5 Watt Audio Amplifier

[clemsontiger]

Im a student and I need to design an amplifier using an opamp, and I dont know what opamp to use. I have to deliver a 5 Watt output to an 8 Ohm load also with a flat electrical response of 20Hz - 18KHz. What IC should I use? LM383? 741? And Im not even sure how to start designing one of these as far as a circuit goes. Should I just start with a basic inverting configuration? Or non-inverting configuration? Is there a better configuration to use? Should I have to consider heat sink issues?

[ante]

Hi clem,

The LM383 and 741 are not going to deliver 5W, you should try something like the TDA2003 or similar. You will find application examples in its datasheet.

[audioguru]

Hi Clemson,
I am sorry to correct those other guys, but the LM383, TDA2003 and TBA810 are all designed to deliver a fairly low output voltage to a 4 ohm speaker when supplied by a 12V car battery. To get 5W into 8 ohms requires a signal that is nearly 18V peak-to-peak. Since an amplifier is not perfect and has voltage losses, to output 18V p-p into 8 ohms it must operate with at least a 22V supply. That is too high for those amps.

I tried a search for a suitable amp on National Semi's newly redesigned website but got many errors. I asked for mono amps but got stereo ones, I asked for 5W/8 ohms but got 2W/4 ohms, etc.

Philips and ST Micro have many amp ICs. Try a search on their sites.

[MP]

Not sure who you think you are correcting, but Clem does not state that he needs to power this from a 12 V car battery.

MP

[audioguru]

Hi MP,
Clemson needs 5W into 8 ohms. I was stating that the "car radio" amps that were recommended by others can't do that, and I don't think that he wants to bridge them.

[clemsontiger]

You make a good point, I have to use a 12 volt power supply. Its in the specifications for my design. I apologize for the confusion.

[audioguru]

Hi Clemson,
Your 12V supply requirement certainly makes a big difference to the design of your amplifier. As I said earlier, to get 5W into 8 ohms, you need nearly 18V p-p, which is not normally available from a 12V supply.
Therefore you need two amplifiers, connected in a "bridge" configuration, where one amplifier drives one of the speaker wires and the other amp drives the other speaker wire with reversed polarity, effectively doubling the voltage across the speaker. Car radios do it this way.
There are many dual amplifier ICs available that will easily give 5W bridged into 8 ohms. They are not opamps, are they allowed? You could always design your own dual amplifier with 2 opamps and 4 darlington transistors, if you must use opamps. The amplifier ICs and opamps with transistors are analog and are therefore inefficient, so will need a heatsink.

If allowed, I would go "high-tech" and blow away the instructor and the others in the class. I would use one of the latest "class-D" audio power amp ICs available from Texas Instruments. Their TPA3001D1 provides about 6.5W into 8 ohms at low distortion when operating on a 12V supply. With its 85% efficiency, it doesn't need a heatsink. However, like most modern electronic parts, its package is surface-mount, so its 24 pins are very small and crowded. It would take good skill to solder the thing. Check the datasheet and see if you can get a sample IC from TI:
http://www-s.ti.com/sc/ds/tpa3001d1.pdf

[clemsontiger]

Thanks for the help. I dont HAVE to use opamps. But I must simulate the circuit to show a frequency response of 20Hz-18KHz, and a power delivered of 5W to an 8 Ohm load. And I knew that using transistors would be extremely difficult to do. So in my proposal, I decided I would use opamps/ic's. To simulate this program all I have is PSPICE which doesnt half work and Im not real proficient with it either. Does anyone have any ideas as to how to go through with the simulation?