Hi,
To make things a little clearer here is the diagram Martin72 is talking about.
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The BNC connector attached to R1 is the DC input, which allows you to set the DC current via e.g. a LAP Powersupply, DAC, 9V battery.
The BNC connector below it can be used with a function generator that gives you a negative DC signal modulated with an AC signal.
A combination of both inputs is also possible, it's your party. :-)
Keep in mind the eventual interaction and the large RC time of C1 of 47uF.
If you don't want to modulate at low frequencies, you can also make this capacitor C1 4.7uf, adjust for what makes sense for you.
I added a Stand By switch and that is S2, if the contact is made then the -input of the NE5534a is pulled to the positive power supply by the orange or the red LED depending on the function.
With large signals on the inputs maybe R11 of 6K8 is not good enough, make it 3K3 or so.
The 80C Clikson take from a temperature value that fits your cooling element for the MOSFets and the power you want to dissipate.
I have good results with the modern PC Processor coolers, mount the MOSFets directly on the cooling element and mount the cooling element isolated in the case.
This gives the best cooling for your MOSFets.
I have tried to squeeze as much performance out of this circuit as possible and that the circuit also still maintains good stability.
But there are plenty of "tune" possibilities to experiment with.
The limitations are mainly in the speed, that is the speed of the NE5534a, the compensation by means of C7 of 560pF and the Drain/Gate capacities at a large AC swing on the Drains.
Not every load is a power supply with a good capacitor across its output terminals.
I also tested this Dynamic Load with a 4 Ohm and an 8 Ohm resistor without Snubber circuitry.
And taking that into account, I determined C7 to be 560pF.
But you can still test some faster Power Supply's, and then there are several possibilities.
Use the input that R1 is connected to, it does not have a 100Khz low pass filter.
And second, use the normal modulation input through C1 and then make C7 smaller but not lower than 180pF.
Otherwise the phase margin becomes too small and you could sometimes get generation problems.
The purpose of C3 of 330pF is, to not feed the NE5534a too fast edges that it cannot handle.
As a tip, don't go too far with boosting the speed.
This circuit is intended for my normal wiring length of about 30cm between the Dynamic Load and the D.U.T.
And of course twist the wires, about 1 twist per Inch is OK.
This then still results in a paracitic inductance, that if this inductance is driven too fast by steep edge from the Dynamic Load can produce resonance.
How do you measure the properties of e.g. a linear power Supply.
I use the following signal for this purpose, 400Hz with 10% Duty Cycle.
The slopes are then 50 to 1uSec this depends on my application.
The minimum current I draw is then 10% of the maximum current of the Power Supply and the maximum current is 95 to 100%.
I use a battery scope to measure the error signal on the power supply terminals.
Clean your banana plug sockets just like your banana plugs with e.g. IPA!
And I don't mean beer by that! :-)
Connect the scope via a coax cable to the + and - connection, so e.g. the holes for wire connection in the banana plugs.
And connect de Dynamic Load via Banana plugs.
Don't use banana plugs you can stack! WRONG!
Or actually do it once the right way as I describe and then once the wrong way which you can see a lot via Youtube, they really know what they are doing :-) and see the difference on the scoop.
A different and verry fast Dynamic LoadFor very fast test pulses, I use a circuit by Jim Williams that was in one of his Application Notes.
Then I do not use a connecting cable but use an almost direct connection via a bannaan coupler, this is to keep the inductance as low as possible.
Then I can test up to 1MHz block pulse, but every mm of wire already gives abberations on the signal to be measured.
So increasing the speed comes with al kind of different limitations and it is good to learn and realize this. :-)
I designed this circuit about 10 years ago, now I would do some experimentation with MOSFets that have smaller internal paracitic capacitances and are also suitable for linear operation.
But I still think this circuit is suitable for the hobbyist to have fun experimenting with and get good results.
Oh well, I chose the NE5534a specifically for this application!
Low Noise and sufficient output current with reasonably good bandwidth and it costs little.
And the offset is the trim and it remains reasonably stable.
So do not think I put in a uA741 , TL071, LM324, no, no,no. :-)
With kind regards,
Bram
Translated with DeepL.com (free version) <= So I hope you will take that and my dyslexia into account.
PS
Some test pictures
This is the current pulse measured at the BNC output, that is, the voltage across the Source resistors from a measurement at a linear HP6237A power Supply.
The Dynamic Load draws a clean pulse from the Power Supply under test, no abberations are visible.(twisted pair wiring used).
The maximum current is slightly above the maximum current the Power Supply can deliver, this is possible due to the energy contained in the capacitor across the output terminals of the Power Supply.
By making the current pulse even larger or making the duty cycle longer you can gain insight about the current behavior of your D.U.T.
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This photo shows the result of the current pulse used.
This is the voltage across the output terminals of the HP Power Supply.
The Phase Margin and Gain margin of this power Supply is optimally set, by the way, this is the 18V output that can deliver a maximum of 1-Ampere.
If the pulse drawn by the Dynamic Load becomes even faster, then the peak amplitude will become even higher.
The reason is largely the inductance of the capacitor across the output terminals and the measurement point of the scope.
I usually mount my measurement cable on the back of the banana terminals to avoid suffering from the inductance of the connection terminals.
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