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Calculating Cathode Decoupling Capacitor

PostPosted: 05 Aug 2013, 13:02
by Augustica.com
Adding a cathode bias resistor to the first tube of the cathode follower caused negative feedback and reduced tube's gain. Lets design measures that will prevent this feedback.

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Because the output signal of the first tube of the cathode follower is derived from changing Ia flowing through R4, and Ia also flows through R3, there will also be a voltage across R3. In order to restore full gain, we must suppress the feedback voltage produced at the cathode using a decoupling or bypass capacitor C3. The bypass capacitor C3, together with resistor R3, will form a local low-pass filter.

Let's calculate the resistance between the anode of the first tube and the ground. Let's title this resistance Rk.

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Mu for 6922 was previously calculated here and is equal to 32.5. And Ra for 6922 was previously calculated here and is equal to 3.25.

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In parallel with 360 Ohm cathode bias resistor R3 this gives us a total resistance Rk of 245 Ohm.

To calculate capacitance of the cathode bypass capacitor C3 we will use the following formula where f is the filter's cut off frequency which we select to be equal to 1 Hz.

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Accordingly C3 has a value of 650uF. The closest standard value to 650 uF is 680 uF.