Constant Current Bias in Differential Amplifier:

In the differential amplifiers discussed so far, we have used the combination of R_E and V_EE to produce emitter dc bias current. Alternatively, constant current bias circuit can also be employed to set up the emitter dc bias current. Actually, the constant current bias in differential amplifier circuit is considered better as it provides current stabilization and, therefore, assures a stable operating point for the differential amplifier.

Dual-input, balanced-output differential amplifier with a resistive constant current bias is depicted in Fig. 20.19. By comparing the two circuits (circuits shown in Figs. 20.2 and 20.19) it is noticed that the emitter resistor R_E has been replaced by a constant current transistor Q₃ circuit. The dc collector current in transistor Q₃ is established by resistors R₁, R₂ and R₃ and may be determined as follows :

Voltage at the base of transistor Q₃,

By voltage divider rule, neglecting base loading effect

Voltage at the emitter of transistor Q₃,

Collector current,

Substituting the value of V_E3 from Eq. (20.42) in above equation, we have

Because the two halves of the differential amplifier are symