1, unipolar constant current source circuit design
It is extremely simple to get a stable current output, and the easiest way to do this is to use a current mirror: two identical transistors (made from the same chip and thus identical in process, size and temperature) are connected, as shown in Figure 1.
Both devices have the same base-emitter voltage, so the output current into collector T2 is equal to the input current into collector T1.
This analysis assumes that T1 and T2 are identical and isothermal, and that their current gains are so high that the base current can be ignored. It also ignores the early voltage, causing the collector current to change as the collector voltage changes.
These current mirrors can be composed of NPN or PNP transistors. When n transistors are connected in parallel to form T2, the output current is n times the input current, as shown in Figure 2A. If T1 consists of M transistors and T2 consists of N transistors, the output current will be n/m times the input current, as shown in Figure 2B.
If the early voltage effect is significant, a slightly more complex Wilson current mirror can be used to minimize this. The 3 transistor and 4 transistor versions are shown in Figure 3. The 4 transistor version is more accurate and has a wider dynamic range.
When a transconductance amplifier (voltage_in/current_out) is required, it consists of a single-supply operational amplifier, a BJT or FET(MOSFET is usually the best choice because it does not have a base current error), and a precision resistor that defines the transconductance value, as shown in Figure 4.
The circuit is simple and inexpensive. The voltage on the MOSFET gate can be set to the current and R1 in the MOSFET, so that the voltage V1 on R1 is equal to the input voltage VIN.
If a current mirror is required in a single chip IC, it is best to use a simple transistor current mirror. However, with discrete circuits, the high price of matching resistors (high price due to limited demand, not manufacturing difficulties) makes the operational amplifier current mirror in Figure 5 the cheapest technology. The current mirror consists of a transconductance amplifier and an additional resistor.
Current mirrors have relatively high and sometimes nonlinear input impedance, so they must be supplied with current by a high-impedance current source (sometimes called a rigid current source). If the input current must have a low impedance current absorption capability, use an operational amplifier. Figure 6 shows two low ZIN current mirrors.
With a basic current mirror and current source, the input and output current polarity is the same. Typically, the emitter/source of the output transistor is grounded directly or through a detection resistor, and the output current flows into the load from the collector/drain, with the other terminals connected to the DC power supply. This is not always convenient, especially when one of the terminals of the load needs to be grounded. As shown in Figure 7, this problem does not exist if the circuit is constructed using the emitter/source of its DC power supply.
If current or voltage is input to the reference ground, level conversion must be used. Multiple circuits can be implemented; The system in Figure 8 can be used in many situations. This simple circuit uses a ground current source to drive the current mirror on the DC power supply, thereby driving the load. Note that the current mirror may have gain, so the signal current does not need to be as high as the load current.
2, bipolar constant current source circuit design
So far, the circuits we’ve discussed are unipolar: current flows in one direction, but bipolar current circuits are possible.
The simplest and most widely used is the Howland current pump, as shown in Figure 9.
There are many problems with this simple circuit: it requires high precision resistance matching to achieve high output impedance; The input source impedance increases the R1 resistance, so it must be very low to minimize the matching error; The supply voltage must be much higher than the maximum output voltage; And the CMRR performance of the operational amplifier must be relatively good.
Today, high-performance instrumentation amplifiers are inexpensive, so it is extremely convenient to use a bipolar current source consisting of an operational amplifier, an instrumentation amplifier, and a current detection resistor, as shown in Figure 10.
This type of circuit is simpler than the Howland current pump, does not rely on a network of resistors (except for those of the integrated instrumentation amplifier), and the voltage swing is within 500 mV of each source.
The circuits we have discussed so far have been amplifiers with precise current outputs. Of course, they can be used with fixed inputs to provide a precise current source, but it is possible to build a simpler two-terminal current source. The low current reference voltage source ADR291 has A standby current of about 10 μA and A typical temperature coefficient of 20 nA/°C. As shown in Figure 11, with the addition of load resistance, the reference current in the 3 V to 15 V power supply range is (2.5/R + 0.01) mA, where R is the load resistance in k ω.
If accuracy is not an issue and only a rigid unipolar current source is required, a depletion type JFET and a resistor can be used as a current source. As shown in Figure 12, this configuration is not very stable under temperature changes and the current may vary considerably from device to device for a given value of R, but it is simple and inexpensive.
Recently, I needed to design a power supply for some leds. Some of my engineer friends thought I would have trouble designing a variable current source for dimming leds.
In fact, I simply modified my laptop’s “black brick” power supply (which I bought at a flea market for a few cents). Figure 13 shows a simple modification of the power supply circuit to provide a constant current to the LED. Using small output current, it can be fixed output voltage normal operation.
To obtain a variable current, apply a reference voltage, either from the black brick or local, to the potentiometers represented by P1 and P2. The OPA2 and MOSFET output a small current through R1, on which a voltage drop is generated. The load current flows through the detection resistance. If the voltage on the detected resistance decreases due to the load current exceeding the voltage drop on R1, the OPA1 output will rise, covering the voltage control in the brick and limiting its output voltage to prevent the output current from exceeding the limit.
Learning source: ADI official website
Journal download: download.csdn.net/download/m0…