The video amplifier in the diagram is a well-known design. Simple, yet
very useful, were it not for the ease with which the transistors can be
damaged if the potentiometers (black level and signal amplitude) are in
their extreme position. Fortunately, this can be obviated by the
addition of two resistors. If in the diagram R3 and R4 were direct
connections, as in the original design, and P1 were fully clockwise and
P2 fully anticlockwise, such a large base current would flow through T1
that this transistor would give up the ghost.
Moreover, with the wiper of P2 at earth level, the base current of T2 would be dangerously high. Resistors R3 and R4 are sufficient protection against such mishaps, since they limit the base currents to a level of not more than 5 mA. Shunt capacitor C4 prevents R4 having an adverse effect on the amplification.
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