Test Circuit

Figure 1 shows a typical test circuit for evaluation of the

LM2427 This circuit is designed to allow testing of the

LM2427 in a 50X environment such as a pulse generator

oscilloscope or network analyzer The 4950X resistor in se-

ries with the output of the LM2427 forms a 1001 voltage

divider when connected to a 50X-input oscilloscope or net-

into 50X

THEORY OF OPERATlON

The LM2427 is a triple channel transimpedance amplifier for

CRT’s suitable for SVGA XGA IBM and Macintosh display

resolution monitors The LM2427 is pin-to-pin compatible

with the LH2426 and CR5527 CRT drivers The device is

packaged in the industry standard 12-lead SIP TO-220

molded plastic power package The heat sink is electrically

isolated and may be grounded for ease of manufacturing

and RFIEMI shielding

Applying an input current to the LM2427 will result in an

input converts the device into a voltage amplifier with a

The LM2427 is a two stage amplifier configured in a push-

pull configuration (see schematic on front page) Q2 is bi-

ased by resistors R4 and R5 Q1 gets its bias through a

5700X feedback resistor and the input biasing current The

bases of Q1 and Q2 are capacitively coupled and therefore

Q2 will be actively driven

The emitter resistors of Q1 and Q2 are bypassed with small

capacitors This increases the gain of the stage for high

frequencies and increases the bandwidth of the amplifier

Emitter followers Q3 and Q4 isolate the input stage from the

output capacitance load and minimizes the circuit sensitivity

to load capacitance

The power supply pin is intemally bypassed If low frequen-

cies are present in the power supply line an electrolytic

capacitor is recommended

Application Hints

The LM2427 is designed as a triple power amplifier for deliv-

ering red green and blue video signals to a cathode ray

tube (CRT) It can provide a 50V output swing and energize

a 12 ns pixel at a CRT cathode with 8 pF of capacitance

As with any CRT driver when designing a video amplifier

board with the LM2427 careful attention should be paid at

reducing stray capacitance along the entire video signal

path This is especially important in the path between the

output of the CRT driver and the cathodes because any

additional capacitance load will increase rise and fall times

and will result in reduced picture quality

INPUT NETWORKS

The voltage gain and the response of the amplifiers can be

set by adding an R-C network to the input

A 430X resistor in series with the input will set the voltage

the system (see

Figure 2a )

Bypassing the resistor with a capacitor of about 47 pF will

restore the rise and fall times but will result in some over-

shoot

(Figure 2b)

Adding a resistor in series with the 47 pF capacitor will re-

duce the overshoot but also increases the rise and fall

times

(Figure 2c)

The addition of a second capacitor offers a compromise

between the above networks by improving the rise and fall

times at the expense of some overshoot

(Figure 2d)

Suggested values for the resistors and capacitors are

shown however optimum values may differ depending

upon the stray inductances and capacitances present in dif-

ferent board layouts

TLH11967 – 6

FIGURE 2 Influence of Input Networks

on Switching Performance

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