AD8007/AD8008

Rev. E | Page 15 of

20

THEORY OF OPERATION

The AD8007 (single) and AD8008 (dual) are current feedback

amplifiers optimized for low distortion performance. A simplified

conceptual diagram of the AD8007 is shown in Figure 51. It

closely resembles a classic current feedback amplifier comprised

of a complementary emitter-follower input stage, a pair of signal

mirrors, and a diamond output stage. However, in the case of

the AD8007/AD8008, several modifications were made to improve

the distortion performance over that of a classic current feedback

topology.

IDI

–

+VS

–VS

CJ1

CJ2

Q1

Q2

IN–

–

–

–

D1

D2

I1

I2

IN+

I3

I4

IDO

Q3

Q4

Q5

Q6

+VS

–VS

RF

OUT

RG

M2

M1

HIGH-Z

Figure 51. Simplified Schematic of AD8007

The signal mirrors were replaced with low distortion, high

precision mirrors. In Figure 51, they are shown as M1 and M2.

Their primary function from a distortion standpoint is to reduce

the effect of highly nonlinear distortion caused by capacitances,

CJ1 and CJ2. These capacitors represent the collector-to-base

capacitances of the output devices of the mirrors.

A voltage imbalance arises across the output stage, as measured

from the high impedance node, high-Z, to the output node, OUT.

This imbalance is a result of delivering high output currents and

is the primary cause of output distortion. Circuitry is included

to sense this output voltage imbalance and generate a compensating

current, IDO. When injected into the circuit, IDO reduces the

distortion that could be generated at the output stage. Similarly, the

nonlinear voltage imbalance across the input stage (measured from

the noninverting to the inverting input) is sensed, and a current,

IDI, is injected to compensate for input-generated distortion.

The design and layout are strictly top-to-bottom symmetric to

minimize the presence of even-order harmonics.

USING THE AD8007/AD8008

Supply Decoupling for Low Distortion

Decoupling for low distortion performance requires careful

consideration. The commonly adopted practice of returning the

high frequency supply decoupling capacitors to physically separate

(and possibly distant) grounds can lead to degraded even-order

harmonic performance. This situation is shown in Figure 52 using

the AD8007 as an example; however, it is not recommended. For a

sinusoidal input, each decoupling capacitor returns to its ground a

quasi-rectified current carrying high even-order harmonics.

+VS

–VS

RG

499Ω

RS

200Ω

IN

RF

499Ω

GND 1

GND 2

OUT

AD8007

+

+

10µF

10µF

0.1µF

0.1µF

Figure 52. High Frequency Capacitors Returned to Physically Separate

Grounds (Not Recommended)

The decoupling scheme shown in Figure 53 is recommended.

In Figure 53, the two high frequency decoupling capacitors are

first tied together at a common node and are then returned to

the ground plane through a single connection. By first adding

the two currents flowing through each high frequency decoupling

capacitor, this ensures that the current returned into the ground

plane is only at the fundamental frequency.

+VS

–VS

RG

499Ω

RS

200Ω

IN

RF

499Ω

OUT

AD8007

+

+

10µF

0.1µF

10µF

0.1µF

Figure 53. High Frequency Capacitors Returned to Ground at a Single Point

(Recommended)