OPA622AP BURR-BROWN [Burr-Brown Corporation], OPA622AP Datasheet - Page 12

• Manufacturer Part Number: OPA622AP
• Description: Wide-Bandwidth OPERATIONAL AMPLIFIER
• Manufacturer: BURR-BROWN [Burr-Brown Corporation]

CONFIGURATIONS
VOLTAGE-FEEDBACK AMPLIFIER
The OPA622’s internal design differs from a “classical”
operational amplifier structure, but it can nevertheless be
used in all traditional operational amplifier applications. As
with conventional op amps, the feedback network connected
to the inverting input controls closed-loop gain (G
with the OPA622, the resistor R
to the closed-loop gain, optimizing the frequency response
and stability.
The “classical” differential input stage consists of two
identical transistors with an emitter degeneration resistor,
two current sources, and an active load diode. However, the
classical configuration limits the current through the gain
transistor to that supplied by the current sources.
In the new design, a complementary push-pull buffer (emit-
ter follower) replaces one side of the differential stage
without the 0.7V offset. The feedback buffer as a second
complementary emitter follower and the open-loop gain
resistor R
differential stage without the disadvantages of the classical
design. The current charging the parasitic capacitance at the
base of the gain transistor is no longer limited to the fixed
current of the current sources and is proportional to the input
signal. This improvement results in an approximately
10-times better slew rate.
The amplified current through the gain transistor of one of
the buffers is mirrored and becomes the output current. The
high-impedance output of the OTA is now buffered by the
high current output stage, which is designed to drive long
cables or low-impedance loads at full power.
FIGURE 5. Voltage-Feedback Amplifier.
OG
®
connected between the outputs recreate the
4
+In
C
R
OPA622
OTA
OG
: Sets the open-loop gain
: Sets the first open-loop pole
+ V
– V
OTA
CC
CC
5
12
13
OG
is simultaneously adapted
R
OG
8
FB
CL
). But
–In
3
R
12
1
The identical input buffers reduce the input offset to typi-
cally less than 7 V. Closed-loop output offset is typically
due to mismatch of the NPN and PNP transistors in the OTA
mirror 100 V after the output bias current is trimmed.
Figure 5 illustrates the circuit configuration of the voltage-
feedback op amp in a complementary circuit design. The
feedback buffer and the OTA input buffer form the
differential input. Inserting the feedback buffer section trans-
forms the current feedback shown in Figure 4 into the
voltage feedback shown in Figure 5.
The resistor R
the emitter degeneration resistor in a classical differential
stage. Because the R
flat frequency response can be achieved over a wide range
of applications without the need to compensate the amplifier
with a capacitor. In contrast to a current-feedback amplifier,
it is possible to adjust the closed-loop gain using the
feedback resistors and to adjust the open-loop gain indepen-
dently using R
Unlike “classical” operational amplifier structures, the
OPA622 configuration makes it possible to attain a nearly
constant bandwidth for varying closed-loop gains, as well as
improved frequency response and large-signal behavior. In
addition—and also unlike current-feedback op amps—it
provides two identical high-impedance inputs, lower input
offset values, and improved CMRR.
CURRENT-FEEDBACK AMPLIFIER
Figure 4 shows the current-feedback configuration. The
feedback loop is closed from the output to the BUF+
terminal of the OTA section. The shorter feedback loop
10
C
OTA
OG
OG
sets the open-loop gain and corresponds to
R
to optimize the frequency response.
+ V
– V
2
OG
CC OUT
OB
CC OUT
11
6
resistor can be varied externally, a
9
G
V
CL
OUT
= 1 +
R
R
2
1