Often, when making dc and rf measurements, it's necessary to bias a transistor at a precise collector current (IC) and collector-emitter voltage (VCE). In reliability tests, IC and VCE should be maintained constant even though the device's beta may be degrading with time.
Conventional bias can't maintain constant IC and VCE 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 IC and VCE. The device under test (DUT) is connected to op amp A1. The bias supply (VB) 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 VB. The noninverting input is connected to the DUT's collector, and VB precisely sets VCE. In addition, because the noninverting input doesn't draw any current, the IC should come from the collector supply (VC). By choosing VC and RC , the IC be precisely set, independently of beta. However, if VB is varied, IC will change.
To make IC independent of VB, 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 VC + VB. Comparing Figure 2 with Figure 1, it can be seen that IC is now independent of VB. Since IC is now supplied by A2, VC need not provide large currents. With VC = 1 V and VB = 5 V, Q1 will be biased at IC = 1 mA and VCE = 5 V. To prevent Q1 from oscillating, capacitors should be added at the collector and base terminals of the DUT.