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136 Valve Amplifiers If we divide by R k ( 11), we obtain: r 0 a 5 r a 1 R L 1 r a 1 1 ððR L 1 r a Þ=R k ð 1 1ÞÞ As R k tends to N, the right-hand term on the bottom line reduces to zero, resulting in a maximum value of r 0 a : r 0 a % R L 1 2r a This high value of r a will become significant when we investigate the PSRR of the differential pair. If R L .. r a , then the output resistance (only one terminal loaded) is: r out % R L ðR L 1 2r a Þ R L % 2ðR L 1 r a Þ 2 AC Balance of the Differential Pair and Signal at the Cathode Junction