R_10032 Datasheet, PDF(3/10 Page) NXP Semiconductors

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R_10032 Datasheet, PDF (3/10 Pages) NXP Semiconductors – CA-330-11 LDMOS bias module

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NXP Semiconductors

R_10032

CA-330-11; LDMOS bias module

1. Introduction

LDMOS RF power transistors require temperature-compensated gate bias voltages to

maintain constant quiescent drain currents over temperature. Additionally, the bias source

must present a very low frequency to the LDMOS gate across the modulation frequency

range (âvideo bandwidthâ) to minimize nonlinearity and memory effects.

This report describes a bias module for LDMOS RF power transistors. It provides a

low-noise bias supply, temperature compensation, and a very low output impedance to

help with video bandwidth optimization.

2. Summary

The characteristics of the bias module described in this report are summarized in Table 1.

Table 1. Summary of bias module characteristics

Parameter

Value

Supply voltage

10 V to 80 V

Supply current

23 mA typical (no-load)

Output voltage

0 V to 3 V[1]

Output voltage adjustment range

1.4 V typical

Output voltage temperature compensation ï2 mV/ï°C typical[2]

Output voltage stability

ï£ 500 ïV

Output impedance

ï£ 2.5 ï, DC to 100 MHz

Output voltage noise

ï£ 100 ïV RMS, 10 Hz to 100 kHz

Output current

70 mA typical

Dimensions

26 mm ï´ 11 mm ï´ 8 mm

[1] Resistor values may have to be changed for part of range.

[2] Uses external NPN temperature sensing transistor in contact with heatsink.

3. Circuit description

3.1 Temperature compensation

The quiescent drain current IDq (and hence the operating point) of the RF device is set by

adjusting the gate-source voltage VGS with a constant-voltage bias source. In an LDMOS

device, the gate-source threshold voltage VGS(th) is inversely proportional to temperature,

with a slope of about ï2 mV/ï°C. To maintain a constant quiescent current, the voltage

generated by the bias supply should vary as a function of the junction temperature Tj of

the RF device.

It is difficult to track the junction temperature exactly. However, reasonable results are

obtained by monitoring the temperature of the baseplate, which is close to the RF

transistor, with the temperature compensated bias circuit used in this amplifier. This circuit

is shown in Figure 1.

R_10032

Report

All information provided in this document is subject to legal disclaimers.

Rev. 1.0 â 24 July 2012

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