A variety of practical circuit examples based on current output circuit technology

　　Although current mirrors and circuits such as the Howland current source are taught as part of analog circuits, there is still a significant number of engineers who prefer to think in terms of voltage when defining the output of precision analog circuits. This is unfortunate because current outputs offer advantages in many areas, including analog current loop signals (0 mA to 20 mA and 4 mA to 20 mA) in noisy environments and level shifting analog signals for large potential differences without the aid of optical or magnetic isolation techniques. This article summarizes some of the existing techniques and provides several practical circuits.

　　Getting a stable current output is a simple matter. The simplest method is to use a current mirror: Two identical transistors - made from the same chip, so the process, size and temperature are exactly the same - are connected as shown in Figure 1. The base-emitter voltage of the two devices is the same, so the output current into the collector of T2 is equal to the input current into the collector of T1.

　　Figure 1. Basic current mirror.

　　This analysis assumes that T1 and T2 are identical and isothermal, and that their current gain is so high that the base current is negligible. It also ignores the early voltage that causes the collector current to change with collector voltage.

　　These current mirrors can be made of NPN or PNP transistors. If n transistors are connected in parallel to form T2, the output current will be n times the input current, as shown in Figure 2a. If T1 is made of m transistors and T2 is made of n transistors, the output current will be n/m times the input current, as shown in Figure 2b.

　　Figure 2. (a) Multistage current mirror (b) Non-integer ratio current mirror can combine the three T2 collectors to obtain 3IIN

　　If the early voltage effect is significant, it can be minimized using a slightly more complex Wilson current mirror. The 3- and 4-transistor versions are shown in Figure 3. The 4-transistor version is more accurate and has a wider dynamic range.

　　Figure 3. Wilson current mirror T4 is optional, but its use improves accuracy and dynamic range.

　　When a transconductance amplifier (voltage_in/current_out) is needed, it can be constructed using a single-supply op amp, a BJT or FET (MOSFET is usually the best choice because it has no base current errors), and a precision resistor that defines the transconductance value, as shown in Figure 4.

　　Figure 4. Transconductance amplifier VIN-IOUT

　　This circuit is simple and inexpensive. The voltage on the MOSFET gate sets the current in the MOSFET and R1 so that the voltage V1 across R1 is equal to the input voltage VIN.

　　If a current mirror is needed in a monolithic IC, a simple transistor current mirror is preferred. However, if a discrete circuit is used, the high price of matched resistors (higher because of limited demand rather than manufacturing difficulties) makes the op amp current mirror of Figure 5 the cheapest technology. This current mirror consists of a transconductance amplifier and an additional resistor.

　　Figure 5. Op amp current mirror.