Constant current source circuit (current mirror circuit)

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(1) Basic concepts:
The constant current source is a current source whose output current is kept constant, and the ideal constant current source should have the following characteristics:
a) does not change due to load (output voltage) changes;
b) does not change due to changes in ambient temperature;
c) The internal resistance is infinite (so that its current can flow out to the outside).
A circuit capable of providing a constant current is a constant current source circuit, which is also called a current mirror circuit.

(2) Basic principles:
The basic constant current source circuit is mainly composed of an input stage and an output stage. The input stage provides a reference current, and the output stage outputs a constant current required. 1 The basic principle of a constant current source circuit: The constant current source circuit is the basis for providing a stable current to ensure the stable operation of other circuits. That is, the constant current source circuit is required to output a constant current, so the device as the output stage should have a volt-ampere characteristic with a saturated output current. This can be done with a BJT or MOSFET that operates in the output current saturation state.
In order to ensure the current stability of the output transistor, two conditions must be met:
a) its input voltage should be stable - the input stage needs to be a constant voltage source;
b) The output resistance of the output transistor is as large as possible (preferably infinite) - the output stage needs to be a constant current source.
2 Requirements for input stage devices:
Since the input stage needs to be a constant voltage source, a device having voltage saturation volt-ampere characteristics can be used as the input stage. A typical pn junction diode has this characteristic—exponentially rising volt-ampere characteristics. In addition, the diode formed by shorting the source-drain of the enhancement MOSFET also has similar volt-ampere characteristics—parabolic rise. Volt-characteristics.
When a diode is used as an input stage device in an IC, an integrated diode which is suitably connected by a triode is generally used, because the diode can adapt to both the IC process and its special advantages. For these triodes, it is required to have a certain amplification performance, so that the corresponding diodes have better constant voltage performance.
3 Requirements for output stage devices:
If BJT is used, in order to increase its output resistance, it is necessary to try to reduce the Evarly effect (base width modulation effect), that is, to maximize the Early voltage.
If a MOSFET is used, in order to increase its output resistance, it is necessary to try to reduce its channel length modulation effect and shimming effect. Therefore, long channel MOSFETs are generally used here instead of short channel devices.

(3) Basic constant current source circuit example:

Basic constant current source circuit

The upper left picture shows a basic constant current source circuit constructed with an enhanced n-MOSFET. In order to ensure that the gate-source voltage of the output transistor T2 is stable, a constant voltage source should be provided in front of it. In fact, the role of the T1 diode here is to provide a stable gate-source voltage to T2, which acts as a constant voltage source. Therefore, T1 should have a small AC conductance and a high transconductance to ensure better constant voltage performance. T2 should have a large output AC resistance, which requires long-channel MOSFETs and reduces the effects of channel length modulation.
The upper right picture shows a basic constant current source circuit constructed with BJT. T2 is a transistor that outputs a constant current, and transistor T1 is an emitter junction diode that supplies a stable base voltage to T2. Of course, the larger the current amplification factor of T1 and the higher the transconductance, the better its constant voltage performance. At the same time, in order to keep the output current constant (ie, increase the output AC resistance), it is naturally necessary to minimize the base width modulation effect of T2 (ie, the Early effect). In addition, if two base-connected pnp transistors are used to form a constant current source, then the two transistors can be placed in the same isolation region in the IC chip, which will help reduce the chip area, but in order to obtain A better output current constant performance requires special attention to increase the current amplification factor of the lateral pnp transistor.

(4) Expansion of the basic constant current source circuit:
Based on the above basic circuit, it can also be extended:
On the one hand, under the action of the diode constant voltage source (T1), a plurality of output branches (a plurality of transistors connected in parallel with T2) can be connected behind it, so that a plurality of stable output currents can be obtained.
On the other hand, a resistor (set to R1 and R2, respectively) can be connected in series to the source (emitter) of T1 and T2, which can obtain constant output currents of different sizes. Since I (output) / I (reference) = R1/R2 at this time, in this constant current source circuit, the constant current of the output is basically determined by the ratio of the resistance and the amplification factor of the transistor, and the resistance and The absolute size of the amplification factor has little to do with. This property just adapts to the characteristics of the integrated circuit manufacturing process, so this constant current source circuit is a basic circuit in the analog IC.