. The Bell System technical journal . ely straight portionsof g-e characteristics can be verified by inspection of (Fig. 1) where thecharacteristic of the 6AG7 is given. Since a portion of the g-e characteristic is a straight line, the third term coefficient — may be replaced by the ratio -^ where Ae is an arbitrary inter-ne Ae val of grid voltage and Ag the corresponding change in g. In many of thefollowing computations it will be advantageous to use for Ae the total excur-sion of the grid voltage. On the basis of the simplifying assumption of a paraboUc pentode it ispossible to derive the si
. The Bell System technical journal . ely straight portionsof g-e characteristics can be verified by inspection of (Fig. 1) where thecharacteristic of the 6AG7 is given. Since a portion of the g-e characteristic is a straight line, the third term coefficient — may be replaced by the ratio -^ where Ae is an arbitrary inter-ne Ae val of grid voltage and Ag the corresponding change in g. In many of thefollowing computations it will be advantageous to use for Ae the total excur-sion of the grid voltage. On the basis of the simplifying assumption of a paraboUc pentode it ispossible to derive the simple formulae given below which cover the perform-ance of the tube as an amplifier, rectifier and modulator. ELECTRON TUBE PERFORMANCE 317 The Parabolic Pentode Amplifier Over the straight portion of the g-e curve the following relations hold foran input v = P cos pi. The fundamental current is Ip = GP where P is the grid second harmonic current is f2p 1 Ag^4 Ae TYPE 6AG7PLATE VOLTS 300SCREEN VOLTS 300GRID-BIAS VOLTS GRID VOLTAGE, e Fig. 1—g-e characteristic having principally second order modulation over part of therange. To find the level H2 of the second harmonic below the fundamental,prolong the straight portion of the characteristic down to the virtual cutoff£0 (Fig. 1). Then H, = 20 log f 20 log ^Sr^ + 12. For the case when all of the parabolic characteristic is used P = ^ and F2 20 log ^ + 18. If it is desired to express Ih in terms of the output current ip and thespace current Io the following approximate formula may be used: Hi = 20 log J + 18. 315 BELL :iySTEM TECHNICAL JOURNAL This expression neglects the area under the characteristic on the left ofthe Une AEq , Fig. 1. The formula is useful in selecting a tube for closerconsideration. When a tube is used as a preamplifier in a wave analyzer an error of meas-urement may occur if two input frequencies intermodulate in the amplifierto produce a current of the same frequency as the one being measure
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