Realization of Fast Response V/I Conversion Circuit

0 Introduction

In some driving circuits, some device characteristics are linearly related to the current, and they are current-type driving devices. These devices (such as semiconductor lasers) may cause a sharp deterioration in performance or even failure under inappropriate working conditions (for example, some lasers may be damaged by current overshoot in a very short time). Therefore, based on the application of the operational amplifier feedback loop, it is necessary not only to ensure the linearity of the current, but also to eliminate the overshoot problem in the step process. This paper designs a voltage-to-current circuit with fast response by driving the load with current, and simulates the circuit using the actual model in PSPICE, with a simulation response time of hundreds of nanoseconds. Therefore, the design of this circuit has a certain reference value in high-speed networks.

1 Theoretical analysis of voltage-to-current conversion

The feedback circuit built by integrated operational amplifiers can generally be represented by the block diagram shown in Figure 1, and can be classified according to whether the output signal is current and voltage and the feedback signal is voltage and current (can be divided into four categories). This circuit adopts the form of current series negative feedback, and its input and output impedance are:

Among them, Ri and R0 are the input and output resistances of the basic amplifier; A is the gain of the basic amplifier when the load is RL, and AS|RL=0 are the source gains of the basic amplifier when short-circuited, respectively. The basic amplifiers referred to here should include the load effect resistance of the feedback network. It can be seen from the formula that the current series has the characteristics of a transconductance amplifier, that is, it has higher input impedance and output impedance.

2 Use PSPICE to create simulation circuits

Figure 2 shows a circuit diagram of voltage-to-current conversion. The op amp and transistor in the circuit are both conventional product models. The op amp used is a high-speed op amp, and the transistor is a high-frequency device. The high frequency combination of the two enables the voltage-to-current circuit to have a faster response speed and a higher bandwidth. Based on the virtual short and virtual open characteristics of the op amp and R1=R2, R5=R6, the relationship between the load current and voltage can be deduced as follows:

Cl in Figure 2 is a compensation capacitor used to eliminate the overshoot generated by the circuit. R7 and R8 are current limiting resistors that can be used to protect the transistor, and R4 is the assumed load in the actual circuit. Assuming that the input voltage is a pulse with a rise and fall time of 20 ns, the circuit in Figure 2 can be simulated.

3 Simulation Results

If R5 is 1 kΩ, R1 is 470 Ω, and R3 is 0.5 Ω, the theoretical current is I=0.94U. When the input is 0.5 V, the simulation results are shown in Figure 3. It can be seen that in this mode, oscillation overshoot will occur during the rise and fall process, and the larger the current, the more serious the overshoot. When the compensation capacitor is added between pin 2 and pin 6, the overshoot is eliminated, and the simulation results at this time are shown in Figure 4. When the input voltage is changed, the output is basically close to the theory and has a better linear relationship. The simulation results at this time are shown in Figure 5, and the current is close to 1 A.

From the above simulation results, it can be seen that the circuit has fast rise and fall times, and its value is close to 100 ns.

4 Conclusion

This paper introduces a design method for a V/I circuit that can achieve fast response. The circuit uses reasonable compensation capacitors to eliminate overshoot in the circuit. Its response time is hundreds of nanoseconds, and the drive current can be increased. It can be used to drive current-type load devices. The simulation circuit uses the actual device model in PSPICE, so it is closer to the actual circuit. Therefore, this method has a certain practical application reference value for driving high-speed current-type loads.