noise figure of a cascade system

[dot13]

noise of cascade system

i found that in 'RF Microelectronics' of Razavi, the noise figure of a cascaded 2-stages system is given as:
NF(tot)=NF1+(NF2-1)/Ap1
where Ap1 means the power gain of stage one.

so there is a problem: if the Ap1<1, (for example, the LNA used in TV-tuner always need to have a voltage gain from -10~10dB), the total NF will be much large! Isn't that? Does the LNA acts as a Noise Boosting Amplifier at then?

thanks

 

[tapa]

noise figure cascade

An aplifier must have a NF diferent for each gain, so you can't supouse NF constant as you vary the gain. And if an amplifier has negative gain, it is an atenuator... Realy is wors than an atenuator, because as it is active allwais add some extra noise.

TaPa