500 kHz signal amplifier

[xeno]

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?

I had opened several PC SMPS and actually never saw anything like a faraday cage, except the actual metal box it is housed in.
The manufacturers appear to mainly focus on EMI suppression components and don't seem too worried about more shielding and these power supplies are legal in my country. This measure indicates that they want to prevent RF injection into the power grid.
It can however not harm to build a box around the device that acts like a faraday cage.
If i'll drive it from a battery on top of it, it's even better or with proper ferrite chokes.

 

[Sceadwian]

except the actual metal box it is housed in

The metal box of an SMPS power supply is a Faraday cage zeno...

You never stated your specific intent, testing means nothing if you don't state what the device is supposed to actually end up doing. If you're just toying around with the circuits then yes a faraday cage is what you need.

 

[xeno]

How would i convert the oscillator circuit to a single supply circuit?
The suggestions i found were:

- You just need to provide a reference at 1/2 of the power supply voltage and use this reference in place of the ground for the opamp.

- This also means the you will need to cap couple the input and output.

Maybe the caps don't allow the initial transient to build up the oscillation, but
as i got it now the circuit does not oscillate:

**broken link removed**

What could i try to change to make it oscillate?

 

[xeno]

Also with a buffered half supply voltage, the oscillation does not sustain.
I thought maybe the op-amp would load the supply voltage too much.

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Current through Choke and Cap are at the right phase, but too damped:

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[carbonzit]

I think your single-supply solution will work, but you want to use much higher resistances in the voltage divider (like 10KΩ, maybe even 100K) to reduce power supply loading and increase isolation.

 

[xeno]

I think your single-supply solution will work, but you want to use much higher resistances in the voltage divider (like 10KΩ, maybe even 100K) to reduce power supply loading and increase isolation.

Thank you Carbonit.
I tried it also with 100k resistors, then went over to try the buffer.
It might work in reallife, but the sim won't sustain the oscillations for some reason
and i am out of ideas why that would be. Even with R2 set very high, the feedback
doesn't seem to work as in the gain is not enough.

 

[xeno]

Okay, i assume that this is a pure Spice issue and will work with the real op-amps with a single supply.
My next step would be to increase the voltage with a bridged amplifier and the current with the transistors in the following gain stage.
This is working with 2 op-amps AFTER the buffer.
For some reason i can't find a solution to combine the buffer and the mutually inverted op-amps, so that there would be 2 buffer op-amps after the actual oscillators that are set-up to be already 180 degrees out of phase.

**broken link removed**

EDIT: I got it working with just 3 op-amps like this:

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[xeno]

In an attempt to raise the output voltage i tried to apply the class AB push pull topology for the 2 transistors behind the op-amps to build a bridged amplifier.
Unfortunately the output waveform looks wrong and so does the voltage range.

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Could this also be a spice-related issue or is there a mistake in the circuit?
My target is to get 10 Volt at 1 amp into a 10 ohm load out of this amplifier.
Would there be a better amplification possibility better suited to reach that?

 

[xeno]

I got the parts now to start building.
To verify that the circuit will indeed run with half supply voltage,
i assembled it on a breadboard.
As to be expected with a high frequency circuit, the breadboard seems
to influence the oscillation (due to its high capacitance).
I have however been able to get a 1.6 MHz oscillation with a smaller cap
and the sine waveform looks nice, but seems a bit unstable (flickering,collapsing for a fraction of a second).

The buffer stage did not work as expected, that might be due to 1.6 MHz being too
high frequency maybe, that i don't know. There is only a very small voltage rectangular
waveform scoped at its output.

I could not locate a single 15nF capacitor, so i had to take 10nF, 4.7nF and 330 pF.
Next step would be to solder the circuit on a board, to see if it oscillates then.

Still a bit clueless on what power stage to use to amplify the current.

**broken link removed**

 

[alec_t]

Isn't there a feedback resistor missing from IC1?

 

[xeno]

Isn't there a feedback resistor missing from IC1?

Hi Alec_t: Indeed, forgot it in the schematic. It has been corrected. Thanks.

Is it important to place the capacitors very close to the IC on the board?
I remember having read about something like that in regards to radio circuits.
As i understand it, they form an oscillatory circuit with the inductor, so i am not sure
if that is indeed essential for the oscillation to occur correctly.
I do not intend to have that board professionally printed,
it's just to get an idea about component placement.

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[moffy]

You might want to drop the value of the 100k bias resistors and bypass them with an electro and ceramic cap.