Philips Semiconductors B.V.

Low cost DECT Power Amplifier

PH97005

RNR-T45-97-M-0931

Biasing

The biasing part of the PA is centered around a dual NPN transistor, Q3 a and b. The circuit

offers a temperature compensated bias voltage for both the first and the second RF stages.

Unlike the previous demo board PH96060, the circuit is activated w hen Vc is high, which is

beyond 2.4 V. The bias output voltage for the first stage is a little higher than for the second

stage. D ue to series resistor R1, the fist stage is current driven and operates in class A. The

second stage is driven by at the knee voltage (0.7 V) and operates in class AB. Series resistor

R2 protects Q 2 from thermal runaway.

The current design of the PA doesn’t include any measures to reduce adjacent channel

power due to switching transients. The slope of the RF pulse can be too steep and has to

be relaxed to comply to the approval specification. This can be done by placing a capacitor

at the base of Q3a. Experimental investigation has shown that a 10 nF capacitor

approximately offers a 20 dB improvement of the adjacent channel power.

Recommendations for use

Figure 4 shows how the PA should be connected.

Figure 4:Schematic view of the PA connections.

In pulsed condition at nominal supply voltage and output power level, the load can be

mismatched (VSWR

≤ 6 : 1, all phases) without damage.

The PA can also be operated in CW, but only w hen the output is properly matched with 50

Ω.

When a mismatched load is applied w hen the amplifier is operated in CW, this will cause

damage of the final stage.

R1

C1

C2

C3

C4

C5

C6

C7

C8

C9

C10

C11

C12

C13

R2

R3

R4

R5

R6

R7

Q1

Q2

Q3

TL1

TL2

TL3

TL4

TL5

TL6

TL7

TL8

RF in

RF out

GND

3.6V supply

pulse (active high)