Current amplifier circuit? - Page 1
Forum section: electronics / projects

author: pacmann
author_rank: Contributor
timestamp: August 12, 2024, 07:22:31 am
content: A current transformer for my project uses a current output that I want to 2x amplify. The current is bidirectional between +100ma and -100ma. Can I use a current mirror?

author: moffy
author_rank: Super Contributor
timestamp: August 12, 2024, 08:22:32 am
content: You would need at least two, one for each direction, npn and pnp, and probably some level shifting transistors as well.

author: guenthert
author_rank: Frequent Contributor
timestamp: August 12, 2024, 08:36:09 am
content: A current mirror won't yieldexactlythe same current. Such a scheme will introduce gain errors, perhaps non-linearity and temperature dependency. Why not use a different CT with the output current you actually desire? Or a 2nd one with the desired 1:2 ratio?

author: pacmann
author_rank: Contributor
timestamp: August 12, 2024, 09:45:31 am
content: Quote from: guenthert on August 12, 2024, 08:36:09 amCheers. The ct is in an existing project thus cant be changed easily

author: moffy
author_rank: Super Contributor
timestamp: August 12, 2024, 10:08:47 am
content: Quote from: guenthert on August 12, 2024, 08:36:09 amYou can probably hold it to 1% of fullscale, but what sort of bandwidth and voltage range out are you looking at, also what output impedance is needed for the current.

author: magic
author_rank: Super Contributor
timestamp: August 12, 2024, 02:39:14 pm
content: Maybe something could be done with a handful of transistors.What power supplies are available and what will happen to the doubled current?

author: Terry Bites
author_rank: Super Contributor
timestamp: August 13, 2024, 04:35:19 pm
content: You will need to use the recomended load resistor (burden resistor) for the CT.Add an amplifier and use it to drive a bipolar current source.Vin is the voltage across the burden resistor. Set Rg accordingly. Most inst amps should work in circuit 1.You might get away with 3904 and 3906 type transistors at 100mA. But... If they get too hot use the Zetex parts.The second circuit is simpler (cheaper) but needs good resistor matching and or tweaking. Set the gain to suit.Use a low Vos dual opamp for best results.Refhttps://ez.analog.com/amplifiers/operational-amplifiers/f/q-a/559321/op177-improved-howland-current-source---how-to-increase-current-outputWatch out for ouput voltage complance limits at low supply voltages and with high impedance loads.

author: Terry Bites
author_rank: Super Contributor
timestamp: August 14, 2024, 08:51:52 am
content: This circuit is a current mirror propper:The tighter the resistor match and the better the opamp, the higher the accuracy.Again supply voltage limits output compliance. The bipolar transistors can be repaced with small signal (logic level) mosfets.There is a price to be paid for this in lost compliance. At higher supply voltages this is less of a problem.Add C1 if the mirror starts oscillating. Igain=Rbdn/Rset. I'd choose Rbdn to give 0.1-1V full scale at the input.Get a 100 10k's and sort them for the best match or spend a couple of bucks a piece on 0.01% resistors eg RNCF1206TKW10K0A low cost precision amp will improve performance, eg OPA4991

author: tszaboo
author_rank: Super Contributor
timestamp: August 14, 2024, 12:46:17 pm
content: Just to avoid the XY problem:Why would you like to have twice the current?

author: mawyatt
author_rank: Super Contributor
timestamp: August 14, 2024, 01:03:28 pm
content: Here's similar looking Bipolar Current Source to Terry's used for the Base Current in a Curve Tracer Fixture we developed awhile back, but never had time to build even tho PCBs are finishedBest,