LMV242

SNWS014C – APRIL 2004 – REVISED MAY 2013

www.ti.com

APPLICATION SECTION

POWER CONTROL PRINCIPLES

The LMV242 is a member of the power loop controller family of TI, for quad-band TDMA/GSM solutions. The

typical application diagram demonstrates a basic approach for implementing the quad-band solution around an

RF Power Amplifier (PA). The LMV242 contains a 50 dB Logamp detector and interfaces directly with the

directional coupler.

The LMV242 Base Band (control-) interface consists of 3 signals: TX_EN to enable the device, BS to select

either output 1 or output 2 and VRAMP to set the RF output power to the specified level. The LMV242 gives

maximum flexibility to meet GSM frequency- and time mask criteria for many different single supply Power

Amplifier types like HBT or MesFET in GaAs, SiGe or Si technology. This is accomplished by the programmable

Ramp characteristic from the Base Band and the TX_EN signal along with the external compensation capacitor.

POWER AMPLIFIER CONTROLLED LOOP

This section gives a general overview and understanding of how a typical Power Amplifier control loop works and

how to solve the most common problems confronted in the design.

General Overview

The key benefit of a PA control loop circuit is its 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. It is a function of the controller's VRAMP voltage. Based upon the value of VRAMP,

the PA controller will set 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 will be eliminated. Also, non-linearity’s

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, it is crucial, that the

LMV242’s detector is temperature stable.

Typical PA Closed Loop Control Setup

A typical setup of PA control loop is depicted in Figure 25. Beginning at the output of the Power Amplifier (PA),

this signal is fed, usually via a directional coupler, to a detector. The error between the detector output current

IDET and the 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 non-inverting 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

is stabilized when IDET is equal to IRAMP . Lets examine how this circuit works in detail.

12

Submit Documentation Feedback

Copyright © 2004–2013, Texas Instruments Incorporated

Product Folder Links: LMV242