An Analysis of Admittance Neutralization by Means of Negative Transconductance Tubes

The reduction of capacitance by feedback from the anode of a negative transconductance tube has frequently been considered an attractive method for compensating for the drop in impedance at high frequencies of a circuit consisting of R and C in parallel. Experimentally, however, the results of previous investigators have never been exceptional. This paper presents a theoretical analysis of the problem on the assumption that an ideal tube is used for admittance neutralization only. The results indicate that approximately flat response of the circuit up to cutoff is obtained when Cm 1 Cg -2 Rp ( 1 + gmRp) -( 1 + gmRp) where Cmis the feed-back capacitance, Cgis the original shunt capacitance, and Rgis the effective resistance (see Fig. 1(c)). An interdependence between cutoff frequency, Cg, Rg, and the closeness to oscillation is found for a given control-electrode-to-anode transconductance. The effective resistance (and hence the gain of a preceding tube) in the ideal case can be raised several fold over that of the conventional inductance compensated circuit for the same cutoff and shunt capacitance; the greater the increase and consequent improvement, the smaller the safety factor (defined as the fractional increase in transconductance needed to produce oscillation). For the inductance compensated case ω0CgRg=1, while for the tube compensated circuit, the relation is approximately 1 coC, gR, = 1.5 + S where ω0is the cutoff angular frequency and S the safety factor.