minimum feature size transistor definition

However, this linear relationship is not necessarily required. These circuits use the linear relationship between the current in resistor R1 and the voltage across the resistor. The virtual ground at the negative input of the op-amp provides a very low input resistance. In figure 11.1.1 we see the classic operational amplifier implementation of the current to voltage converter explored back in Chapter 4 section 2.

It should be noted that these two stages of the current mirror may have a linear relationship (for example where V OUT = I INR and I OUT = V IN/R) like a resistor. Because in a current mirror the input and output are currents, it is easier to convert the input to a voltage first and then convert a voltage back to a current at the output. The use of feedback and the current to voltage relationship of two terminal elements such as a resistor are most often used when manipulating currents as signals. For example the MOS transistor is generally modeled as a voltage controlled current source and can not be used directly as a current amplifier. Accurate current amplifiers are difficult to directly implement using conventional transistor amplifier configurations which are typically voltage amplifiers. The problem with using this feature directly is that β is not a well controlled value from device to device and can vary with changes in temperature. In Chapter 8 we explored the transistor and you should recall that the BJT device is a current amplifier of sorts (current controlled current source) in that the collector current is β times the base current. We can generalize this basic current mirror structure with this first observation: A current mirror consists of a low impedance input stage connected to a high impedance output current stage.įigure 11.1, Current Mirror (a) Sink (b) SourceĬonceptually, an ideal current mirror is simply an ideal current amplifier with a gain of -1. 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.

Minimum feature size transistor definition