TSH691ID datasheet(6/10 Pages) STMICROELECTRONICS

6 / 10 page

Figure 1 : Typical 300MHz-1000MHz Biasing Circuit

CIRCUIT DESCRIPTION

The TSH691 is 50

Ω input/outputinternallymatched

from 300MHz to 1000MHz. Due to its open-collec-

tor structure, the output RF port must be tied to

VCC2. The pin 8 allows a bias current adjust to set

the output power and the gain. The circuit is pack-

aged in SO8 for thermal dissipation considerations.

MATCHING

Within the 300-1000MHz band, although the circuit

is matched, the output return loss can be improved

by adding a serial inductor (L2) between the RF

output and VCC2 (56nH @450MHz and 10nH

@900MHz). Below 300MHz, using the S-parame-

ters matrix, specific input/outpu t matching net-

works can be calculated to maximize electrical

performances.

DC BLOCKING

Because input/output are respectively internal/ex-

ternal biased, DC blocks (C1, C2) are recom-

mended on both RF ports to guarantee a DC

isolation from the next cells. Above 500MHz, 100pF

is suggested whereas below, 1nF is better and far

below (less than 100MHz), 10nF is prefered.

BIASING

The amplifier can operate in the range of 1,5V to

5V and offers a bias current adjust function (Vbias

pin) which enables the trimming of the RF output

power (AB class Amplifier) by tuning a series vari-

able resistor (Rbias).

When Vbias is wired to the VCC rail, the current

consumptionis maximized getting the best linearity

(A class Amplifier) whereas biasing to Ground, the

IC is set in power down mode.

For higher supply voltage than 4V to reach high

output power, the serial resistor (R1) is strongly

recommended to increase the efficiency of the

amplifier and therefore reduce the thermal dissipa-

tion of the circuit.

DECOUPLING

As with any RF devices, the supply voltage decou-

pling must be done carefully using a 1nF bypass

capacitor (C3, C5) placed as close as possible to

the device pins and could be also improved by

adding a 150nH RF choke inductance (L1). Con-

cerning the Vbias pin, a 10nF decoupling capacitor

(C4) is recommended while placing on board is not

critical. Note that Surface Mounted Devices (SMD)

components are prefered for RF applications due

to the right behaviour in high frequencies while low

inductor values (few 10nH) can be printed on

board.

APPLICATIONS INFORMATION

TSH691

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