5

matching resistor to a cable and a 75

Ω load to ground at the

end of the cable provide a 150

Ω DC load. But output

capacitive coupling opens this DC path so an extra pulldown

resistor on the amplifier output to ground is required.

Figure 4 shows a 250

Ω resistor. Capacitively coupling the

output will require that we shift the output offset voltage

about 1V. The output dynamic range will be 0.6V to 3V.

Input capacitive coupling will increase the needed dynamic

range of the amplifier. The standard NTSC video signal is 1V

peak to peak plus 143mV for the color AC peak. The video

signal is made up of the -286mV sync pulse plus the 714mV

picture signal which may very from 0V to 714mV. The video

signal average value for a black picture is about 28mV

(Figure 2) and with a white picture level is about 583mV

(Figure 3). This gives a maximum change in average value

of about 555mV. A direct coupled amplifier with an standard

NTSC video signal needs a dynamic range of 1.143V. But

with input capacitance coupling the dynamic range

requirements are the sum of the 1.143V video plus the

gain of two this doubles to 3.394V. These amplifiers do not

have this much dynamic range so a gain of less than 2 must

be used to avoid waveform compression under all

conditions.

Capacitively coupling the input and output is worse than a

capacitor only on the input. Without any special

compromises you can only take a gain of one. But if the

backmatch resistor is reduced to 36

Ω, reducing the output

+

–

Video

1V

4.7K

RB

+

-

AMP

1K

RF

0.1µF

CB

68K

RA

0.32VB

+

–

V1

5V

0.64VB

V AMP

75

RO

1V

FIGURE 1.

0.714V

0.0V

-0.286V

Ý10µs

5.1µs

3.8µs

1.3µs

53µs

0.6

0.867

0.583V

Gain = 2

1.8V offset

Amp

OutVolts

0

+0.348

1.166

1.8

1.428

2.062

-0.572

+0.062

FIGURE 2. WHITE LEVEL VIDEO

0.714V

0.0V

-0.286V

Gain = 2

1.8V offset

Amp Out

Volts

+1.8

+3.172

+56mV

+1.172

FIGURE 3. BLACK LEVEL VIDEO

53mV

Average BL

+45mV

0.686V

+28mV

Average Picture Value Change — 555mV

EL2111