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AD8330

Rev. D | Page 14 of 36

THEORY OF OPERATION

CIRCUIT DESCRIPTION

Many monolithic variable gain amplifiers use techniques that

share common principles that are broadly classified as translinear.

This term refers to circuit cells whose functions depend directly

on the very predictable properties of bipolar junction transistors,

notably the linear dependence of their transconductance on collec-

tor current. Since the discovery of these cells in 1967, and their

commercial exploitation in products developed during the early

1970s, accurate wide bandwidth analog multipliers, dividers,

and variable gain amplifiers have invariably employed translinear

principles.

Although these techniques are well understood, the realization

of a high performance variable gain amplifier (VGA) requires

special technologies and attention to many subtle details in

its design. The AD8330 is fabricated on a proprietary silicon-

on-insulator, complementary bipolar IC process and draws

on decades of experience in developing many leading edge

products using translinear principles to provide an unprecedented

level of versatility.

Figure 45 shows a basic representative cell comprising just four

transistors. This, or a very closely related form, is at the heart

of most translinear multipliers, dividers, and VGAs. The key

concepts are as follows:

First, the ratio of the currents in the left-hand and right-hand

pairs of transistors is identical, represented by the modulation

factor, x, with values between −1 and +1. Second, the input

signal is arranged to modulate the fixed tail current, ID, to cause

the variable value of x, introduced in the left-hand pair, to be

replicated in the right-hand pair, and, thus, generate the output

by modulating its nominally fixed tail current, IN. Third, the

current gain of this cell is exactly G = IN/ID over many decades

of variable bias current.

In practice, the realization of the full potential of this circuit

involves many other factors, but these three elementary ideas

remain essential.

By varying IN, the overall function is that of a two-quadrant

analog multiplier, exhibiting a linear relationship to both the

signal modulation factor (x) and this numerator current. On

the other hand, by varying ID, a two-quadrant analog divider

is realized, having a hyperbolic gain function with respect to

the input factor, x, controlled by this denominator current. The

AD8330 exploits both modes of operation. However, because a

hyperbolic gain function is generally of less value than one in

which the decibel gain is a linear function of a control input, a

special interface is included to provide either increasing or

decreasing exponential control of ID.

INPUT IS xlD

DENOMINATOR

BIAS CURRENT

(1–x) ID

+–

LOOP

AMPLIFIER

(1–x) IN

NUMERATOR

BIAS CURRENT

OUTPUT IS xlN

G = IN/ID

(1+x) IN

(1–x) ID

Figure 45. Basic Core

COMM

OPHI

INLO

OPLO

INHI

VPSI

VPSO

CMOP

MODE

VDBS

CMGN

VMAG

OFST

EVPOS

BIAS AND

VREF

GAIN INTERFACE

CM MODE AND

OFFSET CONTROL

OUTPUT

STAGES

OUTPUT

CONTROL

VGA CORE

AD8330

Figure 46. Block Schematic

Overall Structure

Figure 46 shows a block schematic of the AD8330 locating the

key sections. More detailed descriptions of its structure and

features are provided throughout the Theory of Operation

section; however, Figure 46 provides a general overview of its

capabilities.

The VGA core contains a more elaborate version of the cell

shown in Figure 45. The current, ID, is controlled exponentially

(linear-in-decibels) through the decibel gain interface at

Pin VDBS and its local common, Pin CMGN. The gain span

(that is, the decibel difference between maximum and

minimum values) provided by this control function is slightly

more than 50 dB. The absolute gain from input to output is a

function of source and load impedance, and depends on the

voltage on a second gain control pin (VMAG), explained in the

Normal Operating Conditions section.

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