age of Q29 There are also four diode drops from the base

of Q30 to the output pin 20 Therefore during blanking pin

20 will be less than 100 mV above ground enabling the

designer to blank at the cathode of the CRT R23 is added

to quickly turn off Q25 by discharging its base when the

blanking signal is removed

Figure 14 also shows the power and ground pins to the

internally connected together A 01 mF bypass capacitor

must be located close to each pin and connected to ground

Further bypassing is done by a 100 mF capacitor This ca-

pacitor needs to be located on the board close to the

10 mF and a 01 mF bypass capacitors must be located

close to pins 22 and 23 Correct bypassing of pins 22 and

23 is very important If the bypassing is not adequate then

the outputs of the LM1205LM1207 will have ringing or

even worse they may oscillate The ground side of the by-

pass capacitors at pins 22 and 23 must be returned to a

ground plane with no interruptions from other traces be-

tween these capacitors and the ground pins 21 and 24 of

the LM1205LM1207

Applications of the LM1205

LM1207

Figure 15 is the schematic of the demonstration board de-

signed at National

Figure 16 is the actual layout of the dem-

onstration board

Note that the schematic shown in

Figure 15 is almost identical to the schematic shown in Fig-

ure 4 The only difference between the two schematics is

that in

Figure 15 each channel has individual adjustments

for both drive and cutoff making this circuit a good design

for monitor applications Each CRT will have a slightly differ-

ent cutoff voltage for each color making it necessary to

provide separate adjustments in order to accurately set the

cutoff for each color The gain of each color of the CRT is

also slightly different if the color temperature of the display

is to be accurately set then each channel of the LM1205

LM1207 must have individual gain adjustments Thus each

channel has its own drive control Once the drive control is

set the gain between the three color channels will closely

track as the contrast is adjusted All the jumpers needed to

design a single sided PC board are shown in the schematic

The resistors and jumpers with no reference designation are

the connections between the PC board and the connectors

mounted on the PC board CN1 thru CN8 are BNC connec-

tors

A30X resistor is in series with each of the video inputs A

voltage surge may occur at these inputs when either the

inputs are first connected to another system or when the

system is powered up before the monitor is turned on If this

voltage surge exceeds the supply voltage (at ground poten-

tial if the monitor is not powered up) of the LM1205

LM1207 or goes below ground current will flow through the

parasitic devices of the LM1205LM1207 This current is

limited by the 30X resistors preventing a potential cata-

strophic failure A 100X resistor is added to the Blank Gate

TLH11881 – 22

FIGURE 14 Simplified Schematic of LM1205LM1207 Video Amplifier Output Stage with Blank Circuit

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