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The Grid Bias. As we mentioned in part 1, positive grid voltage causes the grid act like a tiny plate, attracting few electrons which flow trough external connections back to the cathode. This is known as grid current. In some tube applications, grid current is desired, while in other applications is not and must be avoided. Most amplifier circuits are designed to operate with the grid NEGATIVE relative to the cathode. The voltage that causes this is called BIAS VOLTAGE. The GRID BIAS is a constant DC that is placed in the external circuit between the grid and cathode and is always in series with the input signal. Types of Grid Biasing. There are two main types of biasing - Fixed and Self. In a tube circuit that uses Fixed-Bias, the bias voltage is supplied from a separate power source shown in Fig.2-1
The most common type of biasing is called Self-Biasing. There are two methods of self-bias: Cathode Biasing and Grid-Leak Biasing. Cathode Bias. In this type of biasing, a cathode resistor is used. Grid voltage is kept at ground potential with a high value Rg resistor. A voltage drop across the cathode resistor Rk makes the cathode go POSITIVE relative to the grid. The same effect as making the grid NEGATIVE. Fig.2-2 shows cathode biasing method.
Grid-Leak Biasing. Grid-Leak bias makes use ofthe flow of grid current during a portion of the input cycle to develop negative voltage which biases the grid. Refer to Fig.2-3
When the input voltage swings positive, grid current flows, charging the grid capacitor Cg. Next, when the input signal swings negative, the grid capacitor discharges through the path of the signal source and resistor Rg. Since Rg has a high resistance value, the capacitor discharges slowly over the entire negative portion of the input cycle. Therefore, the average grid potential is negative, provided the grid becomes positive for at least a small part of each cycle.
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