Datasheet

Inverting Video Gain Stage
3-2
3.1 Inverting Video Gain Stage
The circuit described in this section is an inverting gain stage with a voltage
divider on the output, shown in Figure 3−1. The voltage gain from J1 to J4 is
simplified in equation 1. Assuming a 75-Ω source impedance, equation 1
indicates the gain when R1 is changed to 78.7 Ω, and R3 and R5 are changed
to 75 Ω. Rt is the termination resistance of the measurement device.
VOUT
IN *
+
R4
R2
ǒ
Rt
Rt ) R5
Ǔ
+*0.5
R5 is used to match the output impedance of the amplifier to the line being
driven and the instrument taking measurements. For short transmission line
length, R5 can be replaced with a jumper.
R5 can also be used to isolate the amplifier from large capacitive loads.
Figure 3−1. Inverting Video Gain Stage
J4
VOUT
−
+
U1
THS4221
4
3
1
6
2
+VS
−V S
J1
IN−
R4
1.3 kΩ
R5
75 Ω
R3
0 Ω
R1
78.7 Ω
R2
1.3 kΩ
(1)