# universal buffer/modulator cct

#### julesison

Joined Nov 29, 2015
7
hi, and of course my first line has to be to apologise for my ignorance and secondly the possibility that this has been covered somewhere i couldn't find it.
i need to have a signal generator drive any number of different hf diy transformers that i want to experiment with. so i don't know what impedance the cct has to match only that the frequency will be from 20khz to 100khz max. is there not a "universal" impedance matching, "buffer" type device?
i attach a rough pic of what seems to me the perfect "cct". it would be basically a cct that allowed a signal generator to "modulate" the dc from a bench top power supply. for the sake of convenience it wouldn't have to swing + and -, it could be a square or sine all above zero. the power supply can be the current limiter through it's current output adjustment. i just need something that shows a high impedance to the sig gen output and that doesn't care about what's connected to it's output as it's current limited via the power supply anyway. surely this is the easiest way to be able to test "any" winding/transformer? sorry again if my simplistic approach is off but logically it seems sound?

#### MikeML

Joined Oct 2, 2009
5,444
To drive transformers, especially ones with iron or ferrite cores, you need a no-DC-component waveform. This implies either a push-pull bipolar amplifier with split-power supplies, or at least a unipolar amplifier with a large DC blocking capacitor.

If it were me, I'd go with a power opamp.

What magnitude of current do you expect to have to drive into the transformer?
What peak-to-peak voltage do you expect to have to drive into the transformer?

#### julesison

Joined Nov 29, 2015
7
To drive transformers, especially ones with iron or ferrite cores, you need a no-DC-component waveform. This implies either a push-pull bipolar amplifier with split-power supplies, or at least a unipolar amplifier with a large DC blocking capacitor.

If it were me, I'd go with a power opamp.

What magnitude of current do you expect to have to drive into the transformer?
What peak-to-peak voltage do you expect to have to drive into the transformer?

hi mike,
- they will all be ferrites of one type or another.
- i was planning on anywhere between 3 - 12 volts at around 100 - 200mA.
- i was thinking of a module like the attached one. that way i don't have to build anything. it's not that i'm lazy it's just the transformer testing is my immediate goal, so i'd rather spend my time on that.
- the unknown load impedance is not an issue for the "average" audio amplifier?
- how do i calculate the blocking capacitor please?
cheers

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#### MikeML

Joined Oct 2, 2009
5,444
I found the datasheet on the Kemo website. I would have to assume that it has an internal DC blocking capacitor. I would put a 4Ω 10W resistor in series with your ferrite transformer for initial testing, and that way you cant hurt the module...

I suspect that this module has an internal frequency roll-off at 6db per octave above 20kHz, so you will notice a drop in output amplitude as you sweep from 20kHz on up.

#### Alec_t

Joined Sep 17, 2013
11,398
the "average" audio amplifier
..... won't be of much use above 20kHz. I think you'll need an above-average one.

#### julesison

Joined Nov 29, 2015
7
hi guys, thanks very mush for your practical help. sometimes on forums people are more interested in showing how much they know instead of helping. if i use the below linked amp that goes up to 65khz it should see out the hf problem? also no dramas with needing a dc blocking cap? as it has no short cct protection if i added 6 ohms at 15-20 watts i should be ok? thanks again

#### julesison

Joined Nov 29, 2015
7

#### DickCappels

Joined Aug 21, 2008
6,457
That amplifier does not have a specification for its upper frequency limit. If you really need 100 KHz, ot would be a pretty big risk of the $119. Thread Starter #### julesison Joined Nov 29, 2015 7 That amplifier does not have a specification for its upper frequency limit. If you really need 100 KHz, ot would be a pretty big risk of the$119.
sorry, i meant to say i had amended the highest frequency to 50khz. so this amp (or one like it) would do if i used resistors to limit the output current? how do i calculate that?

#### DickCappels

Joined Aug 21, 2008
6,457
You still don't know how the amplifier would perform at 50 kHz.

Calculate the resistor? You could use V/Z = I if you know the effective inductance (when driving a primary, the inductance would appear to be lower when the secondary is loaded). The best approach would probably be to use a current probe or a shunt and monitor the current while adjusting the amplitude.

In the days when I designed transformers for my products I tested them by putting them into the circuit for which they were intended and checked their performance under actual operating conditions.

#### crutschow

Joined Mar 14, 2008
25,116
The Jaycar website that comes up when I click your link in post #7, it states the upper frequency response for the amp is 65kHz so it should work fine for testing at 50kHz.