# Class D output filter high impedence

#### artmaster547

Joined Jan 6, 2016
409
Hi all
I have an issue I want to use a class D amplifier to drive a piezoelectric load, I am modelling this as just a resistive load of 400 ohms, (37.5W, voltage swing between +175V and -175V). Could anyone suggest what kind of filter I should use on the output, the class D FETs will be driven at a minimum frequency of 100kHz and I need a AC sinusoidal output of 1kHz? I have seen it being traditionally with an LC filter when calculating these values I got an extremely large inductance of 59mH, and wondered if anyone had any suggestion if I could use anything else that uses components of smaller magnitudes and are more accessible?

Kind Regards
Art

#### Hymie

Joined Mar 30, 2018
1,115
With the parameters you have given (+/-175V switching at 100kHz into a 400Ω resistive load), using LTspice - with a filter made up of a 10mH inductor followed by a 0.1uF capacitor (to 0V) reduces the switching voltage to around 5V (peak to peak) at the load.

If you play around with these values (or add a second filter stage), you could reduce this voltage further.

#### artmaster547

Joined Jan 6, 2016
409
With the parameters you have given (+/-175V switching at 100kHz into a 400Ω resistive load), using LTspice - with a filter made up of a 10mH inductor followed by a 0.1uF capacitor (to 0V) reduces the switching voltage to around 5V (peak to peak) at the load.

If you play around with these values (or add a second filter stage), you could reduce this voltage further.
By reducing the switching voltage do you mean the filter attenuates? So rather than the load seeing +/- 175V it sees +/- 5V?

#### Hymie

Joined Mar 30, 2018
1,115
By reducing the switching voltage do you mean the filter attenuates? So rather than the load seeing +/- 175V it sees +/- 5V?
Yes.

#### artmaster547

Joined Jan 6, 2016
409
Ah I see I really want to avoid this attenuation is there a way to stop attenuation is this only possibly by having the larger inductor value and small capacitor value I mentioned earlier?

#### Hymie

Joined Mar 30, 2018
1,115
If you want to avoid this attenuation, just apply the output +/- 175V across the load.

#### artmaster547

Joined Jan 6, 2016
409
If you want to avoid this attenuation, just apply the output +/- 175V across the load.
However I want it to be smoothed as in sinusoidal

#### Hymie

Joined Mar 30, 2018
1,115
The D class amplifier is switching the output at high frequency; the audio frequency modulates the high frequency such that at 1kHz, the above filter is applying virtually no attenuation, so the 400Ω load will see 1kHz (350V peak to peak) – but the high frequency component is removed.