Hello,
I want to build an Aleph X with 8 MOSFETs and I think it is not needed to have
two pairs of 4 matched MOSFETs.
My idea :
You have to match only the types in the same position on both halfs of the
amplifier. With this type of matching you get two identical output stages
and that is enough.
What kind of errors this idea have ?
Regards
Ralph
I want to build an Aleph X with 8 MOSFETs and I think it is not needed to have
two pairs of 4 matched MOSFETs.
My idea :
You have to match only the types in the same position on both halfs of the
amplifier. With this type of matching you get two identical output stages
and that is enough.
What kind of errors this idea have ?
Regards
Ralph
8 FETs per amp means paralleling 2 MOSFETS at each of the four corners of the circuit.
It is correct that the devices in parallel do not have to be matched perfectly, but if they are too different in Vgs, you get unequal current sharing, so that one gets more dissipation than the other.
And if I am not wrong, all 8 matched will give you minimum distortion and thermal stability. Just assume a polynomial as transfer function (Id vs Vgs) and do some simple algebra.
Patrick
It is correct that the devices in parallel do not have to be matched perfectly, but if they are too different in Vgs, you get unequal current sharing, so that one gets more dissipation than the other.
And if I am not wrong, all 8 matched will give you minimum distortion and thermal stability. Just assume a polynomial as transfer function (Id vs Vgs) and do some simple algebra.
Patrick
Hi rsbay,
You need to match the transistors that are connected in parallel as closely as you can. As EUVL says, the match does not need to be perfect. There is a great deal of information on this site and on Passdiy about the meaning of "as closely as you can" as used here.
It helps to reduce offset if the gain transistors on either side of the circuit are matched to each other. Matching the CCS transistors to the gain transistors is not important IMO.
Cheers,
Graeme
You need to match the transistors that are connected in parallel as closely as you can. As EUVL says, the match does not need to be perfect. There is a great deal of information on this site and on Passdiy about the meaning of "as closely as you can" as used here.
It helps to reduce offset if the gain transistors on either side of the circuit are matched to each other. Matching the CCS transistors to the gain transistors is not important IMO.
Cheers,
Graeme
Hello,
I fear you didn't understand what I want :
I want to match at least only a pair at a time and put both MOSFET's at the
same position in the both halfs of the amp.
Lets say I'm able to match a pair perfectly, than at least both sides are
equal and I have no differential offset.
Regards
Ralph
I fear you didn't understand what I want :
I want to match at least only a pair at a time and put both MOSFET's at the
same position in the both halfs of the amp.
Lets say I'm able to match a pair perfectly, than at least both sides are
equal and I have no differential offset.
Regards
Ralph
What you want maybe isnt exactly what the amp want's
I am sure the 2 gentlemen, answering your question understood the question Q2 and Q11 must match even if using pairs (or triples, whatever)
If you plan to use pairs you need 4 matching fets for those positions (gainfets).
The CCS fets doesnt necessarily need matching across Q1 and Q10, although it wouldnt hurt. The pairs for those positions would still need matching.
Steen
- Status
- This old topic is closed. If you want to reopen this topic, contact a moderator using the "Report Post" button.