DC Bias

Most integrated-circuit amplifiers use transistors as loads instead of R_c, as shown in Fig. 6.2 .

Fig6.2: Difference Amplifier with Active Load

When transistors are used instead of resistors, the load is said to be active. In Fig. 6.2 transistors Q1 and Q2 form a differential pair. The active load for the differential pair consists of (PNP) transistors Q3 and Q4 connected in a current mirror configuration. The output is taken single-endedly from the collectors of Q2 and Q4. The circuit is biased with constant current I_C6 from the current mirror composed of transistors Q5 and Q6. Assuming Q1 and Q2 are identical, then the following equations apply. >From KCL it is clear that

I_E1+I_E2= I_EE

Then this bias current is split between Q1 and Q2 so that

I_C1+I_C2= I_EE   Now, let us assume Q1 and Q2 are identical, then the the DC bias current I_EE is split evenly between them so

I_E1+I_E2=( I_EE)/2

We now make the excellent approximation that I_E1=I_C1=I_C2. Now, we neglect base currents, and continuity leads to

I_C1=I_C3

Finally, since Q3 and Q4 form a current mirror,

I_C3=I_C4