# Amplify current without modifying voltage

#### Johnathan Crone

Joined Aug 29, 2018
2
Hi there,

I have a 3Vac signal which has a low current of around 50uA or so. I wish to amplify the current to a high enough level that it can drive a 32R load, probably around 250mA or higher. Any suggestions on the best way to go about this? My power supply (rails) are 5Vdc and -5Vdc. I have a LM386N-4 at my disposal but the limited voltage rails are posing a problem in trying to use it. I am willing to try a new component/design if anyone has a better suggestion.

Thanks.

#### -live wire-

Joined Dec 22, 2017
912
Is it 3VAC RMS or Peak? Either way, just look for a rail to rail CMOS power op amp. Look for an output of at least a few 100 mA. Rail to rail means it can go all the way to + or - of the supply. But what is the frequency? If it is high, you have to consider the slew rate and gain bandwidth product. Also, what is the waveform? Sine, triangle, square, or other?

#### Johnathan Crone

Joined Aug 29, 2018
2
Is it 3VAC RMS or Peak? Either way, just look for a rail to rail CMOS power op amp. Look for an output of at least a few 100 mA. Rail to rail means it can go all the way to + or - of the supply. But what is the frequency? If it is high, you have to consider the slew rate and gain bandwidth product. Also, what is the waveform? Sine, triangle, square, or other?

It's 3V amplitude sorry, so 6V peak-peak. The frequency of the input wave can vary from 1Hz up to 10kHz and the type of wave can be: sine, square, sawtooth, reverse sawtooth or triangle. Thanks, I'll have a look and see what components I can find.

#### Dodgydave

Joined Jun 22, 2012
9,837
Try a Tda2030/40 power amp.

#### ebp

Joined Feb 8, 2018
2,332
3 V into 32 ohms will result in a current of 94 mA. 5 V would yield 156 mA. If you want 250 mA into 32 ohms, you require voltage gain for a signal of 8 volts. Given the output swing of most power amp circuits, you would likely require supply rails of at least 10 volts for 8 V signal amplitude.

The waveforms described will require high bandwidth in the amplifier. It depends entirely on how accurately you want the waveforms to be reproduced, but a bandwidth of 100 kHz or more will likely be necessary.

#### -live wire-

Joined Dec 22, 2017
912
It's 3V amplitude sorry, so 6V peak-peak. The frequency of the input wave can vary from 1Hz up to 10kHz and the type of wave can be: sine, square, sawtooth, reverse sawtooth or triangle. Thanks, I'll have a look and see what components I can find.
You can never get a 100% true square, due to the slew rate. So think about how much it needs to look like a square. If there is a slew rate of x volts per μS, then the square wave will take 6V/x μ seconds. For a 10 kHz square wave, that's 50 μS per half cycle. So if the slew rate is just a few V per μS, then you will have some serious distortion. There are formulas for calculating the minimum slew rate for the other waveforms, but if you can easily handle a square wave, then you shouldn't have issues with other waveforms.

Additionally, if you want any sort of gain, you will need to have a high enough gain bandwidth product. GBP = gain * frequency.