Current limiter using current differential transconductance amplifier and its application

Generally, current limiters are a modified method for dealing with nonlinear problems. In analog signal processing applications, current limiters are basic elements in the design of nonlinear components and networks (such as nonlinear resistors, irregular oscillators, precision rectifiers, and piecewise linear function approximation generators).

Now, there is a device called a current differential transconductance amplifier (CDTA), which is a new current mode active device with two current inputs and two output currents. This device is synthesized by using the differential characteristics of a modified differential current conveyor (MDCC) and a multi-output transconductance amplifier to achieve current mode signal processing. Therefore, active filters, oscillators, and amplifiers are realized using CDTA as an active element.

Other applications of nonlinear CDTA are expected, especially in precision rectifiers, current-mode Schmitt triggers, and current-mode multipliers. Unfortunately, there are few examples of CDTA applied to synthetic current-limiting circuits.

This paper describes the application of CDTA as a basic active element for designing a simple current limiter. The breakpoints and slope of the transfer curve can be programmed using the transconductance gain gm of the CDTA. To demonstrate the versatility of the proposed tunable current limiter, nonlinear applications of the current limiter in a programmable current mode precision full-wave rectifier and a step-by-step linear function approximation generator are presented.

Circuit Description

The circuit representation and equivalent circuit of CDTA are shown in Figure 1. The following equations represent the terminal relationship of CDTA:

Vp=Vn=0

from=ip-in

ix=gmVz=gmZziz

Where Zz is the external impedance connected to the output Z. Usually, the gm value can be linearly adjusted within a range of tens using the supply bias current/voltage, which makes the designed circuit parameters controllable. It is emphasized here that electronic controllability becomes very important in the design performance index changes and integrated circuit form.

Figure 2 shows the basic building block of the CDTA-based current limiter proposed by the author and the corresponding transfer characteristic curve. In the circuit shown in Figure 2a, iin is the input current and IB is the breakpoint current. If iin≤IB, the diode is open circuited. Since no current flows through the diode D1 (ID1=0), the output current becomes zero (iout=0). When iin>IB, the output current iout will flow through the diode D1. Therefore, the relationship between the output current iout of the circuit and the breakpoint current IB and the gain gm is as follows:

From equation (2), it can be seen that the breakpoint and slope of the transfer characteristic curve can be electrically controlled by the value of the tuning current IB and the gain gm, which gives more flexibility in the design of nonlinear function approximation. In addition, based on the same principle, the circuits of Figures 2b-d have similar transfer characteristics to equation (2), which depends on the CDTA output connection and the diode.

application

The current limiter shown in Figure 2 can be effectively used to implement a current mode precision full-wave rectifier (see Figures 3 and 4). The multiple output current follower stage is a circuit that generates multiple output currents in the same direction from the input current (iin). According to the previous description, the CDTA-based current limiter is juxtaposed with gml-1=gm2=gm, and the input and output current relationship of these two new circuits is obtained:

This means that the circuits shown in Figures 3 and 4 operate as positive and negative current mode full-wave rectifiers, respectively. It is worth noting that the slope of the transfer curve can still be controlled by tuning CDTA1's gm1 to control the negative slope of the transfer curve and CDTA2's gm2 to control the positive slope of the transfer curve.

The current limiter shown in this article can realize a precise linear function approximation circuit. As an example, Figure 5a shows the desired transfer characteristic, which is composed of three linear segments. The required slopes of each CDTA-based current limiter in Figure 2 are shown in the lower part of Figure 5a. The actual circuit implementation is shown in Figure 5b.

Note that the slopes S0, S1 and S2 of the effective transfer characteristic can be easily determined by adjusting the values ​​of gm1R1, gm2R2 and gmR3, respectively. From the above discussion it is obvious that any nonlinear circuit generator can be synthesized.

Conclusion

This paper proposes a simple method to synthesize current limiters using CDTA as active components. The proposed CDTA-based current limiter can be used as a basic building block for realizing integrated nonlinear function circuits (such as current mode precision full-wave rectifiers, precision linear function approximation circuits). All the resulting synthetic building blocks also have the desired characteristics, that is, by changing the gm value of the CDTA, the positive and negative slopes and breakpoints of the linear segment can be electrically tuned. (Peng Jingxiang)

Figure 1 CDTA circuit symbol (a) and equivalent circuit (b)

Figure 2 CDTA-based current limiter building block

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Figure 3 Positive current mode full-wave rectifier circuit (a) and transfer characteristic curve (b)

Figure 4 Negative current mode full-wave rectifier circuit (a) and transfer characteristic curve (b)

Figure 5 Precision linear function approximation circuit (b) and transfer characteristic curve (a)