DESIGN OF A CLASS A POWER AMPLIFIER Design a power amplifier at 2.3 GHz using a Nitronex NPT25100 GaN HEMT transistor, with an output power of 10 W. The scattering parameters of the transistor for VDS = 28 V and ID = 600 mA are as follows: S11 = 0.593∠178°, S12 = 0.009∠−127°, S21 = 1.77∠−106°, and

Question

DESIGN OF A CLASS A POWER AMPLIFIER

Design a power amplifier at 2.3 GHz using a Nitronex NPT25100 GaN HEMT transistor, with an output power of 10 W. The scattering parameters of the transistor for [latex]V_{DS}[/latex] = 28 V and [latex]I_D[/latex] = 600 mA are as follows: [latex]S_{11} = 0.593∠178°[/latex], [latex]S_{12} =0.009∠−127°, S_{21} = 1.77∠−106°[/latex], and [latex] S_{22} = 0.958∠175°[/latex], and the optimum large-signal source and load impedances are [latex]Z_{SP} = 10 − j3 Ω [/latex] and [latex]Z_{L P} = 2.5 −j2.3 Ω[/latex]. For an output power of 10 W, the power gain is 16.4 dB and the drain efficiency is 26%. Design input and output impedance matching sections for the transistor, and find the required input power, the required DC drain current, and the power added efficiency.

Accepted Answer

First establish the stability of the device. Using the small-signal scattering param-eters in (12.28) and (12.29) gives

[latex]\left|\Delta \right| =\left|S_{11}S_{22}-S_{12}S_{21}\right|=0.579 \lt 1,[/latex]

[latex]K=\frac{1-\left|S_{11}\right|^{2}-\left|S_{22}\right|^{2}+\left|\Delta \right|^{2} }{2\left|S_{12}S_{21}\right| } =2.08\gt 1,[/latex]

showing that the device is unconditionally stable.
Converting the large-signal source and load impedances to reflection coeffi-cients gives

[latex]Γ_{SP} = 0.668∠187°,[/latex]

[latex]Γ_{L P} = 0.905∠−175°.[/latex]

For comparison, using the small-signal scattering parameters in (12.40) to find the
source and load reflection coefficients for conjugate matching gives

[latex] \Gamma _{S}=\frac{B_{1} ±\sqrt{B_{1}^{2}-4\left|C_{1}\right| ^{2}} }{2C_{1}}=0.508\angle 166^{\circ } ,[/latex]

[latex]\Gamma _{L}=\frac{B_{2} ± \sqrt{B_{2}^{2}-4\left|C_{2}\right| ^{2}} }{2C_{2}}=0.954\angle -176^{\circ } [/latex]

Note that these values are approximately equal to the large-signal values [latex]Γ_{SP}[/latex] and [latex]Γ_{L P}[/latex], but not exactly, due to the fact that the scattering parameters used to calculate [latex]Γ_{S}[/latex] and [latex]Γ_{L}[/latex] do not apply for large power levels. We should use the large-signal reflection coefficients to design the input and output matching networks.
The AC amplifier circuit is shown in Figure 12.23.

For an output power of 10 W, the required input drive power is

[latex]P_{in}=P_{out}(dBm)-G(dB)=10\log (10.000)-16.4=23.6 dBm=229 mW.[/latex]

The DC input power can be found from the drain efficiency as [latex]P_{DC} = P_{out}/η =38.5 W[/latex], so the DC drain current is [latex]I_{D} = P_{DC}/VDS = 1.37 A.[/latex] The power added efficiency of the amplifier can be found from (12.89) to be

[latex]\pmb{\eta} _{PAE}=\frac{P_{out}-P_{in}}{P_{DC}}=\frac{10.0-0.229}{38.5}=25 \% [/latex]

Question 12.9: DESIGN OF A CLASS A POWER AMPLIFIER Design a power amplifier...

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