Part 1 [16 marks] – Unilateral matching using L-section LC networks
A radio frequency amplifier uses a RF power transistor with the following S-parameters at 300 MHz:
𝑆11 = 0.7∠30°
𝑆12 = 0∠0°
𝑆21 = 4∠90°
𝑆22 = 0.5∠0°
The RF amplifier uses input and output L-section matching networks to achieve maximum power transfer as shown in the figure below.
The RF amplifier is to operate in a 50 Ω system with a centre-frequency of 300 MHz. In general, there are four possible LC topologies available for matching between a source and load impedance, as shown in the figure below.
However, only two of the available networks will be valid for a specific set of matching requirements. Choose any valid combination of LC networks and determine the values of L & C to unilaterally match the RF transistor between a 50 Ω source and a 50 Ω load impedance. Create an AWR MWO schematic using the “s2p_blk” for the transistor’s S parameters (see relevant AWR video tutorials) and include the input and output matching networks with 50 Ω measurement ports. Simulate over the frequency range 100MHz ≤ 𝑓 ≤ 500MHz and produce the following:
(a) Smith charts of 𝑆11 and 𝑆22. Explain how the plots of 𝑆11 and 𝑆22 demonstrate that you have correctly matched both the input and output of the overall amplifier. [4 marks]
(b) Plots of the input and output voltage standing wave ratios (VSWRs). Explain how the plots of VSWR demonstrate that the amplifier is matched.
(c) Rectangular plot of the magnitude of 𝑆21 in decibels over logarithmic frequency scale.
Evaluate the overall performance of the amplifier by comparing the measured gain at the centre-frequency with the theoretical optimum gain given by, 𝐺𝑎𝑖𝑛dB = 10log ( 1 − | 1 𝑆11| 2 × |𝑆21| 2 × 1 − | 1 𝑆22| 2 )
(d) Investigate the effects of a +50% error in the magnitude of both the source and load impedance. Using appropriate metrics (of your choice), quantify the overall system effects when the amplifier becomes mis-matched.[4 marks]
Part 2 [20 marks] – RF Transistor amplifier design utilizing transistor BFR181W
Design a radio frequency (RF) amplifier based on an Infineon BFR181W low noise, high gain broadband NPN silicon RF transistor. The principal design goal is to achieve maximum power transfer with matched input and output impedances. The bias point details for your circuit and centre-frequency for design optimization are given in Table 1 below.
Set up a simulation in Microwave Office using the AWR Elements tab, Libraries, Parts by Vendor library, selecting transistor BFR181W from “Data” folder.
Select the BFR181W transistor from “Low Noise Si transistors up to 2.5 GHz”. Devices within the “Data” folder are linearized, small-signal models with pre-defined collector current and collector-emitter voltage; in effect these models are pre-biased. Select the requisite bias point (Table 1) using the drop-down option boxes for Vce and Ic (double-click the current or voltage value as appropriate). To design the required RF amplifier, you must first determine the transistor’s S-parameters at the required frequency of operation (centre-frequency). Set-up a single frequency analysis at the centre-frequency and add “New Graph” type “tabular”, adding measurements for all S-parameters. This is simple to do in AWR by using “ports” at the input and output as shown below.
(a)Determine the S-parameters in rectangular form for the bias point and characteristic impedance given in Table 1.[4 marks]
(b)Choosing appropriate plot types, plot the S-parameters over the frequency span,(𝑐𝑒𝑛𝑡𝑟𝑒_𝑓) − 0.3 GHz ≤ 𝑓 ≤ (𝑐𝑒𝑛𝑡𝑟𝑒_𝑓) + 0.3 GHz.[4 marks]
(c)For the bias point given in Table 1, determine whether or not the transistor has any potential instabilities using the relevant stability circles. Design a stabilization network justifying your choice of topology and demonstrating relevant stability circles before and after stabilization.
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