TSH690

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TSH690 DESCRIPTION

The TSH690 is a 2 transistor stages amplifier run-

ning within the 40MHz-1GHz frequency band fea-

turing a gain of 28dB at 433MHz. The TSH690 is

50

Ω input/output internally matched from 300MHz

to 1000MHz. The open collector output requires

an inductive load for the impedance matching and

also to reach an output power of +13,5dBm at 3V

and +18dBm at 4V. A bias control pin allows tun-

ing of current consumption and amplification

mode.

As the matter of fact, when the bias pin is tied to

the supply voltage, amplification is linear (Class A)

while a lower voltage leads to a Class A-B amplifi-

cation featuring a better efficiency. If the control

voltage is grounded, the TSH690 is set in Pow-

er-down mode without current consumption.

MATCHING THE OUTPUT WITH L2

Within the 300-1000MHz band, although the cir-

cuit is matched, the output return loss (S22) can

be improved by adapting the value of the inductor

L2. This inductor is connected between the RF

output and V

L2 = 56 nH gives an output return loss of -19 dB at

450 MHz.

L2 = 10 nH gives an output return loss of - 8 dB at

900 MHz.

In a 433 or 450 MHz transmitter application, L1

and L2 can be optimized to reduce the second

harmonic by choosing L1 = 33nH and L2 = 15nH.

Below 300MHz, using the S-parameters matrix,

specific input/output matching networks 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 iso-

lation 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

consumption is maximized getting the best linear-

ity (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 dissi-

pation 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 capaci-

tor (C4) is recommended while placing on board is

not critical. Note that Surface Mounted Devices

(SMD) components are prefered for RF applica-

tions due to the right behaviour in high frequencies

while low inductor values (few 10nH) can be print-

ed on board.

Figure 4 : Demonstration board schematic