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Another use of the current mirror with the Common Cathode amplifier is shown below. Here the attempt is made at building a DC amplifier. The current mirror only attaches to plate of the first triode and relays the change in current to the resistor that connects to the negative power supply rail. As the first triode sees an increase in current flow, that increase also flows through the current mirror's load resistor, whose output connection consequently moves in phase with the input at the grid. Feedback is taken at the second triode's grid. |
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The circuit below shows how a quasi-current mirror can be implemented. Current sources load both triodes and are circularly DC referenced to each other. In AC terms, they are completely independent, as must be the case as the Cathode Follower works in anti-current phase to the Grounded Cathode amplifier. |
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An often used line amplifier topology consists of one Grounded Cathode amplifier cascading into a Cathode Follower. When the plate resistor equals the cathode resistor, the amplifier exhibits fairly constant current draw in spite of large voltage swings at the output. This a very desirable feature in a circuit, as it means the amplification of the signal will not be imposed upon the power supply noise and re-circulated throughout the amplifier. Still, the circuit's PSRR is only as good as the triode's rp is low in relation to the plate resistor value. Current source loading of Grounded Cathode amplifier and the Cathode Follower would not only make for a constant current draw, but also a much improved PSRR figure. |
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pg. 12 |
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