Keep IC And VCE Constant

Often, when making dc and rf measurements, it's necessary to bias a transistor at a precise collector current (I_C) and collector-emitter voltage (V_CE). In reliability tests, I_C and V_CE should be maintained constant even though the device's beta may be degrading with time.

Conventional bias can't maintain constant I_C and V_CE if there's a wide variation in beta (such as 5 to 500). Moreover, if the emitter must remain grounded for RF measurements, problems arise in maintaining a constant bias.

This circuit (Fig. 1) maintains constant I_C and V_CE. The device under test (DUT) is connected to op amp A1. The bias supply (V_B) is connected to the inverting input (pin 2) of A1. Because transistor Q1 is in the feedback loop with A1, the noninverting input (pin 3) is also at the same potential V_B. The noninverting input is connected to the DUT's collector, and V_B precisely sets V_CE. In addition, because the noninverting input doesn't draw any current, the I_C should come from the collector supply (V_C). By choosing V_C and R_C , the I_C be precisely set, independently of beta. However, if V_B is varied, I_C will change.

To make I_C independent of V_B, a noninverting summing amplifier (A2) is added (Fig. 2). With the component values chosen, the output of A2 will always be at a potential of V_C + V_B. Comparing Figure 2 with Figure 1, it can be seen that I_C is now independent of V_B. Since I_C is now supplied by A2, V_C need not provide large currents. With V_C = 1 V and V_B = 5 V, Q1 will be biased at I_C = 1 mA and V_CE = 5 V. To prevent Q1 from oscillating, capacitors should be added at the collector and base terminals of the DUT.