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AD830

REV. A

–9–

UNDERSTANDING THE AD830 TOPOLOGY

The AD830 represents Analog Devices’ first amplifier product

to embody a powerful alternative amplifier topology. Referred to

as active feedback, the topology used in the AD830 provides in-

herent advantages in the handling of differential signals, differ-

ing system commons, level shifting and low distortion, high

frequency amplification. In addition, it makes possible the

implementation of many functions not realizable with single op

amp circuits or is superior to op amp based equivalent circuits.

With this in mind, it is important to understand the internal

structure of the AD830.

The topology, reduced to its elemental form, is shown below in

Figure 21. Nonideal effects such as nonlinearity, bias currents

and limited full scale are omitted from this model for simplicity,

but are discussed later. The key feature of this topology is the

use of two, identical voltage-to-current converters, GM, that

make up input and feedback signal interfaces. They are labeled

with inputs VX and VY, respectively. These voltage to current

converters possess fully differential inputs, high linearity, high

input impedance and wide voltage range operation. This enables

the part to handle large amplitude differential signals; they also

provide high common-mode rejection, low distortion and negli-

gible loading on the source. The label, GM, is meant to convey

that the transconductance is a large signal quantity, unlike in the

front-end of most op amps. The two GM stage current outputs

IX and IY, sum together at a high impedance node which is char-

acterized by an equivalent resistance and capacitance connected

to an “ac common.” A unity voltage gain stage follows the high

impedance node to provide buffering from loads. Relative to

either input, the open loop gain, AOL, is set by the

transconductance, GM, working into the resistance, RP; AOL =

RP. The unity gain frequency

0 dB for the open loop gain

is established by the transconductance, GM, working into the

capacitance, CC; ω0 dB = GM/CC. The open loop description of

the AD830 is shown below for completeness.

A=1

VOUT

VX2

VX1

IX = (VX1 – VX2) GM

IY = (VY1 – VY2) GM

IZ = IX + IY

VY2

VY1

AOLS =

GM RP

1 + S (CC RP)

Figure 21. Topology Diagram

A=1

VOUT

VX2

VX1

VX1 – VX2 = VY2 – VY1

FOR VY2 = VOUT

VOUT = (VX1 – VX2 + VY1)

VY2

VY1

1 + S(CC /GM)

Figure 22. Closed-Loop Connection

Precise amplification is accomplished through closed-loop op-

eration of this topology. Voltage feedback is implemented via

the Y GM stage in which where the output is connected to the

–Y input for negative feedback as shown in Figure 22. An input

signal is applied across the X GM stage, either fully differentially

or single-ended referred to common. It produces a current sig-

nal which is summed at the high impedance node with the out-

put current from the Y GM stage. Negative feedback nulls this

sum to a small error current necessary to develop the output

voltage at the high impedance node. The error current is usually

negligible, so the null condition essentially forces the Y GM

output stage current to exactly equal the X GM output current.

Since the two transconductances are identical, the differential

voltage across the Y inputs equals the negative of the differential

voltage across the X input; VY = –VX or more precisely

VY2–VY1 = VX1–VX2. This simple relation provides the basis to

easily analyze any function possible to synthesize with the

AD830, including any feedback situation.

The bandwidth of the circuit is defined by the GM and the

capacitor CC. The highly linear GM stages give the amplifier a

single pole response, excluding the output amplifier and loading

effects. It is important to note that the bandwidth and general dy-

namic behavior is symmetrical (identical) for the noninverting and

the inverting connections of the AD830. In addition, the input im-

pedance and CMRR are the same for either connections. This is

very advantageous and unlike in a voltage or current feedback

amplifier, where there is a distinct difference in performance be-

tween the inverting and noninverting gain. The practical impor-

tance of this cannot be overemphasized and is a key feature

offered by the AD830 amplifier topology.

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