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LT1228CS8PBF
(Rev.:Rev_C)

100MHz Current Feedback Amplifier with DC Gain Control

Linear Technology 

LT1228

APPLICATIO S I FOR ATIO

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

for the response with 0.5dB to 5dB of peaking. The curves

stop where the response has more than 5dB of peaking.

Current Feedback Amp Small-Signal Response

VS = ±15V, RF = RG = 750Ω, RL = 100Ω

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. For ex-

ample, in a gain of 100 application, the bandwidth can be

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

capacitor, as shown below. CG must have very low series

resistance, such as silver mica.

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 on the bandwidth. At very high closed-loop gains,

the bandwidth is 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.

VIN

1+

6

CFA

8–

RF

510Ω

VOUT

CG

RG

5.1Ω

LT1228 • TA08

Boosting Bandwidth of High Gain Amplifier

with Capacitance On Inverting Input

49

46

43

40

37

34

31

28

25

22

19

1

CG = 4700pF

CG = 2200pF

CG = 0

10

FREQUENCY (MHz)

100

LT1228 • TA09

1228fc

12

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