A basic constant current source circuit consists mainly of an input stage which provides the reference current and an output stage which outputs the required constant current. The constant current source circuit is the basis for the stable operation of other circuits by providing a stable current. This means that the constant current source circuit is required to output a constant current, so the device used as the output stage should have the voltammetric characteristics to saturate the output current.
This can be achieved by using bipolar junction transistors or gold-oxygen half-field-effect transistors that operate in the output current saturation state. In order to ensure that the output transistor is current stable, two conditions must be met.
Its input voltage has to be stable – the input stage needs to be a constant voltage source;
the output resistance of the output transistor is as high as possible – the output stage needs to be a constant current source.
Four types of constant current source circuits are analysed.
In the modified differential amplifier, a constant current source is used instead of the emitter resistor RE, which sets a suitable quiescent operating current for the differential amplifier circuit, and greatly enhances the common mode negative feedback effect, so that the circuit has a stronger ability to suppress the common mode signal, and does not require a very high supply voltage, so the constant current source and the differential amplifier circuit are simply a perfect match! A constant current source can either provide the right quiescent current for the amplifier circuit, or it can be used as an active load instead of a high resistance resistor, thus increasing the voltage amplification of the amplifier circuit. This usage has a very wide range of applications in integrated op amp circuits. This section will describe common constant current source circuits and their application as active loads.
Mirror constant current source circuits
As shown in Figure 1 is a mirror image constant current source circuit, which consists of two tubes VT0 and VT1 with identical characteristics. Since the c and b poles of tube VT0 are connected, UCE0=UBE0, i.e. VT0 is in an amplified state and the collector current IC0=β0IB0. The current amplification factor β0=β1=β, then the two tubes collector current IC0=IC1=IC=βIB. As can be seen, due to this special connection of the circuit, so that the two tubes collector IC1 and IC0 is a mirror relationship, so this circuit is called a mirror constant current source (IR is the reference current, IC1 is the output current).
The mirror constant current source circuit is simple and widely used. However, when the supply voltage is certain, if the requirement IC1 is larger, the IR is bound to increase, the power consumption of the resistor R increases, which should be avoided in integrated circuits; if the requirement IC1 is small, the IR is bound to be small as well, the value of the resistor R is very large, which is difficult to do in integrated circuits, for this reason, people think of other ways to solve, so that other current source circuits are derived.
Proportional constant current source circuits
The proportional constant current source circuit, shown in Figure 2, consists of two tubes with identical characteristics, VT0 and VT1, and the emitters of the two tubes are connected in series with resistors Re0 and Re1. The proportional constant current circuit source changes the relationship between IC1 ≈ IR, so that IC1 and IR are proportional, thus overcoming the shortcomings of the mirror image constant current source circuit. As with a typical static operating point stabilisation circuit, Re0 and Re1 are current negative feedback resistors and therefore the output current IC1 of the proportional constant current source has higher stability compared to the mirror constant current source circuit.
Slightly variable constant current source circuit
If Re0 is very small or even zero, then Re1 can obtain a small output current by using only a small resistor, this circuit is called a micro-variable constant current source, as shown in Figure 3. The quiescent current of the input stage of the integrated op-amp is very small, often only a few tens of microamps, or even smaller, so the micro-variable current source is mainly used for the active load of the input stage of the integrated op-amp.
Multi-circuit constant current source circuit
The integrated op-amp is a multi-stage amplifier circuit and therefore requires multiple constant current source circuits to provide the appropriate quiescent current to each stage. A single reference current can be used to obtain several different output currents to suit the needs of each stage. The circuit shown in Figure 4 is a multiple constant current source circuit obtained on the basis of a proportional constant current source, with IR as the reference current and IC1, IC2 and IC3 as the three output currents. Since the voltage UBE values between b-e of each tube are approximately equal, an approximate relationship can be obtained: IE0Re0≈IE1Re1≈IE2Re2≈IE3Re3
Once IE0 has been determined, the required current can be obtained at all levels by simply selecting the appropriate resistor.