AD8007/AD8008
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 3. It closely
resembles a classic current feedback amplifier comprised of a
complementary emitter-follower input stage, a pair of signal mir-
rors, and a diamond output stage. However, in the case of the
AD8007/AD8008, several modifications have been made to greatly
improve the distortion performance over that of a classic current
feedback topology.
I1 –
+VS
M1
– I3
D1
IN+
D2
Q1
IDI
IN–
Q2
CJ1
+VS
HiZ
–VS
CJ2
Q3
IDO
Q4
I2 –
M2
RG
– I4
RF
Q5
OUT
Q6
–VS
Figure 3. Simplified Schematic of AD8007
The signal mirrors have been replaced with low distortion, high
precision mirrors. They are shown as “M1” and “M2” in Figure 3.
Their primary function from a distortion standpoint is to greatly
reduce the effect of highly nonlinear distortion caused by capaci-
tances CJ1 and CJ2. These capacitors represent the collector-to-base
capacitances of the mirrors’ output devices.
A voltage imbalance arises across the output stage, as measured
from the high impedance node “HiZ” 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 would 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 cur-
rent “IDI” is injected to compensate for input-generated distortion.
The design and layout are strictly top-to-bottom symmetric in
order 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 sepa-
rate (and possibly distant) grounds can lead to degraded
even-order harmonic performance. This situation is shown in
Figure 4 using the AD8007 as an example. Note that for a sinu-
soidal input, each decoupling capacitor returns to its ground a
quasi-rectified current carrying high even-order harmonics.
RF
499⍀
GND 1
RG
499⍀
0.1F
+VS
10F
+
RS AD8007
200⍀
IN
–VS
0.1F
10F
+
OUT
GND 2
Figure 4. High Frequency Capacitors Returned
to Physically Separate Grounds (Not Recommended)
The decoupling scheme shown in Figure 5 is preferable. Here,
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, one is ensuring that the current returned into the
ground plane is only at the fundamental frequency.
RF
499⍀
RG
+VS
499⍀
RS
200⍀
IN
10F
+
0.1F
AD8007
0.1F
–VS
10F +
OUT
Figure 5. High Frequency Capacitors Returned
to Ground at a Single Point (Recommended)
Whenever physical layout considerations prevent the decoupling
scheme shown in Figure 5, the user can connect one of the high
frequency decoupling capacitors directly across the supplies and
connect the other high frequency decoupling capacitor to ground.
This is shown in Figure 6.
–14–
REV. C