Conversely, given a current sink as the input, the current mirror reflects this current to control current source (figure 11.1b) as a result, now we obtain a current source. Given a current source as the input, the input section of the current mirror looks like a virtual short circuit and reflects (swaps the direction of flow) this current to produce a current sink (the current exiting the mirror) as a result, we obtain a current sink (figure 11.1a).
The ideal block level concept of the current mirror is shown in figure 11.1. The current mirror is often used to provide bias currents and active loads in amplifier stages.
The current being 'copied' can be, and often is, a varying signal current. Another feature of the current mirror is a relatively low input resistance which helps to keep the input current constant regardless of drive conditions. An important feature of the current mirror is a relatively high output resistance which helps to keep the output current constant regardless of load conditions. To best understand this important circuit building block and how it makes use of this relationship we need to deconstruct the circuit into input and output sections and examine each in turn.Ī current mirror is a circuit block which functions to produce a copy of the current flowing into or out of an input terminal by replicating the current in an output terminal. The simple two transistor implementation of the current mirror is based on the fundamental relationship that two equal size transistors at the same temperature with the same V GS for a MOS or V BE for a BJT have the same drain or collector current. The implementation of the current mirror circuit may seem simple but there is a lot going on.
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