Circuit analysis: constant current source circuit composed of op amp and triode

大家好,我是蜗牛兄.Today I will share with you a constant current source circuit composed of op amps and triodes.希望对你有帮助.

This article will explain the circuit from the following five aspects：

图1 Explain the block diagram

一、Basic formulas for op amps and triodes

1.1About the op-amp：

图2 运放

The virtual short and virtual break are basically used in the calculation process of the op amp.,即：

“虚短”是指在分析运放处于线性状态时,The two inputs can be regarded as the same potential,这一特性称为虚假短路,简称虚短.即：

“虚断”是指在分析运放处于线性状态时,The two input terminals can be regarded as equivalent disconnection,这一特性称为虚假开路,简称虚断.即：

1.2About triodes

图3 The direction of the current of the triode

公式如下：

二、电路简介

This is done by the op ampU1A和三极管Q1A constant current source circuit composed of related resistance-capacitance components.

图4 恒流源电路

电阻R2Play a protective role for the op amp,这个值不能太大,一般取值10R左右.（a little experience,The products we design generally have added）;

电阻R3Provide a base current for the transistor;

电阻R4Bit sampling resistor,RL为负载电阻;

电阻R5起缓冲限流的作用,一般选取1K~100K之间（Some circuits do not add this resistor）;

三极管Q1为NPN类型,The voltage needs to be selected according to the actual application、A suitable transistor for the current.

三、原理分析

How does it achieve constant current?？

此电路看似简单,The actual principle is that when the sampling resistorR4when the voltage changes,直接反馈到运放的反相输入端,它与同相输入端电压的差值被运放放大,输出控制三极管的基极电流,改变三极管的内阻,从而改变发射极与集电极间的电压降,从而使采样电阻的电压保持不变,以达到负载电流恒定的目的.

图5  The process of circuit stabilization

Brief description of the two circuit change process diagrams：

图6  Changes due to increased load current

图7  Variation due to reduced load current

因此,负载电流经过采样电阻的实时反馈下最终达成恒定的稳定电流.

实际计算：

If we want to achieve the output1mA的恒定电流,

设Vin=5V,R4=5.1K.

根据虚断,Vin=Vin+;

根据虚短,

又

The base voltage of the triode can be calculated as

即,运放输出电压为5.7V.

输出电流

This load can be achieved1mAconstant current.

四、电路仿真

仿真（后台回复“恒流源电路2”,Get the simulation file）

图8  电路仿真

电流980uA,with calculated value1mA接近.

Disadvantages of the circuit：

虽然,三极管发射极电流与集电极电流近似相等,但实际上,发射极的电流还包含了基极电流.

可以看出,运放输出级使用三极管时,输出电流会产生基极电流分量这一误差.If the circuit accuracy requirements are not met at this time,可将三极管改成MOS管.

MOS管属于压控器件,栅极需要的电流很小.IRL和IR4可以非常的接近,相比三极管而言,电流的精度提升了.

图9  运放+MOS管电路

五、电路对比

Compared with a constant current circuit analysis I posted before《How do these two triodes achieve constant current?？一起来分析一下》

图10  A constant current source composed of two triodes

The two circuit which is better？

其实也不能说哪个好,哪个不好.它们各有利弊,Mainly depends on the use case.

Integrated op amp with high input impedance and low output impedance,用运放设计的恒流源电路引入了反馈,相比三极管式的恒流源,运放式的恒流源有足够的精度和可调性.

Of course, although the constant current source of the op amp has obvious advantages,但也有缺点,运放的VinThe power supply needs to be provided additionally and the price of the op amp is more expensive than that of the triode.

因此,Op amps are used for occasions with high precision requirements+三极管（或MOS管）.对精度要求不高,However, the triode is selected for occasions with high cost requirements+Combination of triodes.

今天的分享就到这里,希望对你有帮助.

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