How to make I/U conversion with 2SK170
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 10th April 2007, 02:23 PM #1 AudioAngel   diyAudio Member   Join Date: Dec 2003 Location: Small City in North Germany How to make I/U conversion with 2SK170 Hello, I'm not shure where to place this question so I placed it here hoping for help. I want to make a I/U-converter with a 2SK170 JFET. I've seen, there are some schematics but what I want to now is the math behind to setup the circuit for another DAC. Can someone explain how to do that? I think it is wise to give a concrete application so let's say my DAC delivers 7.8mA p-p and I want to use +30V for the positive and -30V for the negative voltage rail. So, how to go on? Maybe this question is also interesting for other newbies... Raphael
 10th April 2007, 05:47 PM #2 MaxS   diyAudio Member   Join Date: Mar 2006 Location: France Guten Tag Dude, In some cases, we use an 1 Ω resistor so the voltage is the true picture of current. This resistor was called Rshunt and i remember to have read on the board what their value are given by the datasheet. Cheers, Max. __________________ Ca marche pas ! Tu ne l' as pas branché non plus ....
 10th April 2007, 08:52 PM #3 AudioAngel   diyAudio Member   Join Date: Dec 2003 Location: Small City in North Germany I'm not sure wether this answer is really helpful because it does not explain the circuits based on JFETs to do the I/U but that is what I want to know. Raphael
 10th April 2007, 11:06 PM #4 MaxS   diyAudio Member   Join Date: Mar 2006 Location: France Hi, Well is late, so i 'll made simple. They are two ways to command a transistor, by current, BJT. Or by voltage,FET and their sons. __________________ Ca marche pas ! Tu ne l' as pas branché non plus ....
 10th April 2007, 11:19 PM #5 Nelson Pass   The one and only     Join Date: Mar 2001 There are three ways to use any 3 pin gain device. For I/V conversion, Common Gate is my preferred approach. Take a look at the I/V of the Pass Labs D1 - Common Gate, no feedback, works like glue.
 11th April 2007, 08:05 AM #6 AudioAngel   diyAudio Member   Join Date: Dec 2003 Location: Small City in North Germany Hello Nelson and Max, I know this ways in theory, so, that's not what I want to hear, sorry. What I want to know is how to get the right resistor values to use the SK170 for an I/V-conversion. You see my problem? It is more like coming from theory to practice. Raphael
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Re: How to make I/U conversion with 2SK170

Quote:
 Originally posted by AudioAngel Hello, I'm not shure where to place this question so I placed it here hoping for help. I want to make a I/U-converter with a 2SK170 JFET. I've seen, there are some schematics but what I want to now is the math behind to setup the circuit for another DAC. Can someone explain how to do that? I think it is wise to give a concrete application so let's say my DAC delivers 7.8mA p-p and I want to use +30V for the positive and -30V for the negative voltage rail. So, how to go on? Maybe this question is also interesting for other newbies... Raphael
For a start you need to know a little more about the dac. You need to know the idle current at bipolar zero. You will also need some idea of the Idss of the FET in question.

 18th April 2007, 04:08 PM #8 AudioAngel   diyAudio Member   Join Date: Dec 2003 Location: Small City in North Germany Ok, which DAC? I decided to go for the PCM1794 at the end. But for testing an d building a prototype I want to use the PCM1798, which has only 4mA p-p. For the PCM1798 the datasheet states out a center current of -3.5mA. Concerning the Idss: I don't have an own idea. What I know from reading on the web, I think 4.7mA would be a good choice wouldn't it? Can we go on with these values?
 18th April 2007, 06:47 PM #9 B Cullingford   diyAudio Member   Join Date: Mar 2005 Location: California Idss is not a choice but a property of the FET used - FETs have rather variable specs and so either the circuit has to be designed to allow for this or you have to select. A 2SK170 comes in several versions with a wide range of Idss. So, to contridict myself, you have to choose: an Idss needed based on circuit design and then pick a suitable FET or FETs, or choose a FET and design a circuit to use the Idss of the FET. I would be thinking common gate, and considering the input signal and the input impedance I wanted as starting points.

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