The plate resistance and the amplification factor of a triode are `10 kOmega and 20`. The tube is operated at plate voltage `250 V` and grid voltage `-7.5 V`. The plate current is `10 mA`. (a) To what value should the grid voltage be changed so as to increase the plate current to `15 mA`? (b) To what value should the plate voltage be changed to take the plate current back to `10 mA`?

Explanation

Given: ` r_p = 10K Omega = 10 xx (10^3) Omega, `
` mu = 20, V_p= 250V, `
` V_0= 7.5V, I_p = 10mA `
` (a) g_m = (omegaI_p/omega V_g) [v_p rarr Constant ]`
` omega V_p = (omega I_p/ omegaV_g)`
` = ((15 xx (10^-3) - 10 xx (10^-3))/(mu//r_p))`
` = ((5xx (10^-3))/(20//10 xx (10^-3)))`
` = ((5 xx (10^-3))/(2 xx(10^-3))) = 5/2 = 2.5 `
` r'_0 = + 2.5- 7.5 = - 5V. `
` (b) r_p = ((omegaV_p)/(omegaI_p))[V_g rarr Constant ] `
` rArr (10^4) = ((omega V_p)/(15 xx (10^-3) -10 xx (10^-3)))`
` rArr omegaV_p = (10^4)xx 5 xx (10^-3) = 50V.`
` V'_p - V_p = 50 `
` rArr V'_p = -50 + V_p = 200V ` .