Output resistance of an audio amplifier

[afr123]

Hi,

I am using LM4950 audio amplifier and I am interested to know its output impedance. There is nothing mentioned in the datasheet except that it can provide 7.5W at 8 ohm load (at Vdd = 12V, 10% THD, and 1 kHz). How do I calculate the output impedance?

Regards

 

[frankrose]

It is not so important, TI just don't give it, should be in mOhm range somewhere. But if you have an accurate spice model for the device you can simulate the output resistance.

 

[Audioguru]

The output impedance of a modern audio power amplifier is extremely low so that it damps resonances of the speaker. It is 0.04 ohms or less. The very high open loop gain and the closed loop gain by a lot of negative feedback reduces its output impedance to be so low.
Why do you need to know its output impedance? Your wires connecting its speaker will measure much higher and its speaker will also measure much higher.
The amplifier is an AC voltage source, reducing its load resistance increases its output current until it is destroyed by too much current or by the excessive heat produced.

I think about 60 years ago an amplifier output impedance matched the impedance of the speaker with a vacuum tubes amplifier that had an output impedance matching transformer. Modern amplifiers do not have an output transformer.

 

[afr123]

Some concepts are not clear to me.

I have a piezoelectric transducer and I have to drive that to generate ultrasonic signals. What I am thinking is to drive it by an audio amplifier and a transformer. I am not really sure whether I need impedance matching here. The piezoelectric input impedance is around 900 ohms and I am planning to use 1:10 transformer but still it won't be matched as the source impedance of the audio amplifier is too low. My driving frequency is around 60 kHz.

1. My question is do I need impedance matching?

2. My wire length is not more than 20 cm so I guess it won't act as a transmission line and I don't need impedance matching for minimizing reflections?

3. Is maximizing voltage important or maximizing power or minimizing reflections when it comes to piezoelectric transducers ?

Thanks alot.

 

[Audioguru]

The 1:10 transformer will cause the 900 ohm impedance of the piezo to be 90 ohms on the transformer. If you have the amplifier produce maximum output to the transformer with a 12V battery then the power to the 90 ohms load is about 0.67W.
If you match impedances then you throw away half the power then the piezo gets only 0.33W.

But the amplifier is not designed to drive the high capacitance of a powerful piezo which is worse with the transformer. It can drive a low power 200pF piezo at a low frequency if you add two capacitors.
Your piezo might have a capacitance of 3000pF which would be transformed to be 30,000pF at the output of the amplifier that is not allowed.

Why are you using an audio amplifier that is not designed to drive a high power piezo at a frequency much higher than audio? You need only a couple of output transistors plus a simple driver circuit. This simple circuit will work because it does not need negative feedback for low distortion.

 

[KlausST]

Hi,

The 1:10 transformer will cause the 900 ohm impedance of the piezo to be 90 ohms on the transformer.

Is this a 1:10 impedance transformer?

Usually the ratio is the voltage ratio...thus a 1:10 voltage ratio will become a 1:100 impedance ratio.

900 Ohms --> 9 Ohms.

Klaus

 

[FvM]

Regarding post #4.
1. No
2. No
3. You're starting the design at the wrong end.

Start with intended transducer waveform, signal magnitude, possible relation between source impedance and transducer bandwidth in pulsed applications. Chose the driver amplifier accordingly, add resistive termination if appropriate. In case of a resonant transducer, there's littler advantage in using a linear amplifier. Most transducer drivers are just switchers.