CONSIDERATIONS OF IF PREAMPLIFIER DESIGN 



369 



S, Ns, N„, Nt, Nf, Ni2 = current generators 



A"^ = a voltage generator 



S = signal current associated with gs 



Na = source noise, TVs = yl4KTsBgs 



(In evaluating IF noise figure Tg is usually taken as reference temperature T for 



which F is defined, or 290°K. In a radar receiver Ts is larger than T because of 



noise-noise intermodulation at mixer from local oscillator signal.) 



A^„ = noise current associated with coupling network losses, A''„ = -s^AKTnBgn 



Nt = noise current associated with grid noise of tube, Nt = ^j4KTtBgt 



^ = usually 4 or 5 



gt = grid damping due to finite transit time only 



Nf = noise current associated with feedback such as that due to cathode lead 



inductance: Nf = -yJiKyTg/; y may be between and 1 



A^i2 = noise current associated with output to input feedback conductance: Nn = 



yjiKSTgi^; d usually 1 



A'' = voltage generator representing shot noise in the tube, A^ = -yJ^KTBRn 

 where R„ is the resistance that must be connected between grid and ground to 

 produce the same fluctuation current in the plate circuit of the hypothetical 

 noiseless tube as exists in the actual tube due to shot effect. For the triode, /?„ 

 varies between 



Rn 



2.5 



and R„ 



3£ 



where gm = operating transconductance. 



Fig. 7-7 Equivalent Circuit of IF Amplifier Noise Sources. 



as the bandwidth is reduced. The last term of Equation 7-30, however, 

 may cause the spot noise figure to increase for frequencies removed from 

 the carrier frequency and the average value of F is then increased. To 

 minimize this effect in wide-band applications, care is taken to minimize 

 the increase in this term either by virtue of a small value of i? or, in some 

 cases, by introducing feedback to increase gf. In all cases, IF preamplifier 

 tubes are selected which have a high transconductance-to-input capacitance 

 ratio and small transit time. 



An important consideration is the mixer-IF coupling network. When gf 

 and ^12 are zero, minimum F is obtained for 



,.-yl'-^ + (g. + g. 



(7-31) 



as can be proven by differentiating Equation 7-30. Application of this 

 equation is difficult because ^gt cannot always be easily determined. In 



