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.Ī 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). The solid-state circuit serves to balance the current flow through both triodes. The topology of the circuit Above is not too different in function form the circuit from second Tube Circuit of the Month article. 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. 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.Īnother possible use of the current mirror could be improving on an already first rate circuit, the Common Cathode amplifier. 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. 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. 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.Īn alternative circuit is made up of one diode, one transistor, and two resistors. 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 other words, the current mirror can be used in a non-Class A amplifier without running into the problems that a current source would. 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. 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. The how is easy enough: both transistors see identical bias voltages and bias currents, and thus must conduct equally. That was a description of what happened, but not of how it happened. 10.1109/ Tube CAD Journal,Current Mirrors for Tube Circuits Technical Digest – International Electron Devices Meeting. Transistor matching in analog CMOS applications. Faricelli, “Layout-dependent proximity effects in deep nanoscale CMOS,” IEEE Custom Integrated Circuits Conference 2010, San Jose, CA, USA, 2010, pp. These are the topic of my next blog post. For example, modern processes demand that the designer also insert dummy devices and consider device density. The techniques discussed in this article are just the starting point for achieving good MOSFET matching in current mirrors. Pulsic’s Animate Preview automatically chooses from all the techniques discussed in an intelligent way to create a layout preview of your design in seconds, reducing design iterations. Circuit engineers and layout designers use their knowledge and experience to choose the right set of compromises for their design. All analog layouts are necessarily a compromise between multiple competing requirements matching, isolation, design area and aspect ratio, routing, and much more. There is often no “right” answer in analog layout. 2 Literature Survey Power and operating temperature are those parameters which are very important in circuit design, to maintain their correctness circuit designers must focus on power efficiency. This simplifies the routing process, making it easier to achieve symmetrical routing, but this may be at the expense of device matching due to the channel thickness effect, discussed above. By substituting a current mirror (diode voltage source biasing a current source transistor), this amplifier has a stable biasing situation. In the above case, the source of each MOSFET is highlighted, you can see that they are symmetrical about the center of the image. It is common to flip the current direction around the line of symmetry. Source contacts highlighted to show current direction.Īs mentioned earlier, the current direction through the device also matters.
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