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LT6274/LT6275

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 LTC6252 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 VIN to

VOUT. The LT6275 op amp drives the output based on

whatever is commanded by the middle node, VMID. 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 (AV = –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

VMID

6275 TA08

11k

1µ

10k

1µ

10k

1/2 LT6275

+15V

–15V

VIN

VOUT

LTC6252

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 (AV = –11V/V)

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