Output power setting and pa matching at rf_out – Rainbow Electronics ATA5812 User Manual

Page 18

ATA5811/ATA5812 [Preliminary]

4689B–RKE–04/04

Output Power Setting
and PA Matching at
RF_OUT

The Power Amplifier (PA) is a single-ended open collector stage which delivers a cur-
rent pulse which is nearly independent of supply voltage, temperature and tolerances
due to bandgap stabilization. Resistor R

, see Figure 16 on page 19, sets a reference

current which controls the current in the PA. A higher resistor value results in a lower
reference current, a lower output power and a lower current consumption of the PA. The
usable range of R

is 15 k

Ω

to 56 k

Ω

. Pin PWR_H switches the output power range

between about 0 dBm to 5 dBm (PWR_H = GND) and 5 dBm to 10 dBm (PWR_H =
AVCC) by multiplying this reference current with a factor 1 (PWR_H = GND) and 2.5
(PWR_H = AVCC) which corresponds to about 5 dB more output power.

If the PA is switched off in TX mode, the current consumption without output stage with
VS1 = VS2 = 3 V, T

amb

= 25°C is typically 6.5 mA for 868.3 MHz and 6.95 mA for

315 MHz and 433.92 MHz.

The maximum output power is achieved with optimum load resistances R

Lopt

according

to Table 7 on page 19 with compensation of the 1.0 pF output capacitance of the
RF_OUT pin by absorbing it into the matching network consisting of L

, C

as shown

in Figure 16 on page 19. There must be also a low resistive DC path to AVCC to deliver
the DC current of the power amplifier's last stage. The matching of the PA output was
done with the circuit according to Figure 16 on page 19 with the values in Table 7 on
page 19. Note that value changes of these elements may be necessary to compensate
for individual board layouts.

Example:

According to Table 7 on page 19, with a frequency of 433.92 MHz and output power of
11 dBm the overall current consumption is typically 17.8 mA hence the PA needs
17.8 mA - 6.95 mA = 10.85 mA in this mode which corresponds to an overall power
amplifier efficiency of the PA of (10

(11dBm/10)

1 mW)/(3 V

10.85 mA)

100% = 38.6%

in this case.

Using a higher resistor in this example of R

= 1.091

22 k

Ω

= 24 k

Ω

results in 9.1%

less current in the PA of 10.85 mA/1.091 = 9.95 mA and 10

log(1.091) = 0.38 dB

less output power if using a new load resistance of 300

Ω

1.091 = 327

Ω

. The result-

ing output power is then 11 dBm - 0.38 dB = 10.6 dBm and the overall current
consumption is 6.95 mA + 9.95 mA = 16.9 mA.

The values of Table 7 on page 19 were measured with standard multi-layer chip induc-
tors with quality factors Q according to Table 7 on page 19. Looking to the 433.92
MHz/11 dBm case with the quality factor of Q

= 43 the loss in this inductor is esti-

mated with the parallel equivalent resistance of the inductor R

loss

= 2

× π ×

and the matching loss with 10 log (1 + R

Lopt

loss

) which is equal to 0.32 dB losses in

this inductor. Taking this into account the PA efficiency is then 42% instead of 38.6%.

Be aware that the high power mode (PWR_H = AVCC) can only be used with a supply
voltage higher than 2.7 V, whereas the low power mode (PWR_H = GND) can be used
down to 2.4 V as can be seen in the section “Electrical Characteristics”.

The supply blocking capacitor C

(10 nF) has to be placed close to the matching net-

work because of the RF current flowing through it.