Analysis of constant current source circuit composed of two NPN tubes

2 individual NPN Constitute constant current

T1 and T2 For two NPN triode .R1=1K, Used to simulate the load ,AM1 For ammeter ,R4=100R, Set resistance for constant current .

First of all, it is necessary to judge the working state of the two triodes ？

hypothesis T1 Working in the magnifying area , Then the equation is satisfied Ic=βIb,Ie=Ic+Ib, commonly β Tens to hundreds , Ignore Ib,Ie About equal to Ic.

according to T2 triode Vbe Clamping , got it T1 Emitter voltage , obtain Ie Current ,Ic be equal to Ie,Ic With , The collector voltage has , You can work out Vce It's reasonable , That is, the assumption is true ,T1 Work in the distribution area .

Same thing ,T2 It also works in the magnifying area .

T1 and T2 Mutual restriction , There is no case that one side works in the saturation zone .

Let's look at the simulation results first , Then look at how it works .

3 The voltage at point is T2 BE PN Junction clamp voltage ,0.58V, The constant current we need I=Vbe/R4, by 5.8mA.

working principle ：

When the load is heavier , namely R1 Reduce ,I1 increase ,I2 increase ,R4 The pressure drop on the increases ,I3 increase ,T2 Of CE The equivalent resistance decreases ,I4 and I5 increase ,I6 Approximately constant ,I7 Reduce , Lead to I1 Reduce , This is actually a negative feedback , Play the role of constant current .

R1 increase , Load reduction , Is the same analytical method , No more details here .

Carrying capacity

R4=100Ω, Constant current output 5.8mA, The simulation results show that the load carrying capacity is 1.6K about .

take R4 Reduce to 47Ω, Constant current output 12mA, The simulation results show that the load carrying capacity is 765Ω.

We can come to a conclusion ： The smaller the constant current , The stronger the carrying capacity , The greater the constant current , The weaker the carrying capacity .

Source of the article ：《2 individual NPN A constant current circuit composed of triodes , Mom doesn't worry about my model anymore 》