7

LT1229/LT1230

limited by the gain bandwidth product of about 1GHz. The

curves show that the bandwidth at a closed-loop gain of

100 is 10MHz, only one tenth what it is at a gain of two.

Capacitance on the Inverting Input

Current feedback amplifiers want resistive feedback from

the output to the inverting input for stable operation. Take

care to minimize the stray capacitance between the output

and the inverting input. Capacitance on the inverting input

to ground will cause peaking in the frequency response

(and overshoot in the transient response), but it does not

degrade the stability of the amplifier. The amount of

capacitance that is necessary to cause peaking is a func-

tion of the closed-loop gain taken. The higher the gain, the

more capacitance is required to cause peaking. We can

add capacitance from the inverting input to ground to

increase the bandwidth in high gain applications. For

example, in this gain of 100 application, the bandwidth can

be increased from 10MHz to 17MHz by adding a 2200pF

capacitor.

LT1229 • TA05

–

+

5.1

Ω

510

Ω

1/2

LT1229

Boosting Bandwidth of High Gain Amplifier with

Capacitance on Inverting Input

FREQUENCY (MHz)

1

19

22

25

28

31

46

49

10

100

LT1229 • TA06

34

37

40

43

S

APPLICATI

I FOR ATIO

The LT1229/LT1230 are very fast dual and quad current

feedback amplifiers. Because they are current feedback

amplifiers, they maintain their wide bandwidth over a wide

range of voltage gains. These amplifiers are designed to

drive low impedance loads such as cables with excellent

linearity at high frequencies.

Feedback Resistor Selection

The small-signal bandwidth of the LT1229/LT1230 is set

by the external feedback resistors and the internal junction

capacitors. 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 versus Supply Voltage are done with

a heavy load (100

Ω)andalightload(1k)toshowtheeffect

of loading. These graphs also show the family of curves

that result from various values of the feedback resistor.

These curves use a solid line when the response has less

than 0.5dB of peaking and a dashed line when the re-

sponse has 0.5dB to 5dB of peaking. The curves stop

where the response has more than 5dB of peaking.

Small-Signal Rise Time with

LT1229 • TA04

At a gain of two, on

±15V supplies with a 750Ω feedback

resistor, the bandwidth into a light load is over 160MHz

without peaking, but into a heavy load the bandwidth

reduces to 100MHz. The loading has so much effect

because there is a mild resonance 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 settings; 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