7 General-Purpose Amplifiers For Vhf To Microwave.pdf

Chapter

General-Purpose Amplifiers for

VHF to Microwave

This chapter is devoted to typical IC amplifiers for use in wireless/RF

circuits. The MAX2630 through 2633 are selected as examples. These

ICs operate from a single

5.5-V supply, and have a flat gain

response to 900 MHz. Their low noise figure and low supply current

make the ICs ideal for receiver, buffer, and transmitter IF applications,

including cordless phones, cellular phones, TV tuners, set-top boxes,

land-mobile radios, PCs (personal communicating systems), GPS (glob-

al positioning systems), WLAN (wireless local area networks), and

WLL (wireless local loops).

2.7-V to

7.1

MAX2630 through 2633 Characteristics

Figures 7.1, 7.2, 7.3, and 7.4 show a typical operating circuit, pin con-

figurations, ratings and characteristics, and pin descriptions, respec-

tively. The MAX2630/31 are biased internally, eliminating the need for

external bias resistors or inductors. The MAX2632/33 have a user-

selectable supply current, which can be adjusted by adding a single

external resistor. This allows customized output power and gain,

according to specific application requirements.

The MAX2631/33 feature a shutdown pin that allows the ICs to be

powered down to less than 1

A of supply current. Aside from a single

bias resistor required for the MAX2632/33, the only external compo-

nents needed for this family of IC amplifiers are input- and output-

blocking capacitors and a V CC bypass capacitor.

117

118

Chapter Seven

Figure 7.1 MAX2630 typical operat-

ing circuit ( Maxim, New Releases

Data Book, 1998, p. 10-63 ).

Figure 7.2 MAX2630 through

MAX2633 pin configurations

( Maxim, New Releases Data

Book, 1998, p. 10-63 ).

7.2 External Components for Typical

Operating Circuits

Figures 7.5, 7.6, and 7.7 show typical operating circuits for the

MAX2630, MAX2631, and MAX2632, respectively. In most cases, exter-

nal capacitors for the input and output are required to block dc bias

voltages generated by the amplifiers from interfering with adjacent cir-

cuitry. The blocking capacitors must be large enough to contribute neg-

ligible reactance in a 50-

system at the minimum operating frequency.

Use the following to calculate the minimum blocking-capacitor value:

53,000

C BLOCK (in pF)

where f (in MHz) is the minimum operating frequency.

General-Purpose Amplifiers for VHF to Microwave

119

7.3

Optimizing MAX2630 through MAX2633 Layout

Figures 7.8, 7.9, 7.10, and 7.11 show examples of recommended PC-

board layout for the MAX2630, MAX2631, MAX2632, and MAX2633,

respectively. All layouts use FR-4 with a 31-mil layer thickness

between the RF lines and the ground plane.

In all cases, the V CC pin must be RF-bypassed for correct operation.

This is done by connecting a capacitor between the V CC pin and ground,

as close to the IC package as is practical. Use the same equation giv-

en in Section 7.2 (for dc blocking-capacitor values) to calculate the

bypass-capacitor value. If the PC board has long V CC lines, additional

bypassing might be required. This can be done farther away from the

IC package (if needed).

Figure 7.3 MAX2630 through MAX2633 ratings and characteristics ( Maxim, New

Releases Data Book, 1998, p. 10-64 ).

Figure 7.4 MAX2630 through MAX2633 pin descriptions ( Maxim, New Releases Data

Book, 1998, p. 10-66 ).

Figure 7.5 MAX2630 typical operating circuit

( Maxim, New Releases Data Book, 1998, p. 10-69 ).

Figure 7.6 MAX2631 typical operating circuit

( Maxim, New Releases Data Book, 1998, p. 10-69 ).

Figure 7.7 MAX2632 typical operating circuit

( Maxim, New Releases Data Book, 1998, p. 10-69 ).

General-Purpose Amplifiers for VHF to Microwave

121

Figure 7.8 MAX2630 recommended PC-board layout

( Maxim, New Releases Data Book, 1998, p. 10-70 ).

Figure 7.9 MAX2631 recommended PC-board layout

( Maxim, New Releases Data Book, 1998, p. 10-70 ).

Proper grounding of the GND pin is essential. If the PC board uses

a top-side RF ground, connect this ground directly to the GND pin. For

boards where the ground plane is not on the component side, the best

technique is to connect the GND pin to the plane with a plated

through-hole close to the IC package.

The S

pin (MAX2631/33) does not usually require bypassing,

except in very noisy applications. When RF filtering is needed, use a

bypass capacitor similar to the one used on V CC . Because negligible

current flows into the S

pin, additional RF filtering can be per-

formed with a series resistor.

To set the MAX2632/33 supply current, connect a resistor from the

BIAS pin to ground (Fig. 7.7). To estimate the value for the bias resis-

tor, use the graph of Fig. 7.12.

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