Improving Amplifier Noise Figure

　　Wide spurious-free dynamic range certainly the goal for any IF amplifier. This is particularly true for OTH radar as well as other systems using high resolution digitizers. Recently introduced current feedback amplifiers offer exceptional 3rd order intermodulation intercepts at very low quiescent power levels， but have been plagued by relatively poor noise figures. Teaming these op amps with a simple transformer input coupling yields noise figures less than 7dB with 3rd order intercepts greater than 40dBm （for frequencies 《 10MHz）。 Although not commonly considered for IF amplifiers， wideband， DC coupled operational amplifiers can offer considerable performance advantages at the lower IF （or HF） over standard AC coupled amplifiers. Particularly suitable from a distortion standpoint are a family of recently introduced monolithic current feedback operational amplifiers. Similar to the more common voltage feedback op amps， these parts offer very high non-inverting input impedance， very low output impedance， and a very high open loop gain that is controlled through the use of external resistors to set a well controlled closed loop gain. These amplifiers are unique in that the inverting node presents a low impedance through which the amplifier senses a feedback current as opposed to the more common feedback voltage. （Reference 1）