• Welcome to our site! Electro Tech is an online community (with over 170,000 members) who enjoy talking about and building electronic circuits, projects and gadgets. To participate you need to register. Registration is free. Click here to register now.

mosfet "buffer" amp suggestions?

Status
Not open for further replies.

joolesison

New Member
hi folks, i'm obviously being lazy here but figure i'd risk members wrath (and save killing product) rather than read through posts for a week, apologies in advance.
could someone with knowledge please tell me the simplest way (circuit) to take the signal from a signal generator and feed it into a transformer.
i want to experiment with ferrites/windings etc etc and so need to amplify/buffer the signal from the generator to the experimental transformers, which will be all sorts of inductances and all sorts of reactances.
the frequency will be below 60 khz.
equipment i have is: a 0-30 bench top psu, a gwinstek gfg-8215a signal generator, a scope and multimeters etc
i have some irf540n mosfets, so would like to use those if posssible.
i was initially at least going to keep the signals to single sided square wave.
i’ve never used mosfets and have heard they need different setup/biasing than bipolars and am scared i’ll kill the few semi’s i have before i get it right. thanking you in advance for your help.
 

MikeMl

Well-Known Member
Most Helpful Member
A big consideration with driving ferrites is to keep a DC component out of the excitation to the transformer primary. You effectively need a push-pull power amp with a symmetric output swing, none of which is easy with only NFets and a single power supply.

Can you spring for a second supply (to get split rails), and also buy something like this?
 

joolesison

New Member
i have a +0- power supply and a couple of irf9540n but was hoping i could make the equivalent of an emitter follower circuit with the mosfets and keep it simple? is this not possible?
 

Colin

Active Member
You need to know the output of the generator in mV and its output impedance.
Then you need to know the impedance of the transformer and the turns ratio. This will give you some idea of the output waveform that you will get.
 

crutschow

Well-Known Member
Most Helpful Member
To address Mike's concern about a DC component in the transformer signal, you can couple the output through a large capacitor.
That will guarantee there is no DC current into the transformer, even for an asymmetric waveform in both ±voltage levels and duty-cycle.
 

joolesison

New Member
hi colin, it's a "standard" signal generator 50 ohm output. i thought the beauty/purpose of a buffer amp was it presented a high in/low out impedance that would allow varying loads to be connected? is it not possible? i don't want a lab grade device here just something that allows a bit of latitude for "messing around"?
 

MikeMl

Well-Known Member
Most Helpful Member
hi colin, it's a "standard" signal generator 50 ohm output. i thought the beauty/purpose of a buffer amp was it presented a high in/low out impedance that would allow varying loads to be connected? is it not possible? i don't want a lab grade device here just something that allows a bit of latitude for "messing around"?
The problem with a source-follower, even if you follow it with a huge electrolytic DC blocking capacitor, is that it will actively pull-high like mad (source current), but has very whimpy passive pull-down (current-sinking), which will manifest as severe distortion at 60kHz. You need a true power-amp, like a push-pull audio amp, which can symmetrically actively both source and sink current.

The only way to make the source-follower work is by letting it waste huge power. For example, here is what it takes to get about 1.5W (+-25V) into a 50 Ohm load using a very-inefficient source follower:

51.png

Note the dissipation in M1 and R1. We are pouring 35W in to get ~1.5W out. It is effectively a Class A Power amplifer...
 
Last edited:

Colin

Active Member
With a source-follower, you are going to lose the first 2v to 3v of the signal.

Output voltage can be up to 10 V peak-to-peak. What is yours?
 

joolesison

New Member
Hi MikeMI, i thought of an "off the shelf" audio amp but wouldn't the very low impedance loads be seen as a short circuit and it would shut down?
 

crutschow

Well-Known Member
Most Helpful Member
Hi MikeMI, i thought of an "off the shelf" audio amp but wouldn't the very low impedance loads be seen as a short circuit and it would shut down?
Audio amps generally shut down on overcurrent, so as long as the combination of load impedance and output voltage does not exceed the maximum current rating (which is likely some above it's power rating into an 8Ω load) it should keep operating.
However continuous operation at the maximum current into a low impedance load would likely cause overheating of the output transistors, so you would need to be careful of that.

For example a 50W rated amp into a 8Ω load would have to generate a maximum output current of 2.5A into that load.
If the resistance is less than 8Ω, then you would want to limit the current below 2.5A to keep the heating within limits.
At very low load resistances you would probably limit the the voltage so that the output current doesn't go much over an amp.
 
Status
Not open for further replies.

Latest threads

EE World Online Articles

Loading
Top