That was a description of what happened, but not of how it happened. The how is easy enough: both transistors see identical bias voltages and bias currents, and thus must conduct equally. By tying the collector of the first transistor to its base, the transistor is transformed into a diode, with a voltage drop across its anode to its cathode. This voltage drop equals the voltage drop on the second transistor's base to its emitter, as the transistors used are the same type and usually matched.

  In other words, the current mirror can be used in a non-Class A amplifier without running into the problems that a current source would. As the first tube conducts more, the voltage at the top of the diode's anode increases, which is then relayed to the transistor's base, which in turn increases the voltage on its emitter resistor to match the voltage on the diode's cathode resistor, and thus its current flow. In the presence of a strong music input signal, a leanly run Class AB amplifier will see a large increase in the average current flow through its output stage, but with the current mirror in place, each output tube will bias up to an equal average current. 

   An alternative circuit is made up of one diode, one transistor, and two resistors. It is not as precise as the earlier circuit, but it is easier to understand and provokes half of the transistor embarrassment as the former circuit.

   Where could a current mirror come in handy in the design of a tube based circuit? One use comes immediately to mind is DC application: setting idle current in a push-pull amplifier output stage. Actually a better description is matching the idle current between two output tubes, as the circuit only mirrors current not set it to any precise value.

   Another possible use of the current mirror could be improving on an already first rate circuit, the Common Cathode amplifier. If you have read the two articles from the old GlassWare Tube Circuit of the Month articles on this circuit, you know that I like this circuit a great deal. The topology of the circuit Above is not too different in function form the circuit from second Tube Circuit of the Month article. The solid-state circuit serves to balance the current flow through both triodes. Further it locks the first triode's plate voltage in place, while it allows the second plate to breath freely with a high impedance load.

A current mirror used to match the idle current for two output tube (four or more tubes can be handled by adding more transistors and resistors).

pg. 11

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