Constant Current Source Design

The output current of the constant current source is constant. If this circuit is applied to some input fields, the input device can be effectively protected. For example, this circuit is used in rs422 communication to effectively protect the communication. It can protect against overvoltage within a certain voltage range. The following reference is a constant current source analysis.

The constant current source is the current source with the output current unchanged, and the ideal constant current source is:
A) the load (output voltage) does not change.
B) the environment temperature does not change.
C) The internal resistance is infinitely large.

Circuit symbol of constant current source

Ideal constant current source
The ideal constant current source, the internal resistance is infinite, so that the current can flow out of the outside. The actual constant current source has internal resistance R.
Constant Current characteristics of the transistor:

The transistor characteristic curve shows that the IC in the work zone is affected by IB, while the VCE has a slight impact on the IC.
Therefore, as long as the IB value is fixed, the IC can also be fixed.
The output current IO is the IC that flows through the load.

Current Mirror:
A current mirror is a circuit in which the input current is equal to the output current IO.

Q1 and Q2 have the same characteristics, that is, vbe1 = vbe2, β1 = β2.

Advantages:
The beta of the transistor is affected by the temperature, but the current image is used to mirror the constant current source, not affected by beta, mainly relying on the external resistance R through
Q2 determines the output current IO (ic2 = Io ).

Design of transistor emitter bias

Example 2.

From the left: Base bias

So
Ve = Vb-0.6 = 1.0 V
Because the emitter resistance is 1 K, the current passing through the emitter resistance is

So the current flowing through the load is a stable 1mA
Example 2.

This is a 5.6 V base voltage powered by a voltage regulator diode.
Ve = Vb-0.6 = 0.5 V
Current flowing through the load

Example 3.

This example is different: Use the PNP transistor to supply current to the load circuit. first, a 0.6 V base polar bias (10-3x8.2 = 0.6) is provided using the diode Pressure Drop of 8.2 v ). the 4.7 K resistance is only used to form a path, and there is no need (or will not) a lot of current flowing through this resistance.
Ve = VB +0.6=8.8 v
The voltage difference between the two ends of the 560 ohm resistor of the PNP crystal is 1.2 V, so the current is 2mA
Considerations for applications of crystal constant current source
If you only use one transistor to meet your needs, you can use two transistor frames:

Or
 

Or yes.

Note: The constant current source is a two-terminal part. current Regulating diode (CRD) is also available on the market for small current applications. in high-current applications, you can use an IC voltage regulator to concatenate the resistance or use a MOSFET method.