Datasheet

LT6274/LT6275
15
6275fa
For more information www.linear.com/LT6275
TYPICAL APPLICATIONS
Using the LT6274/LT6275 to Create a Composite
Amplifier with High Gain, High Bandwidth and Large
Output Signal Capability
While the LT6274/LT6275 provide ample slew rate and
large output swing capability, the GBW is not so large
as to achieve high gain, high bandwidth, and high ampli-
tude at the same time. The circuit of Figure 3 harnesses
the high slew rate capability of the
LT6275 by placing it
under control of the LTC6252, an op amp with greater
than 700MHz GBW. The LT C6252 offers high bandwidth
at low supply current, but with limited slew rate and lim-
ited output swing (since it is a 5V op amp). By creating a
composite amplifier adding the LT6275 as a high-voltage,
high-slew secondary op amp, this composite amplifier
enables large output swing at high frequencies with rela-
tively low power dissipation.
Circuit Description
R4
and R1 realize inverting gain of –11V/V from V
IN
to
V
OUT
. The LT6275 op amp drives the output based on
whatever is commanded by the middle node, V
MID
. The
LTC6252 is very fast relative to the LT6275. As a conse-
quence, the LTC6252
controlling first stage can force the
LT6275 output to move quickly by providing sufficient
differential input voltage to the LT6275. With the inverting
input of the LT6275 tied to a DC bias voltage, the LTC6252
needs merely to drive the noninverting input.
Unlike the LTC6252, the LT6275 slew rate increases lin-
early with its differential input voltage. Hence, the LTC6252
benefits from using the LT6275 as a slew enhancer.
Optimizing the Loop
Larger R2 increases the local gain taken by the LTC6252.
Since the total gain is fixed by the global feedback around
the composite amplifier (A
V
= –R4/R1 = –11V/V), raising
the gain in the LTC6252 lowers the gain requirement of
the LT6275, increasing the overall bandwidth of the com
-
posite amplifier. Care must be taken to not take too much
gain in the LTC6252, as the reduction in the LTC6252
bandwidth and the resulting additional phase shift seen
at the output of the LTC6252 can lower the stability mar-
gins of the composite amplifier. Conversely, smaller R2
V
MID
6275 TA08
R
4
11k
C
5
1µ
R
6
10k
R
5
10k
R
1
1k
C
1
1µ
C
2
1µ
R
3
10k
R
2
2k
C
7
3p
1/2 LT6275
+15V
–15V
V
IN
V
OUT
LTC6252
5V
5V
reduces the LTC6252 phase shift, but it also adds to the
gain burden of the LT6275.
R2 was selected to take a gain of 2V/V in the LTC6252,
implying a gain of 5.5V/V being taken in the LT6275. The
5.5V/V gain is required to translate the 5V maximum out
-
put swing of the LTC6252
to the 27.5V maximum output
swing of the LT6275 (when operated at ±15V supplies). It
may be possible to achieve even higher bandwidth in the
composite amplifier if a high speed ±5V (rather than 5V,
0V) op amp replaces the LTC6252 as the first stage, with
the resulting increased first-stage output swing lowering
the gain that has to be taken in the LT6275.
Capacitor C7 in Figure 3 is adjusted to create a favorable
looking transient response. Figure 4 shows the transient
response at the output of the LT6275 as C7 varies. C7 =
3pF was chosen.
DC Biasing
In the circuit of Figure 3, LTC6252 supplies were cho-
sen to be 5V and 0V, which are more practical than split
±2.5V supplies. R5 and R6 form a resistive divider to
bias the noninverting input of LTC6252 and the inverting
input of LT6275 at the middle of this rail, 2.5V. Note that
this approach results in the output of LT6275 having a DC
offset of 2.5V, which reduces the potential peak to peak
output excursion of the composite amplifier since LT6275
is powered up from split ±15V supplies.
Figure 3. Composite Amplifier Using
LTC6252 and LT6275 (A
V
= –11V/V)
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