TQ5121 데이터 시트보기 (PDF) - TriQuint Semiconductor

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TQ5121

3V Cellular TDMA/AMPS LNA/mixer Receiver IC

TriQuint Semiconductor 

TQ5121 Product Description

The TQ5121 3V RFIC Downconverter is designed specifically

for cellular band TDMA applications. The TQ5121 contains a

LNA+Mixer circuit to handle the 800 MHz cellular band. The IF

frequency range covers 70 to 140 MHz with most of the ports

internally matched to 50 Ω simplifying the design and keeping

the number of external components to a minimum.

Operation

Please refer to the test circuit above.

Low Noise Amplifier (LNA)

The LNA section of the TQ5121 consists of a cascaded

common source FETs (see Fig 1). The LNA is designed to

operate on supply voltages from 3V to 5V. The source terminal

has to be grounded very close to the pin, this will avoid a

significant gain reduction due to degeneration. The LNA

requires a matching circuit on the input to provide superior

noise, gain and return loss performance. The output is close to

50 Ω for direct connection to a 50 Ω image stripping filter.

Vdd

Fig 1. TQ5121

Simplified

Schematic of

LNA Section

LOAD

LNA

out

LNA

BIAS

in

BIAS

LNA Input Match

To obtain the best possible combination of performance and

flexibility, the LNA was designed to be used with off-chip

impedance matching on the input. Based on the system

requirements, the designer can make several performance

trade-offs and select the best impedance match for the

particular application.

The input matching network primarily determines the noise and

gain performance. Fig 2 shows a suggested input match using

a series 1.2pF capacitor and a shunt 10nH inductor.

The LNA gain, noise figure and input return loss are a function

of the source impedance (Zs), or reflection coefficient (Γs),

TQ5121

Data Sheet

Fig 2. Suggested LNA Input Match

1.2pF

RF

IN

Pin 7

10nH

Note: These values assume ideal components and neglect board parasitic.

The discrepancy between these values and those of the typical application

circuit are the board and component parasitic

presented to the input pin. Highest gain and lowest return loss

occur when Γs is equal to the complex conjugate of the LNA

input impedance. A different source reflection coefficient, Γopt,

which is experimentally determined, will provide the lowest

possible noise figure, Fmin.

The noise resistance, Rn, provides an indication of the sensitivity

of the noise performance to changes in Γs as seen by the LNA

input.

( ) FLNA = FMIN +

4RN ⋅

Z0 1+

Γopt − ΓS 2

Γopt 2 ⋅ 1 − Γs 2

Components such as filters and mixers placed after the LNA

degrade the overall system noise figure according to the

following equation:

FSYSTEM = FLNA + F 2 − 1

GLNA

FLNA and GLNA represent the linear noise factor and gain of the

LNA and F2 is the noise factor of the next stage. Thus, the

system noise figure depends on the highest gain and minimum

noise figure of the LNA.

Designing the input matching network involves a compromise

between optimum noise performance and best input return loss.

For example, when the TQ5121 LNA is matched for optimum

noise figure (1.35dB @ 880 MHz), the input return loss is

approximately 4dB. On the other hand, when the LNA is

matched for best return loss, the LNA noise figure is

approximately 1.95dB @ 881 MHz. See Table 1 for noise

parameters.

For additional information and latest specifications, see our website: www.triquint.com

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