Calculating current draw of Class A amp

I'm simulating a Class A amp designed to output 0.5Watts into a 4 Ohm speaker. I'd like to power it from a 5V wall wart power supply but I need to calculate how much current the amp is drawing and how much current the power supply needs to be able to source.

The sim below shows the current across the Collector and Emitter from the two output transistors peaking at about 570mA. It's pretty similar across the frequency range from 20Hz to 20K Hz and similar again with a square wave input.

So from that - how do I calculate or approximate the the current requirements of a power supply for this amp or for the amp itself.



Thanks

The current into the load is approximately 0.35A so if you assume the amplifier is about 30% efficient it will draw about 3 times that amount so it needs 1A plus any other current drawn by the amplifier stages and then a safety margin added to it.

Brian.

betwixt said:

The current into the load is approximately 0.35A so if you assume the amplifier is about 30% efficient it will draw about 3 times that amount so it needs 1A plus any other current drawn by the amplifier stages and then a safety margin added to it.

Click to expand...

Thanks Brian - so the 0.35A is about the average of the current draw in the sims I posted - that's what I should be using as my current draw reference?

Then a worst case efficiency rating for the amp of about 30% plus some margin for error?

That pretty close to how you got your figures?

Thanks agin.

I worked out 0.35A as the current needed to deliver 0.5W into 4 Ohms and worked backwards from there. Class A amplifiers draw at least half their peak current even when no signal is present so to allow for peaks you should at least double the current. In reality, their efficiency is lower than 50% and other parts of the amplifier will need current too. The power supply rating you need depends upon the maximum you will draw from it and to some extent how long the current is drawn for, many PSUs can be safely overloaded for short periods without harm. The maximum current also depends on the kind of signal you are amplifying, the amplifier design and to some degree, on the frequency range you are using. For example, at very low frequencies (< 1Hz) there may be long times when the full supply voltage is across the load and the current is very high. 5V across a 4 Ohm load will at most require 5/4 A = 1.25 Amps.

Brian.

betwixt said:

I worked out 0.35A as the current needed to deliver 0.5W into 4 Ohms...

Click to expand...

How to calculate that is the bit I'm missing - (after some more research) did you use:

sqrt(P/R) where P = Power in Watts and R = Speaker/Load in Ohms?

Giving: sqrt(0.5/4) = 0.353...

Thanks - appreciated.

Yes, that was how I did it. Bear in mind that when a loudspeaker is described as 4 Ohms it is a 'nominal' value, it varies greatly across it's frequency range and depends on the air pressure it has to motivate.

Brian.

Picking up on betwixt's point on Loudspeaker Impedances, a minimum Impedance for the Loudspeaker you might use can be easily assessed by measuring the DC Coil resistance.
This will be very close to the minimum Impedance which will be much more useful when trying to assess Power requirements.
From many measurements of this type, I have found that a 4 Ohm Loudspeaker often has a minimum Impedance of 3 Ohms or less!

Mik

You didn't yet mention the top0logy of the intended two transistor class-A amplifier, e.g. transformer push-pull, half-bridge,...