TLH11442 – 8

FIGURE 3 Typical Application Circuit (One Channel)

IMPROVING RISE AND FALL TIMES

Because of an emitter follower output stage the rise and fall

times of the LM2419 are relatively unaffected by capacitive

loading However the series resistors R1 and R2 (see

Fig-

ure 3 ) will reduce the rise and fall times when driving the

CRT’s cathode which appears as a capacitive load The ca-

pacitance at the cathode typically ranges from 8 pF to

12 pF

To improve the rise and fall times at the cathode a small

inductor is often used in series with the output of the amplifi-

er The inductor L1 in

Figure 3 peaks the amplifier’s fre-

quency response at the cathode thus improving rise and fall

times The inductor value is empirically determined and is

dependent on the load An inductor value of 01 mHisa

good starting value Note that peaking the amplifier’s fre-

quency response will increase the overshoot

REDUCING OVERSHOOT

LM2419’s overshoot is a function of both the input signal

rise and fall times and the capacitive loading The overshoot

is increased by either more capacitive loading or faster rise

and fall times of the input signal

Table I shows the overshoot for a typical device with differ-

ent capacitive loads and different input signal rise and fall

times As can be observed from Table I overshoot is large

for large capacitive loads and faster input signal rise and fall

times In an actual application the LM2419 is driven from a

preamplifier with rise and fall times of 3 ns to 7 ns When

driven from LM1203 preamplifier (see application circuit

Figure 6 ) the overshoot is 10% with 12 pF capacitive load

The overshoot can be reduced by including a resistor in

series with LM2419’s output as in

Figure 3 Larger value

resistors for R1 and R2 would reduce overshoot but this

also increases the rise and fall times at the output Frequen-

cy peaking using an inductor in series with the output may

restore the bandwidth

Table I LM2419 Output Overshoot

vs Capacitive Loading for a Typical Device

Input Signal

5 pF

8 pF

11 pF

15 pF

12 ns

4%

6%

7%

8%

7ns

4%

5%

6%

7%

GAIN VS OUTPUT DC LEVEL

Figure 4 shows LM2419’s gain versus output DC level A

the input signal’s DC level is swept As can be seen from

Figure 4 the amplifier’s gain is constant at approximately

and 65V Thus the amplifier’s output response is linear for

output voltage between 35V and 65V If the output voltage

is less than 35V or more than 70V the amplifier’s output

response becomes non-linear (note the change in gain

Fig-

ure 4 ) For optimum performance it is recommended that

LM2419’s output low voltage be at 25V or above For a

signal swing from 25V to 75V LM2419’s linearity error is

measured at 8%

TLH11442 – 9

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