7

LT1227

I

SPLII

F ED S

A

CHE

TIC

S

APPLICATI

I FOR ATIO

The LT1227 is a very fast current feedback amplifier.

Because it is a current feedback amplifier, the bandwidth

is maintained over a wide range of voltage gains. The

amplifier is designed to drive low impedance loads such as

cables with excellent linearity at high frequencies.

Feedback Resistor Selection

The small-signal bandwidth of the LT1227 is set by the

external feedback resistors and the internal junction ca-

pacitors. As a result, the bandwidth is a function of the

supply voltage, the value of the feedback resistor, the

closed-loop gain and load resistor. The characteristic

curves of Bandwidth vs Supply Voltage show the effect of

a heavy load (100

Ω) and a light load (1k). These curves

use a solid line when the response has less than 0.5dB of

peaking and a dashed line when the response has 0.5dB to

5dB of peaking. The curves stop where the response has

more than 5dB of peaking.

At a gain of two, on

±15V supplies with a 1k feedback

resistor, the bandwidth into a light load is over 140MHz,

but into a heavy load the bandwidth reduces to 120MHz.

The loading has this effect because there is a mild reso-

nance in the output stage that enhances the bandwidth at

light loads but has its Q reduced by the heavy load. This

enhancement is only useful at low gain settlings; at a gain

of ten it does not boost the bandwidth. At unity gain, the

enhancement is so effective the value of the feedback

resistor has very little effect. At very high closed-loop

gains, the bandwidth is limited by the gain bandwidth

product of about 1GHz. The curves show that the band-

width at a closed-loop gain of 100 is 12MHz, only one tenth

what it is at a gain of two.

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4 V

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7

2

–IN

+IN 3

CURRENT

SOURCE

BIAS

8

S/D

14k

1

5

1227 SS