HJFETHeterojunction Field Effect Transistor
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The W-CDMA characteristics of the double-doped HJFET with a gate width of 24.6 mm were evaluated using a QPSK modulated signal with a chip rate of 4.096 Mcps at 1.95 GHz under various [I.sub.q] conditions.
The characteristic of the same HJFET was also measured under a higher [I.sub.q] level of 300 mA (three percent of [I.sub.max]) with the same source and load conditions.
Figure 5 shows a comparison of the ACPR1 and PAE of the HJFET as functions of Pout for [I.sub.q] = 80 and 300 mA.
The effect of load match on the HJFET PAE and W-CDMA ACPR1 characteristics was also investigated.
Although ACPR and intermodulation distortion are two different measures, it has been shown that they could be correlated to each other to a certain extent in systems such as North American Digital Cellular and personal digital cellular.[12] Here, the W-CDMA ACPRs of the developed HJFET are compared to its two-tone intermodulation characteristics.
The W-CDMA ACPRs of the HJFET were measured with a QPSK-modulated signal having a chip rate of 4.096 Mcps at 1.95 GHz; the intermodulation distortions were measured using a two-tone input signal at frequencies of 1.948 and 1.952 GHz.
Figure 10 shows the ACPRs and intermodulation distortions of the HJFET under a higher [I.sub.q] level of 300 mA.
To further illustrate the correlation between W-CDMA ACPR1 and IM3 distortions, the load- and source-pull contours of the HJFET were measured.
It is understood that the total distortion at IM3 frequency composes distortions generated by odd-order nonlinearities.[13] Assuming the HJFET is weakly nonlinear (which would be valid under normal nonsaturation operation), its characteristic can be represented by up to the fifth-order nonlinearity.
An HJFET model was constructed using a large-signal equivalent circuit model, EEHEMT1, available in commercial microwave simulators.[14] The two-tone characteristic was simulated using a harmonic balance technique, and good agreement between the measured and simulated results was observed.