You are right about cascoding two J-fet, that's is the one way to get high output impedance, but one might try BJT+ J-fet cascode (such as in Steven circuit http://www.diyaudio.com/forums/attachment.php?s=&postid=330449&stamp=1077312778 or somewhat different configuration from recent Walt Jung's article about CCS). About your suggestion for BF244/245 cascoded J-fet CCS - this two devices look identical (if I remember datasheet correctly), so if that is the case, wouldn't be better to use e.g. BF244A and on top of it an BF244B/C for cascode devices?
aparatusonitus,
JFETs have excellent properties for CCS, you won`t find anything better.
BF244 and BF245 are different. In absence of various types, for a wider range of current, please go for BF244B or C as the cascoding (top) device, any other type would work as the cascoded (bottom) one.
JFETs have excellent properties for CCS, you won`t find anything better.
BF244 and BF245 are different. In absence of various types, for a wider range of current, please go for BF244B or C as the cascoding (top) device, any other type would work as the cascoded (bottom) one.
roender,
in general: they are very sensitive, consequently, easily develop thermal distortions, therefore and for every reason should always be cascoded in common-source configuration, anyway.
As CCS: no difference, however, the advantages are massive, high dynamic impedance, wide bandwidth, fairly low noise, elegant clean biasing.
in general: they are very sensitive, consequently, easily develop thermal distortions, therefore and for every reason should always be cascoded in common-source configuration, anyway.
As CCS: no difference, however, the advantages are massive, high dynamic impedance, wide bandwidth, fairly low noise, elegant clean biasing.
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Lumba,
After many practical experiments, I've come to conclusion that the best CCS, from noise, impedance, thermal stability and precision POV (all of this into a balanced blend), is the combination between a LED and a low noise BJT, cascoded with another BJT with low Cob.
The trick here is to properly use the cascoded transistor, to not turn the whole thing into a oscillator
Cheers,
Mihai
After many practical experiments, I've come to conclusion that the best CCS, from noise, impedance, thermal stability and precision POV (all of this into a balanced blend), is the combination between a LED and a low noise BJT, cascoded with another BJT with low Cob.
The trick here is to properly use the cascoded transistor, to not turn the whole thing into a oscillator
Cheers,
Mihai
Lumba Ogir said:
A cascoded CCS is a very high impedance circuit.
What do you know about my biasing methods? Do you know how much noise a LED can produce?
Blow some air through a j-fet CCS and watch at the output current
.For circuits where VAS current give the bias for the output stage, this type of CCS is a total desaster, not to mention about OLG variation with temperature ...
Don't get me wrong, I'm a big fan of j-fets, but not as a CCSs ... with only one exception, when is used as a current source for LED biasing
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