Application Note TH720xx Transmitters/ TH7122 x Transceivers PA Output Matching
Almost all low-power RF transmitter ICs have an on-chip power amplifier stage where the output pin is either an open collector or open drain. The RF power amplifier stage of all Melexis transmitters is setup with a bipolar transistor that provides an open collector. The collector output must be supplied with a DC voltage to bias the transistor stage. This is usually done through an inductor. The inductor can be either tuned to the operating frequency by means of a parallel capacitor, or the inductor is set to a large value that doesn’t affect the output impedance -known as an RF choke -.
RF designers used to work with S-parameters sometimes have a problem with this approach of undefined output impedances. In some cases, where they are provided, the S-parameters are not the ones of the output device but the complex conjugate values the device should see to deliver maximum output power. The S-parameters ofthe device itself are not very useful because the output stage equivalent circuit is essentially a large resistance in parallel with a small capacitance and little parasitic inductance of the pins. Let’s assume a capacitance of about 2.5pF and a resistance of 10kΩ. Trying to match this impedance to the load -which is usually at 50 to 400Ω -would give very little output because the device would be saturated.
A more useful approach to design the output matching network is to consider the output capacitance, the supply voltage, the saturation voltage of the output stage and the required power output. To avoid saturation of the output stage (which would increase the level of the harmonics), the saturation voltage of the Melexis TH720xx transmitters can be assumed to be 1.2V. While for the TH7122x transceivers a saturation voltage of 0.7V is applicable. The saturation voltage is fairly independent of the supply voltage.
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Application note & Design Guide |
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Please see the document for details |
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English Chinese Chinese and English Japanese |
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JUNE 19, 2017 |
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REVISION 002 |
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39011720000 |
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604 KB |
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