500 kHz signal amplifier

[xeno]

Along with my 50 Hz amplifier, i'd like to build an amplifier for a signal generator XR2206 that outputs a 500 kHz sine wave.
The standard way would probably be to use a class D approach for
efficiency reasons (or maybe that's only for audio and low frequency applications).
I would like to try it with a class AB approach for learning purposes.

So this is the requirements:
- 12V/2A single supply
- no class D
- 500 kHz sine signal amplified to 8-10 Volt and 1 Ampere for a load of 10 Ohms


The challenge here is the relatively high frequency of the
signal , that narrows also the choice of components.
I am not sure if there is Op-Amps that work sufficiently at 500 kHz
even if the slewrate is high.
Found some models:

https://www.electro-tech-online.com/custompdfs/2011/09/AD8000.pdf
https://www.electro-tech-online.com/custompdfs/2011/09/LMH6702.pdf
https://www.electro-tech-online.com/custompdfs/2011/09/LMH6732.pdf
https://www.electro-tech-online.com/custompdfs/2011/09/1028fa.pdf (???)

Since the high-speed types are usually not rail-to-rail-types, the
op-amp input can then be biased near half the single supply voltage.

I am also not sure if "standard" power transistors for the gain stage
would work at that frequency.
So special HF power transistors might be needed; heat-sinked and maybe fan-cooled.

http://www.datasheetcatalog.org/datasheets/166/71087_DS.pdf

To reach the 10 Volt output from a single supply, a bridged
arrangement of 2 amplifiers could be created.

Does anyone have experience with high-speed components like that?
I appreciate any input on this. Maybe even a different approach for that frequency range.

 

[audioguru]

The XR2206 produces a fairly poor sine-wave at a frequency as high as 500kHz.

 

[xeno]

Let me guess, the XR2206 is also garbage?
So MAX038 is discontinued, i am not sure what could give a better
sine at 500 kHz. Any suggestions?

 

[audioguru]

Let me guess, the XR2206 is also garbage?

It is a fairly old function generator. It works fairly well below 100kHz.

I worked with the similar ICL8038 function generator IC about 31 years ago and its low frequency "sine-waves" looked a little odd.

 

[simonbramble]

Wien Bridge Oscillator will give a pretty good 500kHz frequency and op amps are so high in Gain Bandwidth these days, you can construct one around an op amp with a GBW of 50MHz+ without getting into nasties like layout etc

 

[xeno]

@Simonbramble: Thank you. I have always stood back from building a wien bridge oscillator due to the requirement of the small incandescent lamp to stabilize the bridge and the whole thing being so prone to room-temperature influences.
Maybe i should give it a shot and just find out.
There don't seem to be any other options.

EDIT: I have found this (https://www.electro-tech-online.com/custompdfs/2011/09/AN-20.pdf) :

**broken link removed**

**broken link removed**

Not sure if i can modify this circuit to work with a single supply though. Seems to be made for
+12 -12 rails. The unfortunate fact that it is very hard to simulate oscillators in Spice wouldn't
make it easier to adapt this also for 500 kHz.

Well maybe i can adapt this a bit:

**broken link removed**

 

[alec_t]

i am not sure what could give a better
sine at 500 kHz. Any suggestions?

You could try something like this :-

 

[xeno]

@alec_t:

That looks simple, and has no lamp. I assume L1 is just a choke?

**broken link removed**

Could i use U2 for voltage gain with the feedback network or would i need
an additional op-amp stage to do that?
Or using 2 identical bridged push pull stages would care for the correct voltage level maybe.
Assuming that this can be done at that frequency with the right transistor pair.

 

[simonbramble]

This second circuit relies on the phase shift (of 180 degrees) across L1 and C1 at resonance, that is then fed back to the inverting input of the op amp to give 360 degrees loop phase shift. Both circuits should work equally well. The Wien Bridge in the Nat Smelly apps note relies on the JFET to provide a variable gain to keep the oscillator stable, whereas the LC circuit has no such circuitry leading me to believe that the Wien Bridge might offer lower distortion. As an exercise, it is worth going through the Wien Bridge network and working out the transfer funtion. You should be able to determine that at resonance, the circuit needs a gain of exactly 3 to overcome the losses in the feedback network

 

[xeno]

@Simonbramble: Thank you. I will build the simple circuit first and then the wien bridge (since it's not a lot of components needed anyway) to see their distortion level differences.

