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Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
Hello. I am looking for a cheap way to amplify DDS generator output of around 10khz to 1mhz. I would like to measure resonant frequency of a circuit that is similar to a tesla transformer.
I find it strange that there are very few DDS generator amplifier circuits out there, both commercial and as circuit diagrams.
How do I couple the output of a DDS generator to a load?
What is the minimal load resistance that I can directly connect to DDS? How much power do they output? Do they have a protection from short circuit?
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
They are difficult to find because amplifying arbitrary signals over that high bandwidth is extremely difficult. Remember that arbitrary signals are made up of lots of frequency components; if the waveform has a steep slope much of the frequency spectrum will be above the fundamental frequency. at more than about 100 kHz, power devices (BJTs, MOSFETs, diodes, etc) cannot switch fast enough keep up and maintain a satisfactory reproduction of the input signal if the input slews too fast.
High frequency power electronics (transmitters, Tesla coils, ect) work by operating at resonance. As resonance can only occur at a specific frequency, they are limited to sine waves (or damped sinusoids in the case of TCs) and cannot produce arbitrary waveforms.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
I too want a power amplifier (say 1 to 10 W, 100 W possibly) for the frequency range up to > 1 MHz. I've made diy PAs for audio and 10 - 50 MHz ... challenging.
I've not made my 'up to 1 MHz PA' yet as I usually fail on paper. I can not design an efficient (>=50% would be ok), linear (<0.1% thd), simple cheap topology. But I am out of touch with state-of-the-art, there must be an IC for it, preferably 0.1" dil ? (power supplies now switch at what I considered high frequency radio, CPUs operate at microwave carrier frequencies ...)
Ideally I'd like a cheap Chinese ready made pcb because I'm lazy and they usually are cheaper than component cost. (I'm still amazed how small per watt ClassD PAs are)
Registered Member #3859
Joined: Sun May 01 2011, 03:47PM
Location:
Posts: 179
hen918 wrote ...
They are difficult to find because amplifying arbitrary signals over that high bandwidth is extremely difficult. Remember that arbitrary signals are made up of lots of frequency components; if the waveform has a steep slope much of the frequency spectrum will be above the fundamental frequency. at more than about 100 kHz, power devices (BJTs, MOSFETs, diodes, etc) cannot switch fast enough keep up and maintain a satisfactory reproduction of the input signal if the input slews too fast.
High frequency power electronics (transmitters, Tesla coils, ect) work by operating at resonance. As resonance can only occur at a specific frequency, they are limited to sine waves (or damped sinusoids in the case of TCs) and cannot produce arbitrary waveforms.
I am looking for ways to drive a device that is similar to a Tesla Transformer below 1MHz with duty cycle control. I believe that below 1MHz those things are still possible to build.
Registered Member #162
Joined: Mon Feb 13 2006, 10:25AM
Location: United Kingdom
Posts: 3140
tarakan2
At MHz freqencies I can get ferrite cores for transformers at moderate cost and small size, so transformer-coupled push-pull is reasonably straightforward. A power transformer for say 10 kHz to 1 MHz may be quite challenging, large or costly.
At audio frequencies it is normal to use ac and dc coupling without the need for transformers, using the typical half- or full-bridge topology. Most power transistors used in audio power amplifier output stages usually have low gain-bandwidth so become less useful above 100 kHz. I guess that for a up to 1 MHz bridge type linear power amplifier mosfets are required, but common power mosfet intrinsic capcitances become problematic at MHz's.
For moderate output power I'm still considering the IRF510's as they have relatively low gate capacitance, I've used them uo to 14 MHz as reasnably linear PA transistors, and I have stock. Now that this topic has come up I'll have another think, maybe ferrite core push-pull is practical with newer ferrites, mostly not available via my normal sellers ...
The lower the frequency range (bandwidth) required, the easier PA design becomes, so maybe use a conventional audio amplifier to 10's kHz, (I like the LM380) and buy or design your PA to cover from there to MHz
What would happen if I connect a MOSFET gate or a bipolar transistor base (through a resistor) to the output of this generator?
I am not an expert on amplifiers so I am looking for a good circuit that would work up to 1MHz, maybe up to 2MHz to be safe.
I want to drive primary coils.
A few mA perhaps, and nothing particularly useful. A few words on transistors: MOSFETs convert a voltage on the gate into a current through the drain & source. BJTs convert a current to a current. Whilst this is fine; neither a MOSFET nor a BJT exhibits linear characteristics over their operating region, so if you put a voltage of less than a couple of volts onto the gates of most MOSFETs, nothing would happen. Then between 2V and 8V everything would happen, but not proportionally to the input voltage. You can, of course, use various techniques for getting the output of the amplifier to closely resemble the input, involving feedback and many other semiconductors, but the high frequency and the requirement for reproducing arbitrary signals makes it extremely difficult.
To measure the resonance of a TC you could make a 555 monostable, set to produce a pulse of variable width, put the output directly into the gate of a MOSFET (as we are now dealing with on/off transitions, so the non-linearity is not a problem), use this circuit to stimulate the LC into resonating with a damped sinusoid which you could measure the frequency of with an oscilloscope.
Registered Member #72
Joined: Thu Feb 09 2006, 08:29AM
Location: UK St. Albans
Posts: 1659
tarakan2 wrote ...
Hello. I am looking for a cheap way to amplify DDS generator output of around 10khz to 1mhz. I would like to measure resonant frequency of a circuit that is similar to a tesla transformer.
Let's get clear, do you simply want to *measure* your TC, so a volt or so from 50 ohms would be sufficient, or do you want to *power* a TC, where you need tens or hundreds of volts with some serious current behind it?
If the former, then a while ago I designed a commercial instrument that needed to produce 'a volt or so from 50 ohms' from DC to I forget how many 10s of MHz, and the easiest/cheapest way to do it was to use 3 high speed op-amps THS3091 , each driving a 150ohm sourcing resistor, paralleled up at the output. To get to 1MHz, you only need something like (for instance ) THS4052 from TI, a few dollars from digikey. Have a look at the high speed op-amps from Maxim, Analog and TI to see if there's anything better suited. Many of these are also available on small evaluation boards, which (at a price, often not too big) solves the mounting, heatsinking, decoupling problem very easily for a one-off.
Registered Member #11591
Joined: Wed Mar 20 2013, 08:20PM
Location: UK
Posts: 556
tarakan2 wrote ...
Dear Dr. Slack, what does "a volt or so from 50 ohms" mean?
I believe Dr Slack is referring to a resistor value you could use as a current shunt and a voltage you could expect across it. Using ohms law it means a current of 20 mA.
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Cheap amplifier for DDS
