surface 1 or over the equivalent end aperture planes, 2 essentially contain the dominant term Uy(K-cos6)\ kL (K-cosQ) where K — /. 0 [/. g , k = — and ?. g is the rod wavelength. At frequencies outside the stop- ts M Fig. 1A Composite rod structure - 20 Fig. IB Radiation patterns at gigahertz frequencies band, it might be expected that the difference between the uniform-rod and composite-rod patterns could be explained by the diff ere nt dispersion relationship s of the two structures. However, since the mean relative permittivity of the composite ro d is l ess than that of the uniform rod, this indicates that K will be smaller for the composite rod. This was confirmed by measurements which gave a value K = 1-22 for the composite rod compared to K = 1- 33 for the uniform rod at 9-4 GHz. The radiation-pattern equations indicate that the beamwidth of the composite should be greater than the uniform-rod beamwidth. This is seen to be the case at frequencies below the stopband, but the opposite is true for frequencies above the stopband. The equation suggests that the angular posi- tions of the sidelobes should lie closer to the main beam for the uniform rod than for the composite rod, and this was found to be the case. The equation also indicates that the first sidelobe levels should be approximately 4-6 dB great er for the uniform rod, and this is found to be the case above the stopband, but appears to be somewhat less below the stop- band. Since the increased gain property of a composite structure is not a surface-wave-propagation effect, an explana- tion in terms of the modifying influence of the rod structures on the direct radiation from the feed waveguide 3 - 4 appears to be more likely. J. R. BLAKEY 5t h February 1973 Microwave Physics Group epartment of Physics University of Surrey Guildford, Surrey, England References 1 KEILY, D. c : Progress in dielectrics , (Heyw ood, 1961) , 3, pp. 1—45 2 JAMES, j . R.: Theoretical investigation of cylindrical dielectric rod antennas , Proc. IEE, 196/, 114, (3), pp. 309-319 3 BLAKEY, j . R.: Calculation of dielectric-aerial-radiation patterns , Electron. Lett., 1968,4, pp. 46-47 4 AND ERSE N, j . B.: Metallic and dielectric aerials . Polyteknisk Forlag, Lyngby, 1971 uniform rod composite rod 8 -5 9-0 9 5 frequencyjGHz 1 0 0 Fig. 2 Beamwidth and sidelobe levels against frequency for both rods uniform rod / composite rod ELECTRONICS LETTERS 22nd March 1973 Vol.9 No. PRECISION DIFFERENTIAL VOLTAGE- CURRENT CONVERTOR Indexing terms: Magnitude convertors, Integrated circuits, Differential amplifiers new differential voltage-current conv enor is proposed, which achieves high linearity and is substantially temperature independent. The effects of transistor mismatches and of limited current gain are analysed. Experim ental results are given which show a considerable improvement over previous circuits. In m any integrated circuits, for example multipliers, 1 t he first stage is a differential voltage-current convertor, consisting o f a simple differen tial pair with a feedback resistor R E between the emitters. This circuit gives satisfactory performances only if the resistance R E is large compared with the dynamic resistance of the transistors; otherwise the overall transconductance of the circuit is strongly temperature dependent and essentially nonlinear. On the other hand, a high value of R E increases the noise, and this again limits the dynamic range. The circuit proposed (Fig. l a) performs a precision low-distortion voltage-current conversion, even with low values of R E , thus improving the dynamic range. Consider the sum of the voltages around the loop com- prising the signal generator, the base-emitter voltages of T r ls Tr 4 , Tr 2 and Tr 3 and the feedback resistor R E : v ~V BEl - V BE t - iR E + V BE + V BE 3 = 0 Eqn. 1 can also be written as follows: (1) v-iR E + , £2 ] n / £ 3 _ |n / c i _ S2 S3 SI IS where I s are the reverse saturation currents. 14 7