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Application Note
Transmitters TH720xx / Transceivers TH7122x
PA Output Matching
39011 72000 02 Page 1 of 2 AN720xx-PA_Output-Matching
Rev. 001 March/06
VCC
C
out
OUT_PA
LR
L
VCC
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, that 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 of the 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.
The peak output voltage swing is limited by the supply voltage V
cc
minus the saturation voltage V
sat
. It can be
1.8V for the transmitters and 2.3V for the transceivers, if they run from a 3V supply.
The optimum load resistance can be calculated as:
Po
VsatVcc
R
L
×
−
=
2
)(
2
Example:
If we assume a required RF power of P
O
= 6.3mW (or 8dBm), taken from a TH720xx transmitter at
3V supply and with V
sat
= 1.2V, then the optimum load resistance would be R
L
= 256Ω. A slightly smaller load
resistance might be chosen to ensure the output stage does not saturate at supply voltage drops.
The output capacitance C
out
of the transistor is typically at 2 - 3pF. It can be considered in two ways:
1. For simple resonant frequency tuning together with the inductor and an external tank capacitor (in
this case the inductor could be formed by a loop antenna for example).
2. To be included as a capacitor of the output matching network.
It is usually required to adjust the matching network for best results and to compensate for board layout
parasitics.
Power amplifier output stage
