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Balanced Input
The Common Cathode amplifier has really two inputs and one output. One input inverts the input signal at the output and the other preserves the phase relationship between input and output. These two inputs allow for the use of a balanced input signal. The circuit then functions as a balanced to unbalanced converter. (See the article in the Tube CAD Journal on a balanced to unbalanced converter.) In this function it work fairly well, although, strictly speaking, the gain from each input does not match the other. The greater the common cathode resistor Rk, the less difference. The gain from the first grid is given by:
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G1= |
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1 + 2rp / Ra + rp(rp + Ra) / RaRk(µ + 1) |
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The gain from the second grid is given by: |
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G1= |
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1 + 2rp / Ra + rp(rp + Ra) / RaRk(µ + 1) |
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From the above formulas we can see how the greater the cathode resistor value, the less the discrepancy between gains. The difference in gain is roughly given by the following equation: |
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Even if the cathode resistor were to be replaced with a true current source (or a resistor of infinite resistance), the gains may not match, as the mu's between triodes will slightly differ: |
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Although not as evident as the last formula, the higher the mu, the less the unbalance the input gains made by an imbalance in mu's.
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