Op-Amp Constant Current Source

The op-amp constant current source below is designed to eliminate the effects of temperature on the BJT used in the current source. This current source is a very accurate and temperature-independent current source  

The current through R3 is also the emitter current of Ql. Since for a BJT, the collector current is approximately equal to the emitter current, the collector current and load current are held constant at approximately 1 mA. We will run a Bias Point analysis on the slightly modified circuit below to see the voltage Vref and the collector current  

We see that both nodes labeled Vref are at a voltage of 10.01 volts. The negative feedback attempts to keep these two voltages the same. The emitter current is 1.001 mA and the collector current is 997 XA. Since Ic = [P/(P+1)]Ie, we expect the collector current to be slightly less than 1 mA, in this case 3 pA. 

We will now run a linear temperature sweep from -25 to 125°C to see how this circuit is affected by temperature. For the first simulation only the BJT will have temperature dependence. We will use normal resistors (part name R), which are independent of temperature. Note that the Q2n3906 BJT model includes temperature dependence. Set up the DC Sweep as shown  

HSacondaiy Sweep DMonta Cailo/Woret Cate Parametric Sweep Tamperatura (Sweep) 

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