The Grid or 'C' Bias

Author: J.B. Hoag

In many applications of three-electrode tubes it is necessary to maintain the grid at a fixed negative potential with respect to the cathode. This is accomplished by means of grid-bias devices, as shown in Fig. 12 C.

Fig. 12 C. Grid biasing methods: (a) fixed; (b) cathode; (c) leak

A simple, direct method using a C-battery is shown at (a), while the cathode-resistor method is shown at (b). In the latter case, the plate current flows through the cathode-resistor R_c (5 to 5,000 ohms) from top to bottom (in the conventional sense of current flow), and sets up a potential drop which makes the grid negative with respect to the cathode. The value of R_c can be computed with the aid of Ohm's law when the desired grid-bias voltage is known and the total current through R_c is obtained from the characteristic curves (Fig. 12 B) of the triode. C_c is known as the cathode bypass condenser and is used to shunt alternating or h.f. currents around R_c, thus reducing " negative feedback."

In Fig. 12 C(c), the grid leak R_g (10,000 ohms to 10 megohms) is used when the input signal is sufficiently great to drive the grid positive during a portion of its cycle, as in oscillator circuits. When the grid is positive with respect to the cathode, electrons are drawn to it. Flowing through R_g they set up a potential whose negative is at the top of R_g. The grid condenser C_g prevents the grid current from flowing through the transformer shown at the left of the circuit. In order that the grid bias shall be nearly constant, it is necessary that the time constant R_gC_g be large in comparison with the period of the signal voltage. The value of R_g can be computed with the aid of Ohm's law if the desired grid bias and the d.c. grid current are known.