AD844
Rev. F | Page 12 of 20
UNDERSTANDING THE AD844
The AD844 can be used in ways similar to a conventional op
amp while providing performance advantages in wideband
applications. However, there are important differences in the
internal structure that need to be understood to optimize the
performance of the AD844 op amp.
OPEN-LOOP BEHAVIOR
Figure 28 shows a current feedback amplifier reduced to essen-
tials. Sources of fixed dc errors, such as the inverting node bias
current and the offset voltage, are excluded from this model.
The most important parameter limiting the dc gain is the
transresistance, R
t
, which is ideally infinite. A finite value of R
t
is analogous to the finite open-loop voltage gain in a conven-
tional op amp.
The current applied to the inverting input node is replicated by
the current conveyor to flow in Resistor R
t
. The voltage developed
across R
t
is buffered by the unity gain voltage follower. Voltage
gain is the ratio R
t
/R
IN
. With typical values of R
t
= 3 MΩ and
R
IN
= 50 Ω, the voltage gain is about 60,000. The open-loop
current gain, another measure of gain that is determined by the
beta product of the transistors in the voltage follower stage (see
Figure 31), is typically 40,000.
+1
+1
I
IN
R
IN
I
IN
R
t
C
t
00897-028
Figure 28. Equivalent Schematic
The important parameters defining ac behavior are the
transcapacitance, C
t
, and the external feedback resistor (not
shown). The time constant formed by these components is
analogous to the dominant pole of a conventional op amp and
thus cannot be reduced below a critical value if the closed-loop
system is to be stable. In practice, C
t
is held to as low a value as
possible (typically 4.5 pF) so that the feedback resistor can be
maximized while maintaining a fast response. The finite R
IN
also affects the closed-loop response in some applications.
The open-loop ac gain is also best understood in terms of the
transimpedance rather than as an open-loop voltage gain. The
open-loop pole is formed by R
t
in parallel with C
t
. Because C
t
is
typically 4.5 pF, the open-loop corner frequency occurs at about
12 kHz. However, this parameter is of little value in determining
the closed-loop response.
RESPONSE AS AN INVERTING AMPLIFIER
Figure 29 shows the connections for an inverting amplifier.
Unlike a conventional amplifier, the transient response and the
small signal bandwidth are determined primarily by the value of
the external feedback resistor, R1, rather than by the ratio of
R1/R2 as is customarily the case in an op amp application. This
is a direct result of the low impedance at the inverting input. As
with conventional op amps, the closed-loop gain is −R1/R2.
The closed-loop transresistance is the parallel sum of R1 and R
t
.
Because R1 is generally in the range of 500 Ω to 2 kΩ and R
t
is
about 3 MΩ, the closed-loop transresistance is only 0.02% to
0.07% lower than R1. This small error is often less than the
resistor tolerance.
When R1 is fairly large (above 5 kΩ) but still much less than R
t
,
the closed-loop HF response is dominated by the time constant
R1 C
t
. Under such conditions, the AD844 is overdamped and
provides only a fraction of its bandwidth potential. Because of
the absence of slew rate limitations under these conditions, the
circuit exhibits a simple single-pole response even under large
signal conditions.
In Figure 29, R3 is used to properly terminate the input if desired.
R3 in parallel with R2 gives the terminated resistance. As R1 is
lowered, the signal bandwidth increases, but the time constant
R1 C
t
becomes comparable to higher order poles in the closed-
loop response. Therefore, the closed-loop response becomes
complex, and the pulse response shows overshoot. When R2
is much larger than the input resistance, R
IN
, at Pin 2, most of
the feedback current in R1 is delivered to this input, but as R2
becomes comparable to R
IN
, less of the feedback is absorbed at
Pin 2, resulting in a more heavily damped response. Consequently,
for low values of R2, it is possible to lower R1 without causing
instability in the closed-loop response. Table 3 lists combinations
of R1 and R2 and the resulting frequency response for the circuit
of Figure 29. Figure 16 shows the very clean and fast ±10 V
pulse response of the AD844.
V
IN
V
OUT
R3
OPTIONAL
R2
R1
AD844
R
L
C
L
0897-029
Figure 29. Inverting Amplifier
