Uncommon low noise current source circuit base on serie / parallel circuit  - Page 1
Forum section: electronics / general

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 25, 2024, 05:29:14 pm
content: HelloAn interesting circuit built for low noise JFET current source with adjustment by series / parallelRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: November 25, 2024, 10:53:06 pm
content: The JFET constant current circuit was already discussed in a different thread about a JFET based voltage reference.https://www.eevblog.com/forum/metrology/building-your-own-voltage-reference-the-jvr/msg769983/#msg769983It works resonably stable if trimmed well. For good precision one still needs to keep the DS voltage well regulated, especially with FETs that have a higher threshold. A downside it that the FETs set the current to a rather fixed current (e.g. some 1.1 mA for 2N4391 as a resonable choice as a voltage ref.) and one can only trim the current a little via the DC voltage. With a much different current there is thermal drift.

author: TimFox
author_rank: Super Contributor
timestamp: November 25, 2024, 11:10:06 pm
content: I remember an old Tektronix app note for a curve tracer using a storage CRT display.With a JFET in the test socket, it was sprayed with a cooling spray and the operator noted which curve of constantVgsdid not shift with temperature.

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 26, 2024, 07:56:15 am
content: Quote from: Kleinstein on November 25, 2024, 10:53:06 pmHelloThanks , regarding the '' needs to keep the DS voltage well regulated, '' on the italian design they use a stage between the battery and the JFET ( a pair ).I am not expert in electronic design , I try to learn in the same time I doRegardsOS

author: Kleinstein
author_rank: Super Contributor
timestamp: November 26, 2024, 08:21:28 am
content: The 2 italian circuits are using quite different ideas:The first version uses the JFETs as constant current referenes. At a part specific current and with fixed DS voltage the current is reasonable temperature stable (the linear TC crosses zero , but there is a 2nd oder TC), especially in combination with a somewhat stable temperature.Quote from: TimFox on November 25, 2024, 11:10:06 pmOne gets low TC forVgsat about 0.8 V, or with a current to be about 0.8 V from the threshold. So it depends on the current / threshold if one gets a temperature effect.The 2nd italian circuit is more conventional: the battery as voltage reference and than FETs as souce followers. The FETs choosen are rather large with a treshold where the TC is small and thus work as low noise amplifiers. The OP-amp part aims for a fixed DS voltatage to suppress the drain voltage effect and also to keep the power at the fets constant and thus allow a stable temperature. With the battery and likely not that great TC this only a short time stable current source, not at all long time stable.

author: Overspeed
author_rank: Frequent Contributor
timestamp: November 26, 2024, 02:12:21 pm
content: Quote from: Kleinstein on November 26, 2024, 08:21:28 amHelloThanksI do not need long time stable solution as that to make some experiments asap and allow me to find a '' rock solid '' 10 mA / 20 mA precision current source .RegardsOS