Fast 1 MHz, 1 Amp pulse current driver - Page 1
Forum section: electronics / projects

author: D2236
author_rank: Regular Contributor
timestamp: February 23, 2025, 02:41:57 am
content: We needed a fast current pulse generator up to 1 A peak to test some current probes operating from DC to about 1 MHz. After trying various options with existing equipment at hand, which couldn't provide clean and fast enough pulses, I decided to design something simple we could use for testing various probes. The aim was to get rise-times faster than the requirement to test the 1 MHz probes.The circuit shown below reaches switching times of 20 ns or faster depending on the rising or falling slope, giving it a -3 dB BW just shy of 20 MHz. The only special component will be the fast BFG235 or similar transistors needed. Most of these are no longer in production but can still be found if you search. I got lucky, and the ones obtained from eBay seemed to be genuine at least. If anyone knows of other suitable candidates, please drop it in a reply.The input drive is from a standard 50 ohm FG (SDG 2042X) running at 100 kHz with a narrow pulse width set around 100 ns. This is to keep the dissipation in the BFG235 within limits, as it is not mounted to any adequate heat-sink. As a precaution the current limit on the source PSU is set to about 235 mA and worked well with all the testing.The nice thing about the circuit is that it easy to adjust the output current by simply varying the supply voltage. It performs well down to about 150 mA, without significant degradation in switching times.Simulation results seem to indicate that faster switching times should be possible with using a 2SC5551 in place of the BFG235. I am trying to locate some (not fake) devices to test.Update: The correct current probe position is shown corrected in the last circuit diagram.

author: SiliconWizard
author_rank: Super Contributor
timestamp: February 23, 2025, 03:10:33 am
content: Nice all discrete circuit. And the Lecroy scope is nice too!

author: D2236
author_rank: Regular Contributor
timestamp: February 23, 2025, 03:18:37 am
content: Love working with the old 960. We have a few of them and some R&S RTO's in the lab as well, but I like using the 960 where possible.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2025, 03:49:49 am
content: Did you have a question?For better performance I would use a differential pair to switch the current source between two different loads, but of course now the current source operates continuously although it could be active shortly before and after being switched to the test load. This could be all NPN bipolar or all n-channel MOSFET or a combination of both. Switching time is now limited by transconductance, slew rate, and capacitance, but will be much better than trying to control the current source itself.Tektronix used this configuration for some really fast calibrators, but never into amps that I am aware of. There are lots of relatively inexpensive NPN and N-channel RF power transistors to make this work.

author: D2236
author_rank: Regular Contributor
timestamp: February 23, 2025, 04:33:21 am
content: Thanks for your reply and suggestions. I think the FG is also limiting the achievable speed at the moment. The rise-fall times is limited to 8.5 ns minimum for the SDG 4024X.

author: garrettm
author_rank: Frequent Contributor
timestamp: February 23, 2025, 05:30:53 am
content: A charge line pulse generator would be the simplest way to do this. I_pulse = 0.5*V_charge / 50 = 0.01*V_charge ==> 100 V charge voltage for 1 A pulse. The pulse duration is twice the electrical length of the cable. A mercury wetted reed switch can achieve sub nanosecond rise times.Even very crude construction can provide good results. But the best design would use edge mount coaxial connectors with a coaxial load and notches in the PCB for the current probe and the reed switch. A metal tube should go over the reed switch to keep the path coaxial--the tube would be soldered only at the ends with a notch in the PCB around the tube to allow for the trigger coil the fit around the tube. A U shaped notch in the PCB along the edge with a large gauge wire across the U area allows the current probe to sense the current into the load resistor. Aside from the long charge line cable, the whole setup can be made quite compact.I've attached a few photos of a very crude unit I made some time ago.

author: D2236
author_rank: Regular Contributor
timestamp: February 23, 2025, 05:48:20 am
content: Very interesting concept. How expensive is the mercury wetted reed switch that is required?The idea behind the existing design was to have something small, cheap and portable to move around the lab. For now it meets our requirements of checking response times of the 1 MHz current probes. I can see your setup to be installed at a workstation to do performance checks on higher bandwidth probes if the need arises.

author: garrettm
author_rank: Frequent Contributor
timestamp: February 23, 2025, 06:00:00 am
content: I'm not sure how much new a new mercury wetted reed switch costs, but you can get the non-mercury wetted parts for pennies. I used a home alarm door/window sensor for the reed switch shown. I''m sure Mouser or Digikey have proper mercury wetted parts for sale.A proper build with controlled impedances on a PCB can have impressively good performance, if high jitter from the mechanical nature the switching element. As an alternative to the reed switch, a semiconductor switch operating in the breakdown mode can be just as effective, but also has high jitter due to the uncontrolled nature of the switching. There're a lot of good articles on this subject. I think Jim Williams has a article on this for a pulser design he made.

author: garrettm
author_rank: Frequent Contributor
timestamp: February 23, 2025, 06:11:05 am
content: I'm not sure where I got this, but calibration adapters can be used with the charge line pulser to get really clean, fast risetime waveforms. Using one of these simplifies the design greatly, as it acts as both a load and interface for the current probe.Attached are some photos of my current calibration adapter. I also have a Tektronix unit with GR874 connector, but it isn't compatible with my HP probe.

author: D2236
author_rank: Regular Contributor
timestamp: February 23, 2025, 06:24:27 am
content: https://www.ebay.com/itm/326428798297?Ouch!

author: garrettm
author_rank: Frequent Contributor
timestamp: February 23, 2025, 09:21:52 am
content: Yikes! That's highway robbery. I think I paid $80 for the Tek calibrator, and that was as an auction lot with some other goodies.I think a decent calibration fixture can be made using two type N coaxial connectors with a large diameter wire between them and a machined or two-piece folded sheet metal body. This way the load resistor--which is arguably the most expensive part of the fixture--can be a standard coaxial part for good frequency response and low SWR. Keeping everything coaxial and the probe-to-fixture interface as small as possible increases usable bandwidth and minimizes impedance discontinuities.I've been planning on making a few different sized fixtures of this type for my various current probes. I just never seem to remember to do it... I have an old-school 1960s mercury wetted coaxial relay that was used to produce 3kV pulses for an electron microscope. It's perfect for testing broad band current probes and checking bandwidth. I've been meaning to build a driver board for the relay coil. Another thing for the ever expanding project queue.

author: David Hess
author_rank: Super Contributor
timestamp: February 23, 2025, 02:12:46 pm
content: Quote from: D2236 on February 23, 2025, 04:33:21 amIn the Tektronix designs, they had one or more differential stages amplifying the input pulse to achieve the needed slew rate driving the final differential output stage. Everything was using high Ft transistors.

author: moffy
author_rank: Super Contributor
timestamp: February 23, 2025, 07:18:46 pm
content: Quote from: D2236 on February 23, 2025, 04:33:21 amIf a cascode transistor is used it should improve the switching speed.

author: David Hess
author_rank: Super Contributor
timestamp: February 24, 2025, 09:42:02 am
content: Quote from: moffy on February 23, 2025, 07:18:46 pmWhere is the cascode placed to make it faster?The existing circuit works as a current controlled current source without a lot of voltage swing for a cascode to isolate.

author: moffy
author_rank: Super Contributor
timestamp: February 24, 2025, 09:47:00 am
content: Quote from: David Hess on February 24, 2025, 09:42:02 amX1 switches a 1A pulse through a 10R/R10 resistor so there is a 10V swing, the cascode would be between the collector of X1 and the 10R/R10 resistor, though with the current limit set by Q6 R10 is kind of superfluous.

author: D2236
author_rank: Regular Contributor
timestamp: March 06, 2025, 11:27:23 pm
content: After a bit of tweaking some of the values, I got the rise-fall response down to about 15 ns. This performance is well above for what we needed it for.Thanks to all for your valuable input and suggestions that may come in handy in the future.

author: rfengg
author_rank: Regular Contributor
timestamp: April 04, 2025, 08:47:14 pm
content: @garrettm and All,Could I request some help in understanding how what garrettm has made, works?I found a "schematic" on the web for the "charge line pulse generator" (attached) but do not understand how it works.....a) Does the transmission line act as a high Q capacitor in this design?b) When the switch is closed, the voltage source charges the transmission line......but when the switch is opened , where does the charge come from in the transmission line that causes the pulse (green box)?c)How fast should the switch be to "release" this as a short pulse?thanks

author: pardo-bsso
author_rank: Frequent Contributor
timestamp: April 04, 2025, 10:02:17 pm
content: I like your speed-up off solution with L1. Nice approach.