Video signals
contain high frequencies, and they use coaxial cable to transmit and
receive signals. The cable connecting these circuits has a
characteristic impedance of 75 Ω. To prevent reflections, which
cause distortion and ghosting, the input and output circuit
impedances must match the 75 Ω cable.
Matching the input
impedance is simple for a noninverting amplifier because its input
impedance is very high; just make RIN = 75 Ω. RF and RG can be
selected as high values, in the hundreds of Ohms range, so that they
have minimal affect on the impedance of the input or output circuit.
A matching resistor, RM, is placed in series with the op amp output
to raise its output impedance to 75 Ω; a terminating resistor, RT,
is placed at the input of the next stage to match the cable (Figure
3–9).
Figure 3–9.
Video Amplifier
The matching and
terminating resistors are equal in value, and they form a voltage
divider of 1/2 because RT is not loaded. Very often RF is selected
equal to RG so that the op amp gain equals two. Then the system gain,
which is the op amp gain multiplied by the divider gain, is equal to
one (2 × 1/2 = 1).
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