# Design of a current amplifier capable of running inductive coils at low frequencies

#### morgancasale

Joined Nov 21, 2023
1
Hello everyone, I'm a student working on my master thesis involving using a flexible coil to build a haptic wearable device;
this coil has an impedance of 2.2Ohm and needs to be supplied with an AC signal of varying frequency ranging from 1Hz to 500Hz which current needs to go up to 0.5A, so I have to find a way to amplify the current of a signal generator.

The first solution I came up with was to use an of the shelf audio amplifier, but I had no luck finding one that supported below hearing frequency ranges.
I then turned my attention to power op-amps but I found little documentation about how to use them in such a scenario.
I tried to implement some simple transconductance and current amplifiers with the power amplifier we had in our laboratory (L272 and a TDA2006) with little success...
I never worked with power electronics so I must be missing some important theory knowledge.

Morgan

#### crutschow

Joined Mar 14, 2008
34,073
Since this a apparently a battery powered device, I would go with an efficient, switching (Class D) audio amplifier.
The low frequency response is determined by the signal coupling capacitor(s) size, which are usually sized to give a low frequency response of 20Hz, so you would have to increase the values of those by about a factor of 20.
An amp rated for at least 2W into an 8 ohm speaker load should give you the needed current.

If this is just for a demo, and you don't care about battery life, than a power op amp should work.
An example is an OPA564.
You just connect it as a two-resistor inverting amp with the desired gain to give the output you want.
What about that don't you understand?

#### Ian0

Joined Aug 7, 2020
9,531
What is its inductance?

#### BobTPH

Joined Jun 5, 2013
8,686
What is its inductance?
Yes. The higher the inductance, the more voltage you will need to push 0.5A through the coil at 500Hz.

For example, a modest 100mH inductance will require 157V.

#### Ya’akov

Joined Jan 27, 2019
8,975
Welcome to AAC.

It is a haptic device, how does the coil interact with the actuator?

#### Alec_t

Joined Sep 17, 2013
14,234
this coil has an impedance of 2.2Ohm
Is that the DC resistance? The impedance will increase with frequency.

#### Audioguru again

Joined Oct 21, 2019
6,620
A haptic coil will not produce a vibration unless it has a magnet or piece of iron near it.
A loudspeaker has a coil used as an electromagnet that vibrates over a magnet when an AC or DC current is in the coil. A loudspeaker can vibrate at 1Hz to many Hz.

I agree that you must simply calculate the capacitance of the lowpass capacitors to work at 1Hz in the amplifier circuit.

#### MisterBill2

Joined Jan 23, 2018
17,884
An amplifier like my SEISMIC BASS amplifier claims to deliver 100 watts down to 20 Hz. But it is not close to portable at all. So it would need a cable to connect to the wearer of the coil.
And certainly the intense magnetic pulse can be felt. At least I felt the pulses in a rather damaged shoulder during an MRI focused on that area. Evidently the magnetic field pulse moved the iron in the blood in the area. But that magnetic field was probably a lot stronger.