Boosting a Frequency Generator Output

Question

My generator will output a 20Vp-p sine wave, resulting in a 7Vrms AC voltage (10V * 0.7) I want to boost this to >=12Vrms AC and I thought an Op Amp would do that; if I use a gain of x2 that would suffice for my purpose. The problem is I can't get it to work and I suspect I'm doing something stupid that I just can't see. The Op Amp I have is a TI UA741

I don't care about quality nor if the signal is clipped - common mode input voltage range is +-13V at 15Vin and I could increase Vin to +-18V. At the moment, I'm getting a wholly unexpected output which makes me think I'm doing something wrong. I originally thought it was because I'd got the ground wrong but when I change the ground point of the Op Amp output to the Ground of the Function Generator, it still doesn't work AND channel 1 of the PSU oscillates between 0V and 32V!

Is there a better way that I can do this with parts that I might have at hand? Ultimately, I want to be able to variably alter the output voltage level to amounts < 12Vac and alter the frequency (hence why I thought using my frequency generator was a good idea)

three-phase · Accepted Answer

Hello Andrew,

Some of my thoughts for you.

Do you have the ground from the waveform generator connected to the ground for the op-amp? I would have thought all the grounds should be connected together, they are when I build up an amplifier to increase the output on my waveform generators. From your drawing it looks like the ground for the waveform generator is floating.

I am not sure of the 1 kOhm resistor from the op-amp output to the ground, is this providing too much load for the op-amp?

I usually put a resistor in series from the waveform generator into the non-inverting input of the amplifier, to limit current, but I don't think this stops the circuit functioning basically. Same with de-coupling capacitors on the supply rails.

Have you checked that the PSU and waveform generators have isolated outputs and you aren't creating a circulating current somewhere that is causing your supply to current limit and drop the voltage down to 0V? Not sure why it would then want to go up to 32V though.

At the moment your amplifier is set up with a 2 to 1 ratio, and putting in a 20Vpp signal. With the supply only being 30Vpp, you would be overdriving the amplifier. What happens if you decrease the input signal down to 10Vpp or even 5Vpp. You would then be within the limits of the power supply rails and output of the op-amp.

Are you sure the op-amp is good?

It is the same method as I have used in the past, so it should work for you.

Kind regards

shabaz · Answer

Hi Andrew, the 741 (and some other old op-amps like TL0x1 series) behave weirdly enough outside their (often quite limited) specifications personally I try not to use them when experimenting, although they have their advantages like ultra-low cost. What frequency range do you want to work at? Another option (don't know if it is relevant though, depends on if you can find the part or not), is to find a transformer. It will solve any ground issues too, since the output will then be floating. If you're in the audio frequency range then if you can find a audio transformer (vintage equipment! maybe something from some NMOC stash?) that could be usable. Otherwise (say) R/Z1606 will work for about 500Hz upward in the audio range. It is 1:1, but with a centre tap secondary, so the secondary can be used as the primary, using half of the winding, so that it becomes 1:2 ratio. You won't be able to load it very much though. I think an audio transformer can be a great problem-solver for working with signal generators.

michaelkellett · Answer

phoenixcomm You're aiming for 115 * 5 = 575W minimum with a hope for double that. When I needed to do this last I bought a no name mega disco audio amplifier for about £200. It can manage about 1500W out in bridge mode - I couldn't get the parts for the cost of the ready made thing. I used a standard 750W mains toroidal transformer to get from amplifier to (in my case) 240V. If you want to make it yourself (and cheap) the best bet is to start off from which of the expensive bits you have, like output transformer, power supply (if it will run from mains), box, heat sinks. There are loads of MOSFET power amp designs on the web and plenty of very cheap Chinese sources for them, so again you can make or buy. Some of the designs on the web are awful, some look OK, especially for 400Hz, the cheapest use all N channel MOSFETs. This is a typical offering: https://www.aliexpress.com/item/1005001874598103.html?spm=a2g0o.search0302.0.0.20461b7056Lye8&algo_pvid=f86a1e08-1205-40… No idea if it's any good ! If you find an interesting design you should definitely simulate it before buying any bits. This ready made amp from CPC would do nicely but might be a bit pricey: https://cpc.farnell.com/crown/xli2500/amplifier-2-x-750w-xli2500/dp/DP33156 MK

ralphjy · Answer

I agree with Donald, what you trying to achieve is outside the capabilities of a 741. From your description I think you’ve damaged the opamp. You could try a higher voltage opamp like a TI OPA990, but you probably don’t have one of those at hand.

three-phase · Answer

Good to hear you have to working. I believe that the resistor on the output of the amplifier should be in series with the load and is there to limit current if someone shorts out the output of the amplifier. Commoning out the ground references is usually required. As far the the electronics end is concerned, it isn't usually a problem. At the power end, it can be an issue if there is no isolation from the mains to the DC. When I built my high power AC PSU, I used some switchmode PSU boards to provide the DC rails, and of course they aren't isolated. So when I hooked up my waveform generator to the amplifier input, a not so nice current circulated around the waveform generator and PSUs. As MK has pointed out above, the easiest way to do this is to by a power amplifier, but not if you want to cut costs or build the circuit yourself. I guess the uA741 isn't the best, but as long as it can power the load and you aren't looking for any finesse, then I don't see any issues. Kind regards.

Andrew J · Answer

Thanks Donald,

Do you have the ground from the waveform generator connected to the ground for the op-amp? I would have thought all the grounds should be connected together, they are when I build up an amplifier to increase the output on my waveform generators. From your drawing it looks like the ground for the waveform generator is floating.

No, I have it as you see - not connected. I didn't think it right to link it's ground to what I have as GNDREF but I can give that a go in the morning if it isn't going to damage anything. You would connect what I have as Pin1 on the generator to GNDREF?

Have you checked that the PSU and waveform generators have isolated outputs and you aren't creating a circulating current somewhere that is causing your supply to current limit and drop the voltage down to 0V? Not sure why it would then want to go up to 32V though.

It only does that if I connect the non-inverting input and output to the ground of the generator rather than GNDREF.

At the moment your amplifier is set up with a 2 to 1 ratio, and putting in a 20Vpp signal. With the supply only being 30Vpp, you would be overdriving the amplifier. What happens if you decrease the input signal down to 10Vpp or even 5Vpp. You would then be within the limits of the power supply rails and output of the op-amp.

I don't understand what you mean Donald. The output of the generator is 20Vpp with a 0V dc offset so +10V/-10V hence the 7Vac (actually Vac = Vpp/2 * 0.7). Isn't this within the rail limits of +15V/-15V? I could set the inputs to +-18V which may prevent top-end clipping but I don't think that's what you are referring to.

I am not sure of the 1 kOhm resistor from the op-amp output to the ground, is this providing too much load for the op-amp?

Yes it is! Removing it will (temporarily) give a x2 output. But whilst the input stays stable, the output slowly and inexorably drops towards 0V as if the DMM is pulling it down? At least it's a step forward.

It is the same method as I have used in the past, so it should work for you.

That's good to know, at least I'm not a complete idiot

It would seem then that the load was wrong but a test circuit in the datasheet includes a 2K resistor on the output. Any more ideas?

three-phase · Answer

When I built my 240V / 400Hz supply for testing AVRs, I used a PA94 amplifier from Apex Microtechnology. I didn't need as much current as you do. Looking at their website, they probably have an amplifier that you could utilise; Apex Microtechnology amplifier range Make sure you are sat down when checking the price if you find one that meets your requirements. Having built one, I kind of agree with MK that a pre-built audio amplifier is the easiest and most cost effective route to go. Kind regards.

Boosting a Frequency Generator Output

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