Function Generator to Audio Amplifier

Question

I am looking to control a Helmholtz coil with a function generator but I need a waveform amplifier to increase the current and voltage from the function generator. The coil has to be run using low-frequency AC signals from 10 Hz to 100 Hz.

I've seen a few expensive waveform amplifiers but I was wondering if anyone has any experience using an audio amplifier to amplify a function generator and how they did it.

Edit 1: This is a coil setup I'm looking at using Helmholtz Coil. I would need a maximum field of 30 G so that would require a current of about 4 A and with the DC resistance of 2.6 Ω, as they would be run in parallel, a voltage of about 15 V (these values are actually higher than what would be needed I'm just trying to account for the worst-case). As these coils are designed for DC experiments and not AC their inductance and self-capacitance are unknown.

The reason why I am not certain about the coils I will be using is because I would like to optimize them as much as possible so I may need to create my own. However, I do expect to use a low winding, low resistance set up similar to the amazon coils so I will need an amplifier capable of output around 5 A with 20 V

Answer 1

I've seen this question before...

It depends how much your coil looks like a loudspeaker.

If its DC resistance is above a few ohms, then it's a reasonable approximation of a 4 ohms loudspeaker, whose DC resistance is usually around 3 ohms. In this case an audio amp rated for 4 ohms will work fine.

You can use an audio amp from the pawn shop, or a kit from the web, for example. If it doesn't go down to 10Hz you will have to increase the value of the signal coupling capacitors, which limit low frequency cutoff. I'm mentioning a kit because it's easier to modify.

If DC resistance is below a few ohms then you'll need some precautions.

Basically what determines how low an impedance an amplifier can drive is the safe operating area of the output transistors, and their maximum output current. You can't do anything about the max output current besides using more transistors in the output stage, but if your coil has low impedance you can increase the safe operating area by using a lower power supply voltage. Since the power dissipated in the output stage transistors of the amplifier is V*I, for a low impedance load, if you decrease supply voltage, for the same dissipation, more current is allowed.

If you have a couple spare bench power supplies, a LM3886 or TDA7293/7294 kit would be a nice choice, also very cheap. If it doesn't go down to 10Hz, just increase the value of the coupling cap, or replace it with a short.

If you don't mind a bit of extra noise, you can also use a class-D amp kit.

Answer 2

A Helmholtz coil would require a current source rather than voltage source. The output voltage of the audio amplifier is proportional to the signal input voltage.

What you would need is an amplifier whom output current is proportional to the input voltage - voltage controlled current source. This can be done with an operational amplifier, but you should know more datails about the project setup - coil:

• max current
• inductance of the coil
• coil resistance
• do you want DC or AC coupling. DC would give you maximally flat response, but it needs a dual power supply.

A Helmholtz setup is usually used for "focusing" and analyzing the object's magnetic field density with its uniform magnetic field, feed by DC constant current. What is the goal of using AC filed in a Helmholtz setup?

It will probably need a high voltage operational amplifier to drive a high inductance coil, so I doubt the insulation could sustain it.

Answer 3

It's easy enough provided the two items can be matched up.

Obviously, you need an amp that goes down to 10 Hz. Not many audio amps do that, so check the specs very carefully.

More technically, the output of the generator must also be electrically matched to the input of the amp. For example the amp may have an impedance of 1 kΩ and require an input of 100 mV max. So the generator will need an output impedance of 1kΩ or less and an output adjustable to somewhere in the 10-100 mV range. If they are not matched, then you will need some sort of simple interface circuit.