Television Camera Experiments

JFET input. PN photodiodes such as the popular `bullet cell` put out an extremely low-level signal. Yet photodiodes offer advantages over phototransistors. For example, more rapid response to motion. Also, some photodiodes have a peak response in the range of visible light. On the other hand, most phototransistors provide peak response in the infrared. For color reproduction, flat visible light response is essential.
Television Camera Experiments - schematic

Anyway, with photodiodes, a high-impedance preamplifier is even more important than it is with phototransistors. For such applications, the JFET is the amplifier of choice. A JFET is only a little more difficult to work with than is a bipolar transistor. Yet a JFET is more sensitive than either a bipolar transistor or a vacuum tube. Since the output of the photodiode is a high-impedance signal, the JFET is a helpful idea. A JFET adds complexity, but will probably save you a few amplifier stages. Coupling. The high-Z (high-impedance) transistor preamp allows resistance coupling (actually "resistance-capacity" coupling). In their amplifiers, the television pioneers used this same, resistance coupling. Because of their very limited drift, resistance-coupled amplifiers tend to be stabler than direct-coupled amplifiers. The low impedance inputs in normal transistor preamplifiers require electrolytic capacitors in the coupling network. When new, these capacitors are dreadfully leaky. As the capacitors age, the leakage only increases until it destroys the bias voltage on the transistor. At that point, the transistor will block and quit operating. Even before the bias deteriorates, the capacitor can visibly alter television signals. Yet with a high-impedance preamplifier, you can avoid electrolytic capacitors. Instead, insist on premium capacitor, such as the mylar or polyester types. Bypass. Coupling and bypass capacitors don`t...

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