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rumpofsteelskin
Joined: Apr 22, 2009
Posts: 46
Location: brighton, uk
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 Posted: Wed Jul 11, 2012 10:23 am Post subject:
lin/exp transistors... and OTA comparison |
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What are the attributes to look out for when choosing transistors for the exp converter in a VCA? How do they affect the performance? Is there a list of commonly used transistors anywhere? I'm looking for a matched monolithic PNP pair but don't know which one to choose...
Also, whilst i'm on the subject... are there any online resources for comparison of OTAs in synths? I would be interested to see a performance comparison so I know exactly what people mean when they say CA3280 are great, LM13700 not so great. I remember seeing an Aaron Lanterman article on comparison of roll your own OTAs but I can't find it anymore... |
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JingleJoe
Joined: Nov 10, 2011
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| Posted: Wed Jul 11, 2012 11:38 am Post subject:
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lm13700's are current controlled and work with currents rather than voltages and I think the CAxxxx OTAs use voltage which makes things a bit easier.
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JovianPyx
Joined: Nov 20, 2007
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| Posted: Thu Jul 12, 2012 8:31 am Post subject:
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| JingleJoe wrote: | | lm13700's are current controlled and work with currents rather than voltages and I think the CAxxxx OTAs use voltage which makes things a bit easier. |
No. CAxxxx OTAs are current controlled by using the Iabc pin.
Iabc stands for Amplifier Bias Current.
This is all in the datasheets which are easily obtained on the web using Google.
The best way to determine the differences between OTAs (or any part category) is to get the datasheets for the various part numbers and compare paying close attention to the parameters that matter the most to the application being considered.
As for expo converters and the transistors commonly used, many folks have posted schematics on their own websites for these things which will show the part numbers they used. One monolithic I've seen used is the MAT-02. Some of those can be expensive, but it saves time in matching. There are also postings regarding matching of transistors that show schematics for the test bed and instructions on how to use them.
Additionally, there are also good articles on the web regarding how an expo converter works which will point to the parameters that need matching and will help understand how the thing works both for your own enjoyment and for the times when you need to troubleshoot such a circuit.
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Blue Hell
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JingleJoe
Joined: Nov 10, 2011
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Location: Lancashire, England
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| Posted: Thu Jul 12, 2012 1:30 pm Post subject:
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| JovianPyx wrote: | | JingleJoe wrote: | | lm13700's are current controlled and work with currents rather than voltages and I think the CAxxxx OTAs use voltage which makes things a bit easier. |
No. CAxxxx OTAs are current controlled by using the Iabc pin.
Iabc stands for Amplifier Bias Current.
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Thanks for clearing that up  However is it not the case that the LM13700 takes current inputs and outputs a current value rather than voltage*? and the CAxxxx uses a control current like the LM13700, but accepts voltages as inputs, and outputs a voltage too?
*yes, yes, I know, can't have one without the other; ohms law V=IR, the lm13700 to my memory needed a load resistor on the output for the voltage to appear accross as the changing current flowed through it, this would then be buffered and taken as a voltage output. I think.
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JovianPyx
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| Posted: Thu Jul 12, 2012 2:18 pm Post subject:
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All OTAs are essentially the same.
See http://en.wikipedia.org/wiki/Operational_transconductance_amplifier
Note that the inputs are labeled Vin and the output is labeled Iout. This means that it is a voltage input amplifier with current controlled gain and a current type output.
It doesn't matter who makes them or what the part number is, they are all based on the same principle. The main differences between different types are things like input impedance, current delivery capacity, gain range, noise, etc., but the voltage input and current output are part of every OTA design.
Perhaps you are thinking of the LM3900 (Norton Amplifier) which is not an OTA, does not have controllable gain (except through feedback) and is also not an Opamp, but it does have a current input. It was designed for applications in noisey industrial environments, but is something unrelated.
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rumpofsteelskin
Joined: Apr 22, 2009
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Location: brighton, uk
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| Posted: Fri Jul 13, 2012 3:38 am Post subject:
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| JovianPyx wrote: |
The best way to determine the differences between OTAs (or any part category) is to get the datasheets for the various part numbers and compare paying close attention to the parameters that matter the most to the application being considered.
As for expo converters and the transistors commonly used, many folks have posted schematics on their own websites for these things which will show the part numbers they used. One monolithic I've seen used is the MAT-02. Some of those can be expensive, but it saves time in matching. There are also postings regarding matching of transistors that show schematics for the test bed and instructions on how to use them.
Additionally, there are also good articles on the web regarding how an expo converter works which will point to the parameters that need matching and will help understand how the thing works both for your own enjoyment and for the times when you need to troubleshoot such a circuit. |
hi, thanks for your reply. I'm familiar with the articles on expo converters. Whilst looking through some of these transistors I was struck by how cheap you can get matched pairs for these days (especially in surface mount packages) I was wondering why one might go for say an SSM2220 rather than a cheaper alternative, but looking closer I see that some of them are Vbe matched, some of them are hfe matched, the SSM220 is matched to 0.5% while some of them are matched to up to as much as 10%! I suppose that explains the price difference...
on OTAs however sometimes I find it a little difficult to see what people mean. For example, if I wan't to build an OTA based VCO, the main characteristic I'll be interested in is linearity between bias current and output current over a large range (right?). The datasheet for both shows slight variation with temperature for large values of bias current, but the CA3280 also seems to show slight non-linearity for small bias current with temperature - so based on this the LM13700 seems more suitable.
I can't see which values really put the 3280 ahead - it has higher input resistance - but that's not really that important - it has higher slew rate - but both have easily enough to cope with audio frequencies. What is is that I'm missing? |
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JingleJoe
Joined: Nov 10, 2011
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| Posted: Fri Jul 13, 2012 3:56 am Post subject:
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| rumpofsteelskin wrote: | | What is it that I'm missing? |
I've got to say I find myself asking the same question a lot when reading electronic component recomendations or comparisons of one component with another. Which is perhaps why I had the misconception about a significant difference in operation between the LM13700 and CAwhatever 
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JovianPyx
Joined: Nov 20, 2007
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| Posted: Fri Jul 13, 2012 6:45 am Post subject:
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I'll leave the specifics of expo converters to others as I am not an expert.
As for OTAs in VCOs, there are circuits being used today that employ both the LM and CA types. In many cases it appears to be a matter of personal preference or sometimes what is on hand since both make musically useful VCOs. Higher slew rate would suggest possible higher frequency use, but I've never heard complaints about audio high frequency response in any OTA. I've built VCOs using the LM13600 (linear) and they track over some 8 or 9 octaves.
One interesting note about the difference between LM13600 and LM13700 is that the LM13600 Iabc affects the darlington pair output driver. This has been said to cause a "thump" when the IC is used in a VCA, but it is slight enough that I don't hear it in the PAiA Fatman which uses such a VCA. This bias arrangement is not present in the LM13700. So if a VCA is being built and it is desired to use the darlington, the LM13700 is a better choice. The darlington pair is not required, an external driver can be used instead if the builder wants to use an LM13600 (like if you have a pile of them already). Or maybe you like the thump. As I mentioned, personal preference is part of this deal since there is no such thing as a perfect OTA.
The websites that expert designers and builders have often detail the reasons for the selection of parts they chose. It is wise to read read read and then read some more and then select the design that best meets the features and qualities you seek.
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