Example 4.1A 12AX7 has a plate load resistor, Rb, of 220 k ohms and the grid resistor of the following stage, Rcf, is 470 k ohms. Tube manual data on the 12AX7 gives Amplification factor = 100 and plate resistance = 80 k ohms. What is the gain of this 12AX7 amplifier
Solution.First we calculate Rbc.
Rbc = Rb Rcf / (Rb + Rcf) = ( 220 k x 470 k ) / ( 220 k + 470 k ) = 150 k ohms.Now we calculate the gain. Av = mu Rbc / ( rp + Rbc ) = 100 x 150 k / ( 80 k + 150 k ) = 65.2
Example 4.2A triode tube is to be used as a single ended low power amplifier. The output transformer has a primary impedance of 3000 ohms. Think of Rb being 3 k ohms and Rcf as not present. Which of the following tubes will give the greatest voltage gain?
12AT7, Amplification Factor = 60, Plate Resistance = 10900
12AU7, Amplification Factor = 17, Plate Resistance = 7700
12AV7, Amplification Factor = 41, Plate Resistance = 4800
12AX7, Amplification Factor = 100, Plate Resistance = 80000
Solution.The gain for each is given by,
Av = mu Rb / ( rp + Rb )For the 12AT7 it is, Av = 60 x 3000 / ( 10900 + 3000 ) = 12.9For the 12AU7 Av = 4.77
For the 12AV7 Av = 15.8
for the 12AX7 Av = 3.61
The 12AV7 comes in first with 15.8 and the 12AT7 is runner-up at 12.9. Notice that the 12AX7 with its impressive amplification factor of 100 comes in last at 3.61. A high amplification factor may not guarantee a high gain amplifier.
Example 4.3A 12AX7 which is without a cathode bypass capacitor has a plate load resistor, Rb, of 220 k ohms, a cathode resistor, Rk, of 3300 ohms, and the grid resistor of the following stage, Rcf, is 470 k ohms. Tube manual data on the 12AX7 gives Amplification factor = 100 and plate resistance = 80 k ohms. What is the gain of this 12AX7 amplifier
SolutionWithout a cathode bypass capacitor the gain is given by,
Av = mu Rbc / (rp + Rbc + Rk (mu + 1))Remembering from Example 4.1 the parallel combination of Rb and Rcf = 150 k ohms and substituting all values gives, Av = 100 150 k / (80 k + 150 k + 3300 x 101 ) = 26.6BACK