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射频功率放大器设计

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Amplifier Design Tutorial

Introduction

This tutorial will set out the design stages required to design a theoretical microwave amplifier

with the following specification shown in Table 1:

Table 1 Required Specification

Parameter Units

Frequency 1.45 – 1.55 GHz

Gain

12.5 ± 0.2

dB

Noise Figure

≤ 2.0

dB

Output VSWR

≤ 1:1.5 (>13dB)

Gain ripple < 1.5 dB

The first stage in the design process is to pick a suitable device. For X-Band and above GaAs

MESFETS are used while at lower frequencies Bipolar devices are used if noise is not so

critical. Try to pick a device design for the range of frequencies you require. Don’t for example

use an X-band device for an LNA at UHF – you are bound to run into stability problems. Also

pick a device that will give you plenty of gain margin to allow for noise mismatching etc.

For this design an Agilent AT41435 Bipolar transistor has been used. This device has > 14dB

of gain at 2GHz with an associated noise figure of <1.7dB.

To double-check the gain available we can use a simple ‘rule-of-thumb’ estimate by

evaluating

|S21|/|S12|.

At 1.5GHz this will be 4.63/0.063 = 73.5 or 10*LOG(73.5) = 18.6dB

We should easily meet the specification for overall gain and allow for significant output

mismatch to allow for gain equalisation and minimum noise. This estimation needs to be

checked against the stability factor K of the device as this effects the gain and gives an

indication to whether the device is likely to oscillate or not.

The device at 1.5GHz is unconditionally stable with a K of >1.

The ADS simulation shown in Figure 1 has been setup to calculate K factor and plot

minimum noise figure.

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