Need Math Help

[teleamp]

What is the formula for calculating the cathode resistance in a cathode bypassed amp?

MikeY:help:

[Rob S]

What is the formula for calculating the cathode resistance in a cathode bypassed amp?

MikeY:help:

This may help.

http://www.diyguitarist.com/GuitarAmps/PT-Biasing.htm

[Ray Ivers]

Mike,

Here's some info from Randall Aiken's site. I assume you're trying to calculate the exact frequency response of a stage for a particular value of bypass cap?

Ray

Cathode impedance (unbypassed cathode)
It can be shown2 that the resistance seen looking into the cathode (Rk unbypassed) is: Rk' = (Rp+ra)/(mu+1)
= (100K + 62.5K)/(101)
= 1.61KTherefore, the total cathode resistance, or the output impedance if you take the signal off the cathode, is the parallel combination of the cathode resistance, Rk', and the cathode resistor, Rk, as below: R = Rk' || Rk
= 1.61K || 1.5K
= 777 ohmsBy the way, this is the resistance you use if you want to calculate the lower -3dB point of an amplifier stage due to the cathode bypass capacitor. Most people mistakenly use the cathode resistor and the bypass cap to calculate the -3dB point, but that is incorrect. Also, most people mistakenly assume the low frequency rolloff has a -6dB per octave rolloff down to DC. This is also not true, the actual response is quite different.