ise figure station are allowed to operate on adjacent channels be- Certain amplifiers have more inherent electrical noise cause it is likely that their signals will have suffered than others. Manufacturers usually produce a batch of considerable transmission loss by the time they impinge transistors, then classify and name the transistors on the desired channel. according to their inherent electrical noise levels. The inherent noise produced by a transistor is dependent on its Sensitivity general operating conditions, particularly frequency, The sensitivity of a radio receiver is a measure of the temperature, voltage and operating current, and these modulated signal input level which is required to pro- conditions must be specified when its noise level is mea- duce a given output level. A receiver with good sensitivity sured. Engineers use the ratio term noise figure to specify requires a smaller input signal than a receiver with poor noise levels in transistors. sensitivity to produce a given output level. The sensitivity Noise figure is defined as of a small portable receiver (audio output rated at 250 mW) may be quoted as 200 uV/m. What this means is noise figure (N.F.) = at 290 K (2.1.25) that a modulated AM carrier (modulated with a 400 Hz tone and with an AM modulation depth of 30%) will produce an audio output of 50 mW under its maximum If a transistor introduces no noise, then its S/N at both gain conditions when the input signal is 200 uV/m. the input and output is the same; therefore, from Equation 2.1.25, N.F. = 1 or in dB N.F. = 10 log 1 = 0 dB. Hence a 'perfect' or 'noiseless' amplifier has a noise figure of 0 dB. 2.1.9.2 Signal-to-noise ratios An imperfect amplifier has a noise figure greater than 0 dB. For example an amplifier with a noise figure of 3 dB (=2 Any signal transmitted through a communications ratio) means that it is twice as bad as a perfect amplifier. system suffers attenuation in the passive (non-amplify- ing) parts of the system. This is particularly true for radio signals propagati