TomWaterman
Well-known member
Hi peeps,
I've been looking at constant current sources since the little (but very helpful) discussion amongst the 'in another forum' thread.
I'm looking to CCS the outputs of a OPA627 for a headphone amp, and an OPA604 for the mono-summing circuit NYD kindly provided.
I've been reading H+H, the web and a paper from Siliconix AN103.
http://www.vishay.com/document/70596/70596.pdf
I have a few Q's - wondering if Brad, Paul, Feathery, whoever could shed some light.
Basically, its clear to me now that there are issues with the differing types of CCS.
I've narrowed it down to the JFET Cascode or a BJT one with the diodes as suggested in the other thread.
H+H seems to suggest that the BJT CCS will provide the most stable performance but I like the somewhat more simple JFET Cascode (fewer parts).
What would be the real advantage to the BJT type CCS as opposed to the JFET?
I was trying to calculate a source resistor for a 2N5457 NPN Jfet which is commonly available at Farnell and suitable for audio apps.
After going thru H+H, AN103 and 5457 datasheet I'm kind of at a loss.
Basically a couple of questions come up and I've realised I'm a retard!
Here's one circuits' scenario where I'd like to add a CCS.
Between the first stage OPA627 and the 2nd stage OPA627 in a phones amp. There's a 10k attenuator in between the place for the CCS and the 2nd stage.
The input Z of the next section is set to 100k at the mo.
So I calculated the input Z of the next stage + the attenuator, 1/(1/10k+1/100k)=9091ohms.
Lets say going by Pauls example of no lower than 0.5-0.75mA based upon max output (to be well covered on +/-18V rails) 9V pk....
I=9/9091
I=1mA+another 1mA to be safe so 2mA.
Something was mentioned about including the feedback network (of the 1st stage right?). My feedback resistor was going to be 820R....how do I calculate that as well? If I add it into the parallel load it brings it way down and doesn't seem right??
In the AN103 document it claims that the source resistor for the JFET is calculated by Vds/Id.
I'm lost as to where to get Vds from. So I'm kinda stuck knowing how to get the CCS to have an Id(?) of 2mA.
There is another formula which is:
-VGsoff/Id(1-root(Id/Idss))
which gave me an Rs of 275ohms but I had to guess a value for VGsoff as the datasheet says it varies from -0.5 to 6.0V. I chose -3V.
Ooooh another Q? - is the CCS actually more of a current sink than a source?
Thanks for any help explaining to a moron.
Cheers Tom
I've been looking at constant current sources since the little (but very helpful) discussion amongst the 'in another forum' thread.
I'm looking to CCS the outputs of a OPA627 for a headphone amp, and an OPA604 for the mono-summing circuit NYD kindly provided.
I've been reading H+H, the web and a paper from Siliconix AN103.
http://www.vishay.com/document/70596/70596.pdf
I have a few Q's - wondering if Brad, Paul, Feathery, whoever could shed some light.
Basically, its clear to me now that there are issues with the differing types of CCS.
I've narrowed it down to the JFET Cascode or a BJT one with the diodes as suggested in the other thread.
H+H seems to suggest that the BJT CCS will provide the most stable performance but I like the somewhat more simple JFET Cascode (fewer parts).
What would be the real advantage to the BJT type CCS as opposed to the JFET?
I was trying to calculate a source resistor for a 2N5457 NPN Jfet which is commonly available at Farnell and suitable for audio apps.
After going thru H+H, AN103 and 5457 datasheet I'm kind of at a loss.
Basically a couple of questions come up and I've realised I'm a retard!
Here's one circuits' scenario where I'd like to add a CCS.
Between the first stage OPA627 and the 2nd stage OPA627 in a phones amp. There's a 10k attenuator in between the place for the CCS and the 2nd stage.
The input Z of the next section is set to 100k at the mo.
So I calculated the input Z of the next stage + the attenuator, 1/(1/10k+1/100k)=9091ohms.
Lets say going by Pauls example of no lower than 0.5-0.75mA based upon max output (to be well covered on +/-18V rails) 9V pk....
I=9/9091
I=1mA+another 1mA to be safe so 2mA.
Something was mentioned about including the feedback network (of the 1st stage right?). My feedback resistor was going to be 820R....how do I calculate that as well? If I add it into the parallel load it brings it way down and doesn't seem right??
In the AN103 document it claims that the source resistor for the JFET is calculated by Vds/Id.
I'm lost as to where to get Vds from. So I'm kinda stuck knowing how to get the CCS to have an Id(?) of 2mA.
There is another formula which is:
-VGsoff/Id(1-root(Id/Idss))
which gave me an Rs of 275ohms but I had to guess a value for VGsoff as the datasheet says it varies from -0.5 to 6.0V. I chose -3V.
Ooooh another Q? - is the CCS actually more of a current sink than a source?
Thanks for any help explaining to a moron.
Cheers Tom