Differential Pair DC Bias

The differential amplifier is also called the emitter coupled pair, because, as shown in figure 5.1, it has two transistors Q1 and Q2 with their emitters shorted together.

  

Figure 5.2: Differential Amplifier DC Bias

The emitter coupled pair is biased by a DC current source. In Fig. 5.2, this current source is provided by Q3, which is a common emitter biasing configuration. (In contrast to our previous work, the common emitter configuration of Q3 is not being used as an amplifier, but just to provide a constant current to the emitter coupled pair.) Under ideal conditions, Q3 acts as an ideal DC current source which biases Q1 and Q2. The transistors Q1 and Q2 provide the small signal voltage gain, which we will discuss in the next section. Here, we discuss the DC bias. We first ground the two inputs. Now, we provide DC bias by Q3, and define the DC bias current sunk by collector Q3 as I_EE. 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 (101)

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

$$I_{E1}=I_{E2}=\frac{I_{EE}}{2}$$ (102)

We now make the excellent approximation that I_E1=I_C1=I_C2. The DC voltage at the collectors of Q1 and Q2 is then

 

V_C1 = V_CC-I_C1R_C1 (103)

 

V_C2 = V_CC-I_C2R_C2 (104)

If R_C1=R_C2, then V_C1=V_C2. Of course, since the bases of Q1 and Q2 are grounded, then

$$V_{E1}=V_{E2}=-V_{BE}\approx -0.7V.$$ (105)

Just as we did in our other transistor amplifier circuits, for analog operation, we must ensure that Q1 and Q2 are in the forward active region of operation. So, when designing our amplifier, we must make sure that our transistors are not in saturation $(V_C\gt V_B\approx 0.7)$.So, to zero order approximation, we usually choose R_C and I_C so that $V_C \approx \frac{V_{CC}}{2}$. This will help you to obtain virtually maximum swing of your AC signal.