Downloaded from https://www.everyspec.com MIL-STD-1311D METHOD 1306B TRANSCONDUCTANCE 1. Methods of determination. The grid-anode transconductance shall be determined graphically from the slope of the grid-anode transfer characteristic, calculated from measurements of the amplification factor and the anode resistance, or measured directly by means of the voltage-ratio method shown in figure 1306-1 or by an equivalent method. 2. Voltage-ratio method. The voltage-ratio method utilizes a system of transformers, capacitors, and attenuators to supply three independent, properly phased, and adjusted ac voltages (normally 1,000 Hz) from a common source.
The ac (rms) voltage E1, E2, and E3 must be in phase and the output resistance of the attenuators made small by proper design in order to neglect the voltage drop due to electrode currents. The voltage ratio E2/E1 must be accurately determinable. The voltage applied to the grid (whether using an alternating current signal or the incremental bias technique) shall be kept as low as practicable to obtain as true a value of transconductance as possible at the specified test point. When a null is reached, R x E E Sm 1 1 2 = , where R is a fixed resistor of 100,000 ohms. By using the correct voltage multipliers or dividers, the reading can be obtained directly in micromhos. 3. Transconductance.
The grid-anode transconductance shall be within the limits specified. For a change in the value of transconductance with reduced filament or heater voltage, the reading at the reduced voltage shall be taken when the value of transconductance is rising or stable. FIGURE 1306-1. Simplified circuit for measuring transconductance. METHOD 1306B 1306-1 28 March 1975