CV generator, a 0-10V control voltage for analog synths.In this case, you do not need coupling caps since you want a DC output.Might be wrong tough

author: Maximus
author_rank: Contributor
timestamp: May 13, 2015, 02:52:19 am
content: It is a DC circuit and, as PeterFW guessed it is for a synth's VCO.Good catch on the bypass caps.Also I just noticed that I forgot to update a few points that are labeled 'V+'. They should be labeled '15V'.