I would like to ask the teachers: Class D amplifier problem

杨旭煜

I would like to ask the teachers: Class D amplifier problem [Copy link]

 

I would like to ask the teachers: Can a Class D amplifier be used to amplify a DC signal that changes with time?

High power TDA8954TH dual chip Class D digital amplifier board

chunyang

A DC signal that changes with time can be seen as a DC superimposed AC signal with a very low frequency. All power amplifiers, regardless of their type, have bandwidth parameters. If the bandwidth parameters are exceeded, they cannot be used.

maychang

The class-D power amplifier TDA8954T can amplify DC signals, but the board in the picture you posted probably cannot.

chunyang

For DC superimposed on extremely low frequency AC, if the output polarity remains unchanged, the best way is to use a voltage-stabilized power supply circuit, either a linear power supply or a switching power supply, and this can be achieved by controlling its output voltage. Specific means can include using a digital potentiometer or DAC, etc.

杨旭煜

maychang posted on 2019-7-5 16:04 The class D power amplifier TDA8954T can amplify DC signals, but the board in the picture you posted may not be able to do so.

Please tell me, how should this finished power amplifier board be modified so that it can be used to amplify DC voltage?

For example, remove the coupling capacitor at the input.

杨旭煜

chunyang posted on 2019-7-5 16:07 For DC superimposed on extremely low frequency AC, if the output polarity remains unchanged, the best way is to use a voltage regulated power supply circuit, either linear power supply or switching power supply, control...

Hello teacher, the output polarity of the signal changes, and it varies between ±12V.

I used a high-power op amp before, which worked, but it generated a lot of heat.

Therefore, there is a contradiction between heat dissipation and chassis size. If a Class D amplifier is available, the size and weight should be much smaller.

maychang

Yang Xuyu published on 2019-7-5 16:07 Please ask the teacher, how should this finished power amplifier board be modified to amplify the DC voltage? For example, remove the coupling at the input end...

The board in the picture seems to have capacitors connected to both the signal input and output ends, and there seems to be an inductor wrapped around a magnetic ring at the output end.

Changing your board is not a good idea.

maychang

Yang Xuyu published on 2019-7-5 16:07 Please ask the teacher, how should this finished power amplifier board be modified to amplify the DC voltage? For example, remove the coupling at the input end...

Your output voltage needs to vary between ±12V. How much current should be used? You don't have to use a high-power amplifier like TDA8954T, right?

maychang

Yang Xuyu published on 2019-7-5 16:07 Please ask the teacher, how should this finished power amplifier board be modified to amplify the DC voltage? For example, remove the coupling at the input end...

"I previously used a high-power op amp, which worked, but generated a lot of heat"

Power amplifiers such as LM1875 and LM3886 do not work as switches, so they generate a lot of heat.

From this perspective, it is a good idea to use Class D amplifiers to replace power amplifiers such as LM1875 and LM3886.

chunyang

Yang Xuyu posted on 2019-7-5 16:15 Hello teacher, the output polarity of the signal is changing, and it varies between ±12V. I used a high-power op amp before, and I can use...

If Class AA and B amplifiers are available, Class D amplifiers can be used, but they require direct coupling and attention must be paid to the circuit. It is not necessarily necessary to remove the coupling capacitor.

杨旭煜

OK, thank you both teachers. I will ask the seller how to modify it.

maychang

Yang Xuyu published on 2019-7-5 16:07 Please ask the teacher, how should this finished power amplifier board be modified to amplify the DC voltage? For example, remove the coupling at the input end...

Not only TDA8954T, most Class D power amplifiers require positive and negative power supply, and the input is differential input. Single-power Class D amplifiers can only output H-bridge (BTL form). Your board is powered by positive and negative power supplies, and can achieve a single-ended output change between ±12V, but the input must still be differential input.

I guess you are using D/A to control the power output. If so, the single-ended signal output by the D/A must be converted into differential before controlling the TDA8954T. This is different from using LM1875 and LM3886 to achieve power amplification. LM1875 and LM3886 are single-ended input positive and negative signals.

杨旭煜

maychang published on 2019-7-5 17:00 Not only TDA8954T, most class D power amplifiers require positive and negative power supplies, and the input is differential input. Single-power class D amplifiers only...

OK, thank you for your advice. I will study the datasheet first and ask you if I have any questions.

杨旭煜

maychang posted on 2019-7-5 16:29 Your output voltage is required to vary between ±12V, how much current should it be? You don't have to use a high-power amplifier like TDA8954T, right?

I chose TDA8954 based on its output current. A single channel can output 12A,
but TDA8954 has two amplifiers. It is a bit of a waste to use only one channel.
My application circuit has a continuous current of about 5A and a short-term peak current of about 10A.
I wonder if you have any recommendations for a suitable single-channel Class D amplifier. This is more economical.

maychang

Yang Xuyu published on 2019-7-5 17:52 I chose TDA8954 based on its output current. A single channel can output 12A, but TDA8954 has two amplifiers. I only use one channel. It really works...

The output current requirement is 10A, so TDA8954 is suitable.

After thinking about it, I have never seen a single-channel Class D power amplifier that can output positive and negative voltages.

杨旭煜

maychang posted on 2019-7-5 18:13 The output current requirement is 10A, so TDA8954 is suitable. After thinking about it, I have never seen a single-channel Class D power supply that can output positive and negative voltages...

From the data, there is a LM4651/4652, but there is no finished amplifier board for sale.

I guess even if there is, the price may not be cheaper than TDA8954.

maychang

LM4651 and LM4652 are two different chips, LM4651 is the pre-stage and LM4652 is four power tubes.

LM4651 + LM4652 are also two amplifiers, using positive and negative power supplies, and each can have positive and negative outputs.

shaorc

maychang published on 2019-7-5 17:00 Not only TDA8954T, most class D power amplifiers require positive and negative power supplies, and the input is differential input. Single-power class D amplifiers only...

I saw some familiar knowledge points again. Thank you teacher for your guidance.

杨旭煜

maychang published on 2019-7-5 17:00 Not only TDA8954T, most class D power amplifiers require positive and negative power supplies, and the input is differential input. Single-power class D amplifiers only...

Teacher, I have two more questions:

1. TDA8954T has differential input, so a dual op amp preamplifier is needed to convert single-ended to balanced output.

2. How is the gain of TDA8954T adjusted?

maychang

Yang Xuyu posted on 2019-8-30 14:06 Teacher, I have two more questions: 1. TDA8954T has differential input, so it is necessary to add a dual op amp to form a single-ended to balanced output pre-stage. ...

2. How is the gain of TDA8954T adjusted?

TDA8954 has a fixed gain. If you want to adjust the gain, you can only do it in the front stage.