Broadband Class C power amplifiers (PAs) are useful in certain communications bands. Although now integrated into the Advanced Design System (ADS) simulation software from Agilent-EEsof (www.agilent.com), the Touchstone simulation software at one time was a powerful tool for developing and optimizing impedance matching networks for such amplifiers. What follows is a design approach to show how to extract the optimum input and output large-signal impedances for a selected RF transistor, model their behavior with one-port networks, and then develop matching networks over the desired frequency band for operation at the 50-ohm system impedance. To confirm the effectiveness of the approach, a 10-W amplifier was designed and built for 10-dB power gain from 225 to 400 MHz.

Designing wideband microwave PAs is challenging. RF power device parameters change with signal level as well as with frequency, making optimum impedance matching difficult. There is a broad range of techniques used to represent the behavior of the power device. The more complete the representation, the more complicated the model usually becomes.

The large-signal charge-control transistor model^1,2 and the modified Ebers-Moll model³ were used earlier for modeling RF power transistors. Large-signal S-parameters were also employed with an approximate PA design. ⁴ However, because of the difficulty in measuring these large-signal S-parameters, the technique was of limited use. Computer simulations were also used to predict the operation of Class C power amplifiers through numerical analysis. ^5,6 Although this method can give accurate results, the design of Class C amplifiers using this approach is tedious. Fortunately, the development of harmonic-balance design approaches in the mid-1970s greatly simplified the design of nonlinear circuits and large-signal amplifiers. ⁷ The basic limitation of this technique is its complexity and the large amount of mathematics needed with the professional numerical methods required to resolve the circuit.

Because of the nonlinear nature of an RF power transistor, a full two-port device model is not an optimum choice for designing input and output matching networks. In this article, one-port impedance models have been used to characterize the optimum load and source terminations of the power device. Optimum load and source large signal impedances are usually specified by RF device data books at several frequencies in the operating band of the RF power transistor. ⁸ The effective input and output impedances of the RF device can be represented as the complex conjugates of these optimum terminations.

RF power transistor characterization can be performed by measuring the device's optimum load and source impedances with the aid of load-pull tuners over the frequency band of interest. ⁹ This requires one-port representations to predict the complex conjugate of these impedances from the lower band edge (F_L) to the upper band edge (f_H) as shown in Fig. 1. In this case, Z_out = Z* _OL and Z _in = Z*s, where Z_OL is the optimum load impedance and Zs is the source impedance. Figure 2 shows two possible topologies for the modeled impedance networks. ¹⁰ All losses were lumped into a single resistor, which terminates an inductive-capacitive (LC) two-port network.

An analytic synthesis procedure can be used to realize the one-port networks that fit the measured impedance data at both band edges. But instead of performing this tedious task, simulation software such as Touchstone (now ADS) can be used to optimize the circuit elements of the modeling networks to predict performance across the full frequency band of interest.

The maximum available gain will roll off at a negative slope of 6 dB/octave with increasing frequency if a transistor is conjugately matched over a broad frequency range. One of the techniques used to compensate the transistor's power gain variation with frequency is by selectively reflecting some of the power at lower frequencies of the band where the power gain is relatively high. The controlled mismatch imposed in this technique will however degrade the input VSWR at lower-band frequencies. The approximate power gain of the RF transistor is given by11:

where:

f_max = the maximum frequency of oscillation and

γ= a constant related with the slope of the gain roll-off. γis given by:

where: