Analysis and application of four common DC power supply circuits

A basic DC power supply circuit mainly consists of an input stage and an output stage. The input stage provides reference current and output stage supplies required DC current. The constant current source circuit is basis for providing stable current to ensure stable operation of other circuits. That is, DC source circuit must produce DC current, so device as an output stage must have a saturated output current I-V characteristic.

This can be achieved by using bipolar junction transistors or metal oxide half-field transistors operating in output current saturation mode. To ensure stability of output transistor current, two conditions must be met:

The input voltage must be stable - input stage must be a constant voltage source;

The output impedance of output transistor should be as high as possible - output stage should be a constant current source.

Analysis of four DC source circuits:

In advanced differential amplifier, emitter resistor RE is replaced by a constant current source, which not only sets a suitable static operating current for differential amplifier circuit, but also greatly enhances common-mode negative feedback effect, making circuit more reliable Strong ability to suppress common-mode signals and does not require high supply voltage, so DC supply circuit and differential amplifier circuit are a perfect match!

A constant current source can not only provide a suitable quiescent current for amplifier circuit, but can also be used as a resistive load instead of a high-resistance resistor, thereby increasing voltage gain of amplifier circuit. This usage has a very wide range of applications in integrated operational amplifier circuits. This section will introduce general DC power supply circuits and their applications as resistive loads.

1. DC source mirroring

As shown in figure 1, this is a mirror-image DC power supply circuit consisting of two lamps VT0 and VT1 with same characteristics. Since c and b poles of VT0 lamp are connected, UCE0=UBE0, that is, VT0 in extended condition Collector current IC0=β0*IB0. In addition, B-e lamps VT0 and VT1 are connected separately, so their base current is IB0=IB1=IB. If we assume that current amplification factor is β0=β1=β, then collector current of two lamps is IC0=IC1=IC=β*IB. It can be seen that due to special connection of circuit, collectors IC1 and IC0 of two lamps are in mirror image, therefore this circuit is called a mirror DC source (IR is reference current, IC1 is reference current). output current).

The mirror DC power supply circuit is simple and widely used. But with a constant supply voltage, if IC1 is required to be larger, then IR will inevitably increase, and power consumption of resistor R will increase, which should be avoided in integrated circuits; if IC1 needs a smaller size, then IR is bound to be small, and resistance R value is very large, which is difficult to achieve in integrated circuits. For this reason, people think of using other methods to solve it, so that other current source circuits can be obtained.

2. Proportional DC power supply circuit

As shown in Figure 2, this is a proportional constant current source circuit, consisting of two lamps VT0 and VT1 with same characteristics, and emitters of two lamps are connected in series with resistors Re0 and Re1 respectively. The DC link proportional source changes ratio of IC1≈IR, making IC1 and IR proportional, thus eliminating shortcomings of mirror image DC source circuit. As in typical static operating point stabilization circuit, Re0 and Re1 are negative current feedback resistors, so compared to DC mirror circuit, output current IC1 of proportional DC power supply has higher stability.

Third, micro-variable DC source circuit

If Re0 is very small or even zero, then Re1 can obtain less output current only by using less resistance. This circuit is called a microvariable DC source, as shown in Figure 3. The quiescent current of input stage of integrated op-amp is very small , often only a few tens of microamps, or even less, so micro-alternating current source is mainly used to actively load the input stage of an integrated operational amplifier.

Fourth multichannel DC power supply

The integrated op-amp is a multi-stage amplifier circuit, so multiple DC supply circuits are required for each stage to provide adequate quiescent current. The reference current can be used to produce several different output currents according to needs of each stage. The circuit shown in Figure 4 is a multi-channel DC source circuit derived from a proportional DC source, IR is reference current, and IC1, IC2, and IC3 are three-way output currents. Since voltage values ​​UBE between b-e of each tube are approximately equal, an approximate ratio can be obtained:

IE0Re0≈IE1Re1≈IE2Re2≈IE3Re3

When IE0 is defined, all levels only need to select appropriate resistance to get required current