Bias Stability Using Current Mirrors

Current-mirrors are the most common biasing method in integrated circuits, mainly due to the ease of incorporating transistors in the fabrication process in comparison to using bulky, more expensive resistors. An added advantage of using current-mirror biasing is that the bias point is supposedly less susceptible to variations in temperature and fluctuations in circuit component values. To see this effect in action, I decided to compare the temperature-bias stability of two different simple common-emitter circuits in SPICE. The two circuits that were compared are shown below. The circuit on the left is biased with a resistor network, while the one on the right is biased with a basic current mirror.


All the resistors in both circuits are given arbitrary temperature coefficients of 200ppm/deg. C. The SPICE model for the transistors automatically accounts for temperature variations in internal parameters, so I didn't have to specify any temperature coefficients there. A plot of the collector bias current Ic vs Temperature is shown below for both circuits.



Although the variation of Ic isn't much in terms of magnitude for both cases, the graph clearly shows the enhanced temperature stability of the collector current in the current-mirror biased circuit as compared to the resistor-biased circuit. To see the effect of variations in the power supply voltages on the bias current, I used the .SENS command in SPICE, which computed the sensitivity of the specified parameter (Ic) with respect to all the other components and parameters in the circuit, including the voltage sources. However the simulation output file showed similar sensitivities for both circuits, around 5.17E-06 amps per percent change in either +/- 12V source voltage, with no real reduction in sensitivity with the current-mirror biased circuit. The lack of reduction in sensitivity in the current-mirror biased circuit might be due to the specific circuit configuration I used. I'll probably try and compare larger, multi-stage circuits to see if the current-mirror bias performs any better with respect to voltage variations.