Current mirror

Description: Two NPN transistors, specifically models 2N2222 or 2N3403, are recommended for this circuit. Larger "power" transistors may not behave similarly at low current levels. However, any pair of identical NPN transistors can be utilized to construct a current mirror. It is important to note that not all transistors have the same terminal designations or pinouts, even if they appear similar. This will influence how the transistors are connected to each other and to other components, so it is essential to verify the manufacturer's specifications from the component datasheet, which can be easily accessed on the manufacturer's website. There is a possibility that the transistor's package and the manufacturer's datasheet may contain incorrect terminal identification diagrams. Therefore, it is highly recommended to double-check pin identities using the multimeter's "diode check" function. For guidance on identifying bipolar transistor terminals with a multimeter, refer to chapter 4 of the Semiconductor volume (volume III) of this book series. A current mirror can be regarded as an adjustable current regulator, with the current limit easily established by a single resistor. Although it is a relatively basic current regulator circuit, its simplicity contributes to its widespread use. In this experiment, the circuit will be constructed to explore its current-regulating properties and to experience some practical limitations firsthand. The circuit should be built as illustrated in the schematic. An extra 1.5 kΩ fixed-value resistor will be available from the parts list for use in the final part of the experiment. The potentiometer is used to set the current through transistor Q1, which is configured to function as a simple diode, essentially a PN junction. The use of a transistor instead of a regular diode is crucial for matching the junction characteristics of both transistors in the current mirror circuit. The voltage across the base-emitter junction of Q1 is applied across the base-emitter junction of the second transistor, Q2, activating it to conduct current. Since the voltage across the base-emitter junctions of the two transistors is equal, the current through their base terminals will also be equal, assuming identical junction characteristics and temperatures. Matched transistors should exhibit the same β ratios, leading to equal base currents and, consequently, equal collector currents. As a result, Q2's collector current will replicate the current magnitude established through the collector of Q1 by the potentiometer. Essentially, the current through Q2 mirrors the current through Q1. Variations in load resistance (the resistance connecting Q2's collector to the positive side of the battery) do not influence Q1's current or the base-emitter voltage or base current of Q2. With a constant base current and nearly constant β ratio, Q2 will adjust its collector-emitter voltage as necessary to maintain a constant collector (load) current. Thus, the current mirror circuit serves to regulate current at a value determined by the potentiometer, independent of load resistance. However, practical implementation may yield more complex results than theoretical expectations. In the provided circuit diagram, the load circuit of Q2 is connected to the positive side of the battery via an ammeter for straightforward current measurement. Instead of firmly attaching the ammeter's black probe to a specific point in the circuit, five test points, labeled TP1 through TP5, have been designated for measurement.

The current mirror circuit employs two NPN transistors, Q1 and Q2, configured to replicate the current flowing through Q1 into Q2. The circuit's operation hinges on the matching characteristics of the transistors, specifically their base-emitter junctions. The potentiometer allows for the adjustment of the current through Q1, which in turn influences Q2's collector current due to the direct relationship between their base-emitter voltages. The configuration ensures that regardless of variations in load resistance connected to Q2, the current remains stable, demonstrating the fundamental principle of a current mirror.

To build the circuit, the following connections should be made:
- Connect the collector of Q1 to the positive terminal of the power supply.
- The emitter of Q1 should be connected to the emitter of Q2, ensuring both transistors share a common reference point.
- The base of Q1 is connected to one end of the potentiometer, while the other end connects to ground, allowing for adjustable current flow.
- The collector of Q2 is connected to the load, with the ammeter placed in series to measure the current flowing through Q2.
- Finally, ensure that the base of Q2 is connected to the base of Q1 to maintain the same base-emitter voltage across both transistors.

This setup illustrates the fundamental operation of a current mirror, enabling practical experimentation with current regulation and the effects of component variations on circuit performance.Two NPN transistors - models 2N2222 or 2N3403 recommended. Larger "power" transistors may not exhibit the same behavior at these low current levels. However, any pair of identical NPN transistors may be used to build a current mirror. Beware that not all transistors share the same terminal designations, or pinouts, even if they share the same physical appearance. This will dictate how you connect the transistors together and to other components, so be sure to check the manufacturer`s specifications (component datasheet), easily obtained from the manufacturer`s website. Beware that it is possible for the transistor`s package and even the manufacturer`s datasheet to show incorrect terminal identification diagrams!

Double-checking pin identities with your multimeter`s "diode check" function is highly recommended. For details on how to identify bipolar transistor terminals using a multimeter, consult chapter 4 of the Semiconductor volume (volume III) of this book series. A current mirror may be thought of as an adjustable current regulator, the current limit being easily set by a single resistance.

It is a rather crude current regulator circuit, but one that finds wide use due to its simplicity. In this experiment, you will get the opportunity to build one of these circuits, explore its current-regulating properties, and also experience some of its practical limitations firsthand. Build the circuit as shown in the schematic and illustration. You will have one extra 1. 5 k © fixed-value resistor from the parts specified in the parts list. You will be using it in the last part of this experiment. The potentiometer sets the amount of current through transistor Q1. This transistor is connected to act as a simple diode: just a PN junction. Why use a transistor instead of a regular diode Because it is important to match the junction characteristics of these two transistors when using them in a current mirror circuit.

Voltage dropped across the base-emitter junction of Q1 is impressed across the base-emitter junction of the other transistor, Q2, causing it to turn "on" and likewise conduct current. Since voltage across the two transistors` base-emitter junctions is the same - the two junction pairs being connected in parallel with each other - so should the current be through their base terminals, assuming identical junction characteristics and identical junction temperatures.

Matched transistors should have the same ² ratios, as well, so equal base currents means equal collector currents. The practical result of all this is Q2`s collector current mimicking whatever current magnitude has been established through the collector of Q1 by the potentiometer.

In other words, current through Q2 mirrors the current through Q1. Changes in load resistance (resistance connecting the collector of Q2 to the positive side of the battery) have no effect on Q1`s current, and consequently have no effect upon the base-emitter voltage or base current of Q2. With a constant base current and a nearly constant ² ratio, Q2 will drop as much or as little collector-emitter voltage as necessary to hold its collector (load) current constant.

Thus, the current mirror circuit acts to regulate current at a value set by the potentiometer, without regard to load resistance. Well, that is how it is supposed to work, anyway. Reality isn`t quite so simple, as you are about to see. In the circuit diagram shown, the load circuit of Q2 is completed to the positive side of the battery through an ammeter, for easy current measurement.

Rather than solidly connect the ammeter`s black probe to a definite point in the circuit, I`ve marked five test points, TP1 through TP5, for you t