 

[Nigel Goodwin]

Perhaps you should tell us EXACTLY what you're trying to do? - but 500KHz is RF, and presumably an RF solution is probably the best one?.

 

[audioguru]

The very first product produced by Hewlett and Packard (HP) was a Wien bridge audio oscillator that used a vacuum tube and an incandescent light bulb to set the output level. When the frequency was changed then the light bulb cause the output to bounce up and down for a couple of seconds.

 

[alec_t]

Could i use U2 for voltage gain with the feedback network or would i need
an additional op-amp stage to do that?

I don't see why not. I ran the sim in LTSpice with R5 = 200 and got a reasonable looking sine wave (but much higher values were iffy). You've clearly got LTS set up so can easily check.
I haven't checked out distortion figures. What sort of figure are you after? (and out of curiosity what are you going to apply the 500kHz to?).
As Simonbramble mentioned, there is no provision for stabilising the output of that LC oscillator. Is that a problem? What accuracy/stability do you need? And yes, L1 is just a choke.

 

[xeno]

I am reading an e-book about transformers and wanted to
see how a 500 kHz signal will behave when i send it through an aircore transformer
as opposed to a ferrite core transformer, with the aim of getting a fairly good signal on the secondary.
There is a couple of pitfalls waiting in such an application and going through
them helps me learn.
Keeping all wires as short as possible and making a nice circuit board
is important. I am not experienced with stability of the oscillator, i have the feeling
though that it is advisable to ensure stability in such an application.

The op-amp LT1225 is not available from my suppliers. Are there common alternative
op-amp models?
I have found LT1226CN8 which has 1GHz GBW and a slew rate of 400 V/ us
https://www.electro-tech-online.com/custompdfs/2011/09/61411.pdf

Or a more affordable model : https://www.electro-tech-online.com/custompdfs/2011/09/LMH6702-1.pdf
I think i go for this one.

 

[Nigel Goodwin]

I am reading an e-book about transformers and wanted to
see how a 500 kHz signal will behave when i send it through an aircore transformer
as opposed to a ferrite core transformer, with the aim of getting a fairly good signal on the secondary.

Then it's an RF application - air cored is no problem, except for the size of the coils required.

It's usual to use ferrite cores for RF below VHF simply because it makes everything smaller and easier.

 

[ericgibbs]

hi xeno,
You do realise that 500kHz is allocated to some broadcast users.?

Until recently it was the International Distress frequency, I understand that it is still monitored.

http://www.davesergeant.com/mf/index.htm

 

[xeno]

hi xeno,
You do realise that 500kHz is allocated to some broadcast users.?

Until recently it was the International Distress frequency, I understand that it is still monitored.

http://www.davesergeant.com/mf/index.htm

Would there be frequencies that are free for use in the kHz range?
Otherwise i must contemplate about measures like putting the device in
a faraday cage. Not sure if an aircore coil will make such a powerful antenna
to pose like a significant transmitter.
Some people do indeed use those coils to transmit : http://www.aaroncake.net/circuits/fmtrans.asp
I just knew about loop antennas which make a much better receiver than transmitter according to the available descriptions.
But this matter has to be certainly looked at and
appreciate further input on this.

 

[ericgibbs]

hi

look at this link.
**broken link removed**

Extract.
The frequencies 490kHz and 518kHz are currently used for Navtex a maritime advice service for this reason alone the maritime use of the 405-535kHz maritime band should be maintained with 495-515kHz or 495-505kHz3 nominated a heritage section.

Not knowing which country you are posting from.?? I would advise you contact your local licensing authority.

BTW: the HMS Titanic used 500kHz for her distress calls!!!

 

[xeno]

Thank you Eric.
If i construct a faraday cage around the device, would that not
shield the radio waves completely?
As i understand this is being done in SMPS too and they switch up to RF as well with powers of hundreds of watts where i plan to only use 10 watts.
Quote:

Smaller transformer (if used; else inductor) due to higher operating frequency (typically 50 kHz – 1 MHz). Size and weight of adequate RF shielding may be significant.

The inverter stage converts DC, whether directly from the input or from the rectifier stage described above, to AC by running it through a power oscillator, whose output transformer is very small with few windings at a frequency of tens or hundreds of kilohertz (kHz).

The country is germany.
The effectiveness of the shielding could maybe even be tested with
a 500 kHz receiver circuit.

 

[Sceadwian]

A faraday cage would probably work, but for such low frequencies I'm not sure what the construction would require. What's the point of building a device in a faraday cage if you can never use it?