I am currently trying to design an amplifier for the output of a generator. It needs to work at up to 1MHz, and have a negligible distortion, ideally in the ppm range whilst delivering 30 to 40Vpp.
This is my first tentative:
The THD is ~0.01% and there are cheats: spice sources used as ideal buffers, because the base currents of the transistors are too large and too distorted. Additional transistors would be required.
There is also a nasty, unexplained peak in the frequency response, in the region of 200MHz.
This is my first tentative:
The THD is ~0.01% and there are cheats: spice sources used as ideal buffers, because the base currents of the transistors are too large and too distorted. Additional transistors would be required.
There is also a nasty, unexplained peak in the frequency response, in the region of 200MHz.
This one is based on a topology from Groner, and manages ~0.004% THD, but its stability is very marginal and increasing the compensation immediately ruins the linearity:
Check out the circuit of the older Tek FG504 or similar function generators. They have a very linear output amp.
H&H describe this in detail in I think their 2nd Ed.
Jan
H&H describe this in detail in I think their 2nd Ed.
Jan
FETs and/or BJTs designed for RF use will be needed I think - you need individual component gains to hold up at 1MHz, suggesting GBW products of well over 100MHz would be wise. Layout will be tricky, good RF design experience essential, screening cans a requirement too I bet.
I hope the load impedance is a stable 50 ohms. I also presume it is DC-coupled?
You might want to consider using a design capable of much higher power so its running at low level, think of 3rd-order intercept behaviour for instance.
RF design is usually class A or class C - class B is seldom used without transformers, and I don't think you can get low enough distortion with transformers. I suspect you will not get good enough matching of complemetary devices at high frequencies to keep cross-over under control anyway. You need very low inherent distortion plus as much negative feedback as can be stabilized.
I hope the load impedance is a stable 50 ohms. I also presume it is DC-coupled?
You might want to consider using a design capable of much higher power so its running at low level, think of 3rd-order intercept behaviour for instance.
RF design is usually class A or class C - class B is seldom used without transformers, and I don't think you can get low enough distortion with transformers. I suspect you will not get good enough matching of complemetary devices at high frequencies to keep cross-over under control anyway. You need very low inherent distortion plus as much negative feedback as can be stabilized.
Link to thread: Winfield's 100W DC-10MHz 1000V/us amplifier, posted by Winfield Hill. He's a coauthor of the famous textbook "The Art Of Electronics" by Horowitz and Hill.
Output transistors are 5x 2SC4883 and 5x 2SA1859 , biased at approx 85mA per device. In other words, Class-AB. fT at that bias current is about 85 MHz, see datasheet graph below.
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Output transistors are 5x 2SC4883 and 5x 2SA1859 , biased at approx 85mA per device. In other words, Class-AB. fT at that bias current is about 85 MHz, see datasheet graph below.
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Attachments
Was testing some circuits
On a comical note, When I set my function generator to 1MHz
Thought the circuit was instantly oscillating.
forgot to set time scale on scope
Not use to being up that high.
On a comical note, When I set my function generator to 1MHz
Thought the circuit was instantly oscillating.
forgot to set time scale on scope
Not use to being up that high.
Thanks, the circuit is interesting as always with Tek (or HP), but it is mostly speed-oriented (40Mhz) and the linearity, although good is suited to a function generator, not capable of ppm performance.
Yes, I may resort to class A, but with the current conveyor topology, some adaptations will be required
The load will vary from 50 ohm to infinity; the sims are based on the worst case, a short at the output and 50 ohm for the amplifier.
The matching is indeed going to be a problem, and I may need to resort to a non-complementary output. And anyway, for the moment it is just a sim: the reality will probably somewhat harsher.
Yes, it is an inspiration, as are Stochinno designs, but the scope is not quite identical. Anyway, I try to gather as many inspiration sources as possible to arrive at a reasonable design: one that doesn't require exotic devices, large componets count or inordinate amounts of consumed power
They bootstrap (or cascode if you prefer) the output transistors: I have found that their Early effect is a major contributor to the THD
Yup , here. That cascoded circuit is my "go to" VAS. Bombproof , gets cooler and the red led's flash at overload.
I built a modified one with a cascode led's as infrared opto. Fed it to a MCU to count clip events.
Never tried to get it to amplify 1mhz ?? My EF3 limits both bandwidth and slew ... still fast @ 300V/uS.
It is THD for 20kHz. If you need 1MHz, you should use output transistor with fT >= 100 Mhz. Maybe parallel of many 2SC5171.
But gain at 1mhz is problematic. BJT's don't make the best ultra HF amplifiers.
FET's are better here.
OS
FET's are better here.
OS
Start with the distortion your diamond output stage generates at 1MHz by itself, without any NFB applied. Say it’s 0.1% (-60dB). If you want 1ppm (-120dB) with NFB, you‘d need 120-60=60dB of loop gain, at 1 MHz. With the usual dominant pole compensation this is equivalent to the crossover frequency of 1GHz. Commonly available transistors have Ft of up to 150-200 MHz, limiting the crossover frequency to 50-70MHz, so you have to use a more complicated compensation scheme. Even then, dropping from 60dB @ 1MHz to 0dB at 50MHz and retaining a reasonable phase margin is not an easy thing. And of course the actual hardware will need to operate at up to 50 MHz, so VHF construction techniques would be required.
Hi Elvee!
Did you try HF pentode (maybe cascoded) + output cathode (source) follower suplied with ~+800VDC?