7 Application and Implementation

Note

Information in the following applications sections is not part of the TI component specification,

and TI does not warrant its accuracy or completeness. TI’s customers are responsible for

determining suitability of components for their purposes, as well as validating and testing their design

implementation to confirm system functionality.

7.1 Application Information

7.1.1 Power-Control Principles

The LMV242x are members of TI's RF power detector with PA controller family for quad-band TDMA and GSM

systems. Section 7.2 demonstrates a basic approach for implementing the quad-band design example around

an RF power amplifier (PA) using the LMV242. The LMV242x contain a 50dB log RF detector and interface

directly with the directional coupler.

The LMV242x baseband (control) interface consists of three signals: TX_EN to enable the device, BS (LMV242

only) to select either output 1 or output 2, and VRAMP to set the RF output power to the specified level.

The LMV242x give maximum flexibility to meet GSM frequency and time-mask criteria for many different

single-supply PA types, such as HBT or MESFET in GaAs, SiGe, or Si technology. These criteria are met

by the programmable ramp characteristic from the baseband and the TX_EN signal along with the external

compensation capacitor.

7.1.2 Power Amplifier Controlled Loop

This section gives a general overview and understanding of how a typical PA control loop works, and how to

solve the most common problems confronted in the design.

7.1.2.1 General Overview

The key benefit of a PA control-loop circuit is immunity to changes in the PA gain-control function. When a PA

controller is used, the relationship between gain and gain control voltage (VAPC) of the PA is of no consequence

to the overall transfer function. The overall transfer function is a function of the VRAMP voltage of the controller.

Based upon the value of VRAMP, the PA controller sets the gain control voltage of the PA to a level that is

necessary to produce the desired output level. Any temperature dependency in the PA gain control function is

eliminated. Also, nonlinearity in the gain transfer function of the PA do not appear in the overall transfer function

(POUT vs VRAMP). The only requirement is that the gain control function of the PA has to be monotonic. To

achieve this monotonic gain control function, ensure that the LMV242x detector is temperature stable.

7.1.2.2 Typical PA Closed Loop Control Setup

Figure 7-1 depicts a typical setup of a power amplifier (PA) control loop. Beginning at the output of the PA, this

input is input to a detector, typically through a directional coupler. The error between detector output current

IDET and ramp current IRAMP, representing the selected power setting, drives the inverting input of an op amp

configured as an integrator. A reference voltage drives the noninverting input of the op amp. Finally, the output of

the integrator op amp drives the gain control input of the power amplifier, which sets the output power. The loop

stabilizes when IDET is equal to IRAMP.

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LMV242, LMV2421

SNWS014D – MARCH 2004 – REVISED JUNE 2025

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