Biasing of Differential Amplifiers

Constant Current Bias:

In DC analysis of the differential amplifier, we have seen that emitter current IE depends upon value of the bdc. To make the operating point stable IE current should be constant irrespective of the value of bdc.

For constant IE, RE should be quite large. This increases the value of CMRR also but if RE value is increased to quite large value, IE  (quiescent operating current) decreases. To maintain the same value of IE, emitter supply VEE  should be increased. To obtain very high value of resistance RE and constant IE, current, current bias is used.

Figure 1

Figure 1, shows dual input balanced output differential amplifier by using a constant current bias. The resistance R_E  can be replace by the constant current transistor Q₃. The DC collector current in Q3 can be established by R₁, R₂, & R_E.

By applying the voltage divider rule, the voltage at base of Q3 is

As the two halves of differential amplifiers are symmetrical, each has half of the current I_C3.

The collector current, I_C3  in transistor Q₃  is fixed as no signal is injected into either the emitter or base of Q₃.

Besides supplying the constant emitter current, constant current bias provides a very high source resistance also since the AC equivalent or DC source is ideally an open circuit. Thus, all the performance equations obtained for the differential amplifier by using the emitter bias are also valid.

As seen in the I_E  expressions, current depends upon the V_BE3. If the temperature changes, V_BE changes and current IE also changes. To improve the thermal stability, a diode is placed in series with the resistance R1as shown in the figure 2.

Figure 2

It helps to hold the current I_E3 constant even if the temperature changes. By applying KVL to the base circuit of Q₃.

Thus, the current I_E3 is constant and independent of the temperature due to the added diode D. In the absence of D the current would vary with temperature because V_BE3 decreases approximately by 2mV/° C. The diode has same temperature dependence and hence the two variations cancel each other and I_E3 doesn't vary appreciably with the temperature. Since the cut - in voltage V_D of diode is approximately same value as the base to emitter voltage V_BE3  of a transistor the above condition cannot be satisfied with one diode. Therefore 2 diodes are used in the series for V_D. In this case common mode gain reduces to zero.

 
At times zener diode can be used in place of the diodes and resistance as shown in the figure 3. Zeners are available over the wide range of voltages and can have the matching temperature coefficient

The voltage at base of transistor Q_B is

Value of R₂  is selected so that I₂  » 1.2 I_Z(min)  here I_Z  is minimum current needed to cause the zener diode to conduct in reverse region, which is to block the rated voltage V_Z.

Current Mirror:

The circuit in which output current is forced to equal the input current is said to be a current mirror circuit. Therefore in a current mirror circuit, the output current is a mirror image of input current. The current mirror circuit is shown in the figure 4.

Figure 4

Once the current I₂ is set up, the current I_C3 is established automatically to be approximately equal to I₂. The current mirror is the special case of constant current bias and current mirror bias requires of constant current bias and thus can be used to set up the currents in differential amplifier stages. The current mirror bias requires few components than the constant current bias circuits.

As Q₃ and Q₄ are identical transistors current and voltage are nearly same

For the satisfactory operation the two identical transistors are essential.

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