Designing a DAC + op amp + AB amplifier with some specific characteristics - Page 1
Forum section: electronics / projects

author: luizete
author_rank: Newbie
timestamp: August 06, 2025, 04:28:53 am
content: Hey people. I'm designing a circuit that requires some special characteristics, and I don't have much experience with this.I need those characteristics:Waveform: triangular and/or trapezoidalTiming: ramps from 0 V to 40 V in 2 microseconds, held at 40 V for 2 microseconds, and then ramps back down to 0 V in 2 microsecondsAmplitude: 0 V to 40 VCurrent: output around 2 A, with occasional peaks of 3–5 ASo far, I've defined the following:Microcontroller: Pi Pico 2 (RP2040)DAC: AD5428 in unipolar mode, with a reference voltage of –4.096 V (generated by an LM4040BIM3 and inverted using an ICL7660M)Op-amp #1: connected directly to the AD5428 output, acting as a buffer (?)Op-amp #2: connected to the output of the opamp #1, with a gain of 10 (to produce output from 0 V to 40 V)Class AB amplifier for driving the load (capacitive load, ~200-400pf)From what I've studied, I believe I need five transistors: two NPN, two PNP, and one NPN for the bias.I have some questions:Choosing the right components. I understand some of the theory (like gain-bandwidth product, slew rate, settling time for op-amps), but I'm still lost, here are so many options. I also have no clue which transistors are best suited for this application.I need the components to be available in China, since I'm ordering the PCB with all componentsI'm not sure if my logic is solid and functional. Does this signal "flow" I've designed make sense?Could someone help guide me through this?Thanks!

author: magic
author_rank: Super Contributor
timestamp: August 06, 2025, 05:35:10 am
content: Quote from: luizete on August 06, 2025, 04:28:53 amThat's trapezoidal, not triangular. Are there any other variants, like as above but without "held at 40V for 2μs"?How close to 0V does the 0V need to be, does the amplifier need to sink current or only source, do you have negative supplies available?In general, I'm not sure if you need a DAC and opamps. A simple common emitter inverting amplifier could be driven with a square wave and have its slew rate limited to 20V/μs by means of a suitable feedback capacitor. This might suffice if the rate of change doesn't need to be variable.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 07, 2025, 04:51:16 am
content: Quote from: luizete on August 06, 2025, 04:28:53 amWith that DAC and slew, you will have about 40 samples per 2us, or 1v steps, so cross over effects are not looking important.You may get away with a simpler design of a 'good' OpAmp (perhaps OPA552 or OPA596) + 'good' HFE NPN+PNP emitter follower pair. (perhaps 2SC4552 / 2SA1744 )Worth a try?If the load is very reactive, you may need to check the SOAR, and bump to slower but larger parts like 2SC5200/2SA1943Alternatively, designs that implement some gain in the power stage, can reduce the OpAmp requirementsSome examples are here - the divider from OUT to the OpAmp defines a nominal voltage gain for the output stage.https://www.eevblog.com/forum/chat/favorite-op-amp/msg2349186/#msg2349186

author: PCB.Wiz
author_rank: Super Contributor
timestamp: August 08, 2025, 09:53:36 pm
content: 20V/us is in the ballpark of Audio amplifiers, and if you can bridge connect the load, you gain a bit more slew rate.Parts like LM3886 claim 19V/us, and this is a TI app note for an old LM4702 Amp Driver chip you can still buy.https://www.ti.com/lit/an/snaa045a/snaa045a.pdfThat has examples for EOL MOSFETS and some MOSFETS you can still buy, like IRFP240, IRFP9240More modern would to be look at Audio Amp lateral mosfets like EXICON ecx10p20 ecx10n20 - nice parts, if a little $$$. Low charge and well behaved thermally.Spice confirms they need much less drive energy than IRFP240, IRFP9240.Also new are OpAmps like OPA596, high slew rate and high voltage, (85V 100V/µs) but modest drive, suited to drive lower charge MOSFETS. Too weak to drive single transistor stages.You should check your actual load and model that, that's an important part of the spec !An alternative is to have DACs define UP/DOWN set currents, and use the pico 2 to select direction and duration.That gives trapezoid outputs into large C loads, without overshoot issues feedback amplifiers will have, but it is less flexible than 'waveform playback'.