Amplifier TV specs for reception

[Leiser]

Hi all,

I have some customer specifications for a receiver TV amplifier. Noise figure is stated as a max. of 4dB. Does anyone know why is it higher than for the FM band which is 1.5dB?

And a second question. Can anyone guide on the possibility of doing a TV receiver amplifier using an impedance converter? I need 12-8dB of gain.

Thanks!

 

[VEBill]

FM band is 88-108 MHz. TV is 54-400ish(?) MHz (keeps getting cut back, not sure where it ends these days). In general, the higher the frequency, the more difficult it is to achieve a low NF. But neither of those NF limits appear to be very challenging. Ham radio LNAs for the 430-450 MHz band are generally a small fraction of 1dB (like 0.2dB for example).

An 'impedance converter' is not necessarily an amplifier. It might just be a transformer.

 

[Leiser]

Hi,

I was reading your comments and then I realized that noise must be related to the bandwidth in which is measured. TV bandwidth is, as you mentioned very high, from 47MHz Band I in Europe to 862MHz, so I guess it is not possible to have noise figure values of 1dB in such a big BW.

The problem I am expiriencing with the TV amplifier is that the linearity requirement is quite hard to meet using discrete transistors, i.e, avago ATF-531P8, 60dB for Pin 100dBuV. I heard that there might be a different approach, using an impedance converter, but that´s all I know. How can I design an amplifier with 12dB gain, 4dB noise figure and that linearity specs? any hints?

 

[VEBill]

Noise is obviously (being noise) spread over a bandwidth and the noise energy is therefore more-or-less proportional to the bandwidth.

But the Noise Figure (or Noise Factor) is a measure that is not directly affected by the bandwidth.

For example, ham radio UHF band preamplifiers easily achieve 0.2dB NF, but they would NOT be improved to 0.002dB or 0.0002dB or 0.000002dB NF by adding narrow band filtering.


Linearity simply means keeping both in-band and out-of-band signals from exceeding the headroom of your devices. It's not unheard of to use high voltage rails in the LNA (sort-of building the LNA as if it were a power amplifier stage). Adding an attenuator at the front end can help, although its loss chews into your NF allowance.