Intermodulation in an Amplifier and Third Order Incercept

[Natalie Johnson]

Hi,

I have measured a Transfer function curve for an amplifier, showing Input power vs output power. It shows non-linear increases for a portion of the transfer curve (at higher input power).

I also measured 3rd order intermodulation products during the input power range where it gives non linear output. I measured 3 Amplifiers.

On one Amplifier, the ratio of gradients was 3:1 (intermodulation power vs input power : output power vs input power) but the other two were not the theoretical 3:1 . They were 6:1 and 7:1. But from all three I obtained the correct Third Order Incercept as the manufacturers datasheet.

Why might amplifiers have intermodulation higher than the theoretical 3:1 ? If they cant, it must be an error with my measurements?

Thanks

 

[sophiecentaur]

The harmonic performance will depend on the amplifier which depends on the output device and the circuit design. Third order intercept assessment is only useful for comparing amps with mild non linearity. What amp type are you measuring?

 

[Natalie Johnson]

My measurements are based on a computer model. The computer model takes a waveform an amplifies it according to the manufacturers transfer curve of the amplifier. The amplified waveform is correct.

The intermodulation products are captured from a fast Fourier transform.

So basically, as long as I get the third order intercept correct - my model must be correctly obtaining the levels of intermodulation products. From what your saying, amplifiers in theory do not always have 3:1 ratio?

 

[sophiecentaur]

From what your saying, amplifiers in theory do not always have 3:1 ratio?

Why should they? The third order intercept is a fair way to asses an amplifier where the transfer characteristic can be characterised with a simple polynomial. You can's assume that the result for all amplifiers will be the same.
Another way of assessing amplifier linearity is to measure intermodulation products from a two or three tone input signal. That can be very useful and repeatable for comparing amplifiers of different technologies.
It all depends.