Learning Event 2:
DESCRIBE THE THEORY OF THE PREAMPLIFIER CIRCUITRY
1. Preamplifier. For every pickup tube in a studio camera head there is always
preamplification circuitry. Studio color cameras have three pickup tubes;
consequently, they have three preamplifiers.
2. Operation of a Preamplifier. What does preamplification do for the camera
head? First of all, it receives a high current low voltage input from the target
of the pickup tube. This DCcoupled signal is changed within the preamplifiers to
a stable voltage output which goes to its respective video amplifier circuitry
input (fig 12 foldout located at the end of Lesson 3).
3. Circuitry of a Preamplifier. The most common examples of circuitry of a
preamplifier are the following (fig 12).
a. The bias adjust circuit permits adjustment of the preamplifier video
output to zero volts DC at black level. This normalizes the DC bias levels for the
entire preamplifier.
(1) What is bias? Bias is a negative or positive voltage that either
turns on, turns off, or regulates the voltage and current output of the electronic
device.
(2) Black level is a constant voltage that video references to.
b. The midrange and low frequency circuits are common in preamplifiers.
They are used to shape the video wave form and reduce unwanted peaking caused by
stray capacitance.
c. The video gain is set according to the manufacturer's manual. The video
gain control biases the video amplifier stage of the preamplifier stage, so it will
conduct and amplify the video signal to the manufacturer's specifications.
4. Basic Operation. The video signal is sent from the target of the pickup tube
to the input of the preamplifier (fig 13).
3
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