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DC Power Amplifier
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PostPosted: Wed Sep 01, 2010 11:44 am    Post subject: DC Power Amplifier Reply with quote

Hi
I want a schematic for a power amplifier which works in the band of 0Hz to 1 KHz. Any useful links are appreciated.

Thanks
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torbjorn



Joined: 07 Jun 2007
Posts: 370
Location: Sweden

PostPosted: Thu Sep 02, 2010 11:42 am    Post subject: Reply with quote

Most audio power amplifiers have in fact a DC coupled output stage, so many such designs should be directly useable. To get any better advice, you should tell somewhat more about your application, for instance:
-Required output power/current/voltage?
-Load impedance?
-Single ended or balanced output?
-Will the load be especially "difficult" in any way (for instance, heavily capacitive or inductive, "live" load with back EMF like a servo motor with flywheel)?
-Has the amplifier to be fool proof regarding short circuits, sudden disconnection of load, connection of supply with wrong polarity etc?
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PostPosted: Fri Sep 03, 2010 3:16 pm    Post subject: Reply with quote

Dear torbjorn

I tried with one audio amplifier but it distorts the square wave signals especially at very low frequency of 1 or 2 Hz.
-the required output current of 25 Amp, 800Watt
-the load impedance 1-2 ohms
-single ended
-inductive load
-I don't need a complicated design, only within the above specification. The important issue is that it should not distort the signal (square wave) at very low frequencies.

Thanks and regards
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Gatomas



Joined: 13 Sep 2010
Posts: 1
Location: Texas

PostPosted: Mon Sep 13, 2010 3:51 pm    Post subject: Re: DC Power Amplifier Reply with quote

A square waveform involves high frequency components and at such low rate and high frequencies I wonder if a solid state rely would do the trick.
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Gatomas
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JD406



Joined: 16 Sep 2010
Posts: 2

PostPosted: Thu Sep 16, 2010 6:42 pm    Post subject: Reply with quote

Can you be more specific about your application? Exactly what are you trying to drive?

Whilst most audio amplifiers have DC coupled outputs, very few if any will operate at DC since they are usually AC coupled at some stage in the signal path. Thus you should expect significant distortion of square waves at frequencies below about 10 Hz.

A simple half or full bridge arrangement from the power supply might be more appropriate and easiest to do (Solid state relay idea of Gatomas). If you decide to go that way, ensure you use good (short and thick) wiring layout and diodes to catch any fly-back emf in the load is inductive.

Fir ideas on how to do bridge circuits check out the application notes and technical documents here:
http://www.irf.com/indexnsw.html
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PostPosted: Wed Sep 22, 2010 12:18 pm    Post subject: Reply with quote

Hi JD406

It sounds a good idea to use a bridge circuit with ssr. My question is : can this arrangement provide bipolar squarewave? Can you please decribe the circuit if it is possible.

Thanks
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JD406



Joined: 16 Sep 2010
Posts: 2

PostPosted: Wed Sep 22, 2010 6:28 pm    Post subject: Reply with quote

Anonymous wrote:
Hi JD406

It sounds a good idea to use a bridge circuit with ssr. My question is : can this arrangement provide bipolar squarewave? Can you please decribe the circuit if it is possible.

Thanks


A full bridge drive will provide bi-directional drive, i.e. it will drive current into a load in both directions and produce no drive (off). These three drive states can be applied to make bipolar drives. Depending on what you need and the nature of the load, there are various configurations that you could try. A half bridge (simplest) might be suitable if you have + & - supplies but if you only have a single supply then a full (H) bridge will probably be required.

Note: Bridge arrangements can only provide fixed voltage outputs at the supply level (unless modulated which adds a lot of complexity - see digital amplifier [D-class]).

This link is to a simple full bridge circuit that could be used as a starting point: http://www3.telus.net/chemelec/Projects/H-Bridge/H-Bridge-1.png
[http://www3.telus.net/chemelec/Projects/H-Bridge/H-Bridge.htm]

I did not design this circuit, not have I used it but I cannot see any major flaws in it.

It is suitable for controlling the direction of motors/solenoids and similar. There is no current limit so it would be sensible to add a fuse in the supply line. Heatsinks will likely be necessary on the main MOSFETs and if the drive current is calculated to be close to the MOSFET limit then extra devices can be placed in parallel to share it.
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PostPosted: Mon Sep 27, 2010 3:35 pm    Post subject: Reply with quote

Hi JD406

I think the DC supply and the H-bridge will give a good bipolar ouput voltage but the current shape will not be good since I am driving an inductive load.
My question: is there any kind of addition to this circuit to improve the shape of the current waveform? something like negative current feedback used in amplifiers or any other arrangement?
What is importnat to me is that the current should be a square waveform.

Many thanks
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Guest






PostPosted: Mon Sep 27, 2010 7:27 pm    Post subject: Reply with quote

Anonymous wrote:
Hi JD406

I think the DC supply and the H-bridge will give a good bipolar ouput voltage but the current shape will not be good since I am driving an inductive load.
My question: is there any kind of addition to this circuit to improve the shape of the current waveform? something like negative current feedback used in amplifiers or any other arrangement?
What is importnat to me is that the current should be a square waveform.

Many thanks


You will need a quite sophisticated system to achieve that kind of output. It will need a control loop with current sensing. That would probably be more of a project than you’d want to take on.
However, there is one simple option that you might try, assuming you can find a suitable power supply. If you get a PSU with current limiting, you could set the limit, at your required output, and use it with the full (H) bridge arrangement. Many bench power supplies have this facility but at the power levels you are looking at they will be expensive. However, this places the current limiting complexity in the power supply leaving the switching drive simple.
The supply will also need to have a much higher maximum voltage output than for a resistive load. Inductive spikes at the switching transitions may present a problem and require suitable snubbing circuitry.
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Guest






PostPosted: Tue Oct 19, 2010 7:07 am    Post subject: Reply with quote

I agree that square waveform involves high frequency.
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