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LT1969

network is bootstrapped. This network can also be placed

between the inverting input and an AC ground.

Another compensation scheme for noninverting circuits is

shown in Figure 4. The circuit is unity gain at low frequency

and a gain of 1 + RF/RG at high frequency. The DC output

offset is reduced by a factor of ten. The techniques of

Figures 3 and 4 can be combined as shown in Figure 5. The

gain is unity at low frequencies, 1 + RF/RG at mid-band and

for stability, a gain of 10 or greater at high frequencies.

Output Loading

The LT1969 output stage is very wide bandwidth and able

to source and sink large currents. Reactive loading, even

isolated with a back-termination resistor, can cause ring-

ing at frequencies of hundreds of MHz. For this reason, any

design should be evaluated over a wide range of output

conditions. To reduce the effects of reactive loading, an

optional snubber network consisting of a series RC across

the load can provide a resistive load at high frequency.

Another option is to filter the drive to the load. If a back-

termination resistor is used, a capacitor to ground at the

load can eliminate ringing.

Line Driving Back-Termination

The standard method of cable or line back-termination is

shown in Figure 6. The cable/line is terminated in its

characteristic impedance (50

Ω, 75Ω, 100Ω, 135Ω, etc.).

A back-termination resistor also equal to to the

chararacteristic impedance should be used for maximum

pulse fidelity of outgoing signals, and to terminate the line

for incoming signals in a full-duplex application. There are

three main drawbacks to this approach. First, the power

dissipated in the load and back-termination resistors is

equal so half of the power delivered by the amplifier is

Figure 5. Combination Compensation

1969 F05

CBIG

= 1 AT LOW FREQUENCIES

= 1 +

AT MEDIUM FREQUENCIES

(RC || RG)

= 1 +

AT HIGH FREQUENCIES

Figure 4. Alternate Noninverting Compensation

1969 F04

< 15MHz

πRGCC

RG ≤ RF/9

= 1 (LOW FREQUENCIES)

(HIGH FREQUENCIES)

= 1 +

Figure 6. Standard Cable/Line Back-Termination

1969 F06

RBT

CABLE OR LINE WITH

CHARACTERISTIC IMPEDANCE RL

(1 + RF/RG)

RBT = RL

APPLICATIO S I FOR ATIO

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