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LT6274/LT6275

6275fa

For more information www.linear.com/LT6275

TYPICAL APPLICATIONS

Noninverting Amplifier Slew Rate and Step Response

Figure 1 shows a noninverting amplifier with closed-loop

gain of 11V/V. The closed-loop bandwidth of this ampli-

fier is approximately GBW/11 (GBW = Gain-Bandwidth

Product). For a step input, the output follows an expo-

nential curve:

VOUT = VINITIAL +AV • VINPUTSTEP • 1–e

– t

⎛

⎝

⎜

⎞

⎠

⎟

⎛

⎝

⎜

⎜⎜

⎞

⎠

⎟

⎟⎟ (1)

where τ = time constant associated with the closed-loop

bandwidth.

The maximum slew rate occurs in the beginning of the

output response:

VOUTSRMAX = AV • VINPUSTEP •

(2)

Keep in mind that the closed-loop bandwidth and the

closed-loop gain are related (τ = τo AV), so Equation (2)

is simplified to:

VOUTSRMAX = VINPUTSTEP •

τo

(3)

where τo = time constant associated with the LT6274/

LT6275 GBW.

Interestingly, Equation (3) reveals that the maximum slew

rate is nominally related only to the input step size and the

op amp’s inherent GBW. Closing the loop to implement

AV > 1 gain configurations slows down the response, but

increases the excursion. The resulting maximum slew rate

remains the same.

The LT6274/LT6275 feature ample slew rate capability

with low power consumption. Because the input stage

architecture allows high slew rate with low input stage

quiescent currents, the overall power consumption when

amplifying pulses is very low; additional power is only

drawn from the supplies during the highest slew rate

moments of the exponential response.

Since GBW of the LT6274/LT6275 is 40MHz, Equation (3)

suggests that the maximum slew rate in a step response

whose output swings 25V (implying VINPUTSTEP = 25/11

= 2.27V) is 571V/µs. The LT6274/LT6275 high slew

capability ensures that the output response is never slew

rate limited despite the very high excursion.

Figure 2 shows the output response to varying input step

amplitudes. Note that none of the exponential responses

is limited by the initial slew rate (which increases with

increasing amplitude).

As a particular example, with AV = +11V/V, 15V output

excursion, and 40 MHz GBW, Equation (3) predicts a

maximum slew rate of 343V/μs. Measurement on the cor-

responding curve in Figure 2 shows 390V/μs, which is in

good agreement with the prediction. As another example,

with an 18.5V output excursion, the predicted maximum

slew rate is 423V/μs; measurement shows 460V/μs.

As the peak to peak voltage of the input step changes, the

maximum initial slew rate changes. The 63% rise time

of the closed loop response, however, does not change

(as seen in Figure 2), because the closed loop bandwidth

stays constant for all input amplitudes.

6275 TA06

200

1/2 LT6275

+15V

–15V

VIN

VOUT

AV = +11

50ns/DIV

–15

–12

–9

–6

–3

6275 TA07

Figure 1. LT6275 Configured in a Noninverting Gain of AV = +11V/V

Figure 2. Noninverting Amplifier Step Response (AV = +11V/V)

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