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Old 10th April 2007, 01:23 PM   #1
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Default 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
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Old 10th April 2007, 04:47 PM   #2
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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.
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Old 10th April 2007, 07:52 PM   #3
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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
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Old 10th April 2007, 10:06 PM   #4
MaxS is offline MaxS  France
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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.
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Old 10th April 2007, 10:19 PM   #5
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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.

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Old 11th April 2007, 07:05 AM   #6
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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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Old 11th April 2007, 07:42 AM   #7
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Default 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.
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Old 18th April 2007, 03:08 PM   #8
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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?
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Old 18th April 2007, 05:47 PM   #9
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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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