External amplifier for laboratory generator. - Page 1
Forum section: electronics / testgear

author: MikeP
author_rank: Regular Contributor
timestamp: March 08, 2020, 07:52:44 am
content: By the will of fate, a new generator appeared on my table.Unfortunately, this device has a rather modest output voltage. Therefore, I decided to create an external amplifier. To my surprise, there was only one way - THS3491. Here is a link to this chip.http://www.ti.com/product/THS3491Four chips on a small printed circuit board with two connectors. The board is installed directly on the generator. I am completely satisfied with such a device.However, I ask the community to share their experiences and their own options for solving the problem. Thanks.

author: nctnico
author_rank: Super Contributor
timestamp: March 08, 2020, 05:33:11 pm
content: I have been thinking of a similar project for a couple of years and use several ADA4870s in parallel. The THS3491 is mighty interesting too though.

author: MikeP
author_rank: Regular Contributor
timestamp: March 08, 2020, 07:33:48 pm
content: Thanks. Then I will slowly draw a diagram.

author: nctnico
author_rank: Super Contributor
timestamp: March 08, 2020, 10:46:02 pm
content: Still the thermal design is going to be tricky. A rough initial analyses indicates that you can get about 300mA from one THS3491 while supplying it from +/-15V because of the thermal limits. I'd like to have at least several Ampere of out put current. Say around 5A. That would mean using 17 -make it 20- THS3491 in parallel.

author: MikeP
author_rank: Regular Contributor
timestamp: March 09, 2020, 07:32:59 pm
content: Quote from: nctnico on March 08, 2020, 10:46:02 pmYou're scaring me. Why do you need this terrible device?!I was thinking about thermal design. In reality, nothing extraordinary is needed to operate at 50 Ohms x 4. Moreover, at the highest frequencies permissible voltage decreases to 2Vpp / 500 MHz. And before that, the generator itself is quite good.

author: Micke
author_rank: Regular Contributor
timestamp: March 09, 2020, 08:16:43 pm
content: Will not help OP with getting higher output voltage, buy may fit others if higher currents are needed...This is designed for DC up to like 140kHz @8ABut on the other hand, might be easy to increase the voltage compliance, if the current is lowered?I needed this for calibrating current ranges on multimeters. Bought the 2N6438 and 2N6341 transistors on Farnell.I built one according to article, and made a PCB. Used VISHAY VHP-3 and VHP-4 current shunts of values 0.1, 1and 10selected by a rotary switch.https://www.nist.gov/publications/wide-band-transconductance-amplifier-current-calibrationsIt works really well, but on higher currents and tens of kHz, there are some oscillations, have not bothered to track down the root cause yet, but will try base resistors or ferrites on output transistors. One (big) problem may be that output transistors are not on the PCB but connected with wires (output transistors on a big heatsink)The .asc is the LTSpice simulation, for completeness I attach the transistor models, and also the Gerber files.

author: MikeP
author_rank: Regular Contributor
timestamp: March 10, 2020, 07:22:56 am
content: Thanks, very interesting article. Especially the part with homemade resistors.Regarding your design. It may be dangerous. Shorten the length of the conductors.

author: dzseki
author_rank: Frequent Contributor
timestamp: March 10, 2020, 08:08:18 am
content: I have plans for a similar project, I'll use antique OPA654 (TO-3 case) devices, because I have a dozen of those NOS in tube...

author: Henrik_V
author_rank: Regular Contributor
timestamp: March 10, 2020, 08:24:43 am
content: Place the power BJTs on one edge of the PCB, use massive bracket(s) to mount heat sink(s) ... I would go for processor heat sinks ...Layout with a a dedicated star ground , regarding the currents ..May the current force be with you(Reminds me to an audio amplifier design build 30 years ago.. with a changed feedback we ended up with a GBP of 140 MHz ...)

author: TurboTom
author_rank: Super Contributor
timestamp: March 10, 2020, 10:54:28 am
content: If relatively low peak-to-peak voltage at high currents will suffice (30Vpp and 10A max), I can suggestthis designaraound the LT1210 high speed power Operational amplifiers. Cascading the ten units require some decent component matching, though. The reward is a power bandwidth of at least 10MHz.There may be other power operational amplifiers available that suit this approach as well or even better.

author: montemcguire
author_rank: Regular Contributor
timestamp: March 10, 2020, 11:06:18 am
content: Quote from: nctnico on March 08, 2020, 10:46:02 pmThis is a 900MHz amplifier. Who needs 5A @ 900MHz? I wonder if some other amplifier might make more sense? Or are you making a precision microwave oven?

author: nctnico
author_rank: Super Contributor
timestamp: March 10, 2020, 11:33:06 am
content: Quote from: montemcguire on March 10, 2020, 11:06:18 amWell at 20Vpp out you are left with a bandwidth of around 100MHz. With all the parasitics and the need to slow the amplifiers down if they are used in parallel the end result will probably have less bandwidth. An amplifier like this (with a sub 1 Ohm output impedance) would be useful as a 2 quadrant power supply, test rig for current probes, driving low impedance loads, etc. The PCB design will be challenging though.

author: MikeP
author_rank: Regular Contributor
timestamp: March 10, 2020, 05:32:13 pm
content: Quote from: TurboTom on March 10, 2020, 10:54:28 amTom thanks. I have not seen this conversation.I have to apologize - I was not clear enough in my question. I'm talking about the generator SDG6000X with a 500 MHz. Alas, the output voltage at the upper limit is only 640 mVpp.

author: tkamiya
author_rank: Super Contributor
timestamp: March 10, 2020, 06:43:08 pm
content: What you are talking about, is basically creating a transmitter. 5A going into 50 ohm load, would be 250V, thus 1250 watts. Amateur Radio operators doing EME do 1.5KW range. It usually entails whole bunch of smaller amps and divider / combiner type configuration. It's certainly do'able but doing so in instrument grade stuff would be a whole different ball game. Narrow band application will be not so hard. If you really want these in instrument grade, you can have them custom made for some major money.By the way, assuming 50% effeciency (class B), it will require 2.5KW power supply. Instrument grade is A class at 35% ish effeciency. That'll be 3.6KW power supply.With this kind of RF power, it's quite dangerous. Bodily injury is certainly possible. Early experimenters in EME made amplifiers that did 500 watts at 430MHz. An article written by him said he lost his vision for few days, and mostly recovered with intensive medical care. I recall he was using 4CX250B (vacuum tube) in parallel.Are you sure you got the numbers correct? Do you REALLY want 6A?

author: nctnico
author_rank: Super Contributor
timestamp: March 10, 2020, 07:02:03 pm
content: Quote from: tkamiya on March 10, 2020, 06:43:08 pmNot 5A from a 50 Ohm source. 5A from a < 1 Ohm source (at DC).Safety is a good point but given the slew rate limitation of the chips the maximum swing (and power) at 430MHz will be much less. According to a very quick & rough calculation: around 6Vpp so 1.4Vrms. @5A this would mean 7W from the amplifier and with matching you might get 3.5W into an antenna.It would be nice if you could built a hefty RF amplifier from a few simple amplifier chips.

author: tkamiya
author_rank: Super Contributor
timestamp: March 10, 2020, 07:07:08 pm
content: I was addressing OP.Yeah, it would good to have few chips and do that kind of power. EME from handie talkie!

author: MikeP
author_rank: Regular Contributor
timestamp: March 10, 2020, 07:30:26 pm
content: Quote from: tkamiya on March 10, 2020, 07:07:08 pmNo my friend. You missed some of the little things in the conversation. The idea of exit 5A does not belong to me. This is the design of a respected nctnico. In fact, it could be an amazing tool. And such a device can be created by said means.My idea repeats the design from the TI THS3491 based. And here I want to find the answer to the question: is there an alternative?

author: thmjpr
author_rank: Regular Contributor
timestamp: March 10, 2020, 11:54:36 pm
content: I've made a similar device with intent to useTHS3091. Lower power and low rails (+/-8V via USB). Ended up fittingTHS3201instead as it was cheap (+/-5V out with +/-7.5V rail). There are many devices in this SOIC8 size with various bandwidth ranges, with and without powerpad.Tried OPA548 but its bandwidth is not great, and crossover distortion was horrendous. Very simple to setup though for good power levels, and robust.Next try will be with LT1210 and external transistor buffer. I didn't know cascading that many 1210's was possible as TurboTom link, is there a schematic?Will add some additional notes shortly on the components.

author: schmitt trigger
author_rank: Super Contributor
timestamp: March 11, 2020, 12:08:04 am
content: Back in the 80s a company would advertise heavily on Electronic Design and EDN magazines.They would have some humongous power opamps housed in cases similar to the steel TO3, with some badass specs.However for the life of me, I can’t remember the company name.I suspect they would be very expensive.I remember now:https://www.apexanalog.com/

author: thmjpr
author_rank: Regular Contributor
timestamp: March 11, 2020, 07:12:25 am
content: Created a thread here for the 2x board:https://www.eevblog.com/forum/projects/function-generator-booster-for-mso5000/