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TQ5122

3V Cellular TDMA/AMPS Receiver IC With Power-Down

TriQuint Semiconductor 

TQ5122 Product Description

The TQ5122 3V RFIC Downconverter is designed specifically

for cellular band TDMA/AMPS applications. The TQ5122

contains LNA, Mixer and LO buffer circuits matched to the 800

MHz cellular frequency band. The IF frequency may be selected

between 85 and 150 MHz. Most RF ports are internally

matched to 50 Ω simplifying the design and minimizing the

number of external components. The TQ5122 also includes a

power–down mode switch which allows current saving during

standby and the non-operating portion of the TDMA pulse.

Operation

Please refer to the test circuit above.

Low Noise Amplifier (LNA)

The LNA section of the TQ5122 are cascaded common source

FET’s, see Figure 1. It is designed to operate on DC supply

voltages from 2.7V to 5V. The source terminal must be

grounded as close as possible to Pin 8 to avoid significant gain

reduction due to degeneration. The LNA requires an input

matching circuit to obtain best noise figure, gain and return loss.

The LNA output is close to 50 Ω for direct connection to a 50 Ω

image reject filter.

Vdd

LNA

in

BIAS

LOAD

LNA

out

BIAS

Figure 1. Simplified Schematic of LNA Section

LNA Input Match

The designer can make some Noise Figure and Gain trade off

by varying the off chip LNA input matching circuit values and

topology. This allows the TQ5122 to be optimized for specific

system requirements.

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

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

TQ5122

Data Sheet

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 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

+

4 RN

Z0

⋅

1+

Γopt

Γ2

opt

− ΓS 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 + F2 − 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. The system

noise figure is a compromise between the highest gain and

minimum noise figure of the LNA. See Table 1 for noise

parameters.

Table 1. TQ5122 Noise Parameters

Freq. |Gopt| / Gopt Fmin Rn

MHz

830

0.88

34.5

0.75

51.2

880

0.83

37.8

0.89

50.4

930

0.82

40.0

0.97

50.0

LNA Output Match

The output impedance of the LNA was designed for 50Ω. The

internal 50Ω match eliminates the need for external

components at this port. It also improves IP3 performance and

power gain.

The output of the LNA is intended to be connected directly to an

image reject filter. Depending on the filter, additional

components may be needed to better match to the LNA output.

Some image reject filters may require a series inductor to

smooth the frequency response and improve overall

performance.

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

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