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Four types of constant current source circuits

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2022-11-14 10:22

In the improved differential amplifier, the emitter resistance RE is replaced by a constant current source, which not only sets a suitable static working current for the differential amplifier circuit, but also greatly enhances the common mode negative feedback effect, making the circuit have a stronger ability to suppress common mode signals, and does not need a very high power supply voltage. Therefore, the constant current source and the differential amplifier circuit are a perfect match! The constant current source can not only provide a suitable static current for the amplifier circuit, but also act as an active load to replace the high resistance value resistor, thus increasing the voltage magnification of the amplifier circuit. This method is widely used in integrated operational amplifier circuits. This section will introduce the common constant current source circuit and its application as an active load, so as to provide knowledge for subsequent learning.
1、 As shown in Figure 1, the mirror constant current source circuit is a mirror constant current source circuit, which is composed of two tubes VT0 and VT1 with identical characteristics. Because the c and b poles of the VT0 tube are connected, UCE0=UBE0, that is, VT0 is in an amplified state, and the collector current IC0= β 0*IB0。 In addition, b-e of tubes VT0 and VT1 are connected respectively, so their base current IB0=IB1=IB. Set current amplification factor β 0= β 1= β， Then two tube collector current IC0=IC1=IC= β* IB。 It can be seen that this special connection of the circuit makes the two collector IC1 and IC0 mirror image, so this circuit is called a mirror image constant current source (IR is the reference current, IC1 is the output current). Fig. 1 Image constant current source circuit Image constant current source circuit is simple and widely used. But when the power supply voltage is constant, if IC1 is required to be large, IR will inevitably increase, and the power consumption of resistance R will increase, which should be avoided in integrated circuits; If IC1 is required to be small, IR is bound to be small, and the value of resistance R is large, which is difficult to do in integrated circuits. Therefore, people have thought of using other methods to solve this problem, and thus other current source circuits are derived. 2、 The proportional constant current source circuit is shown in Figure 2 as a proportional constant current source circuit, which is composed of two tubes VT0 and VT1 with identical characteristics. The emitters of the two tubes are connected in series with resistors Re0 and Re1 respectively. The proportional constant current circuit source changes the relationship between IC1 ≈ IR, making IC1 and IR proportional, thus overcoming the shortcomings of the mirror image constant current circuit. Like a typical static operating point stabilizing circuit, Re0 and Re1 are current negative feedback resistors, so the output current IC1 of the proportional constant current source has higher stability than that of the mirror image constant current source circuit.
When Re0=Re1, IC1 is still equal to IR, but the IR of this circuit is specified by Formula (2-4), which is smaller than that of Formula (2-2). It is generally used in the input stage of the preamplifier.
Fig. 2 Proportional constant current source circuit 3. Micro variable constant current source circuit According to Formula (2-3), if Re0 is very small or even zero, Re1 can obtain smaller output current by using only smaller resistance. This circuit is called micro variable constant current source, as shown in Fig. 3. The static current of the input stage of the integrated operational amplifier is very small, often only tens of microamps, or even smaller. Therefore, the micro variable current source is mainly applied to the active load of the input stage of the integrated operational amplifier. Fig. 3 Micro variable constant current source circuit 4. The integrated operational amplifier of multi-channel constant current source circuit is a multi-stage amplifier circuit, so the multi-channel constant current source circuit is required to provide appropriate static current for each level. A reference current can be used to obtain multiple different output currents to meet the needs of all levels. The circuit shown in Figure 4 is a multi-channel constant current source circuit based on the proportional constant current source. IR is the reference current, and IC1, IC2 and IC3 are three output currents. Since the values of voltage UBE between b-e of each tube are approximately equal, an approximate relationship can be obtained
IE0Re0≈IE1Re1≈IE2Re2≈IE3Re3(2-6)
When IE0 is determined, the required current can be obtained by selecting appropriate resistance at all levels.
Figure 4 Multiple Constant Current Source Circuit

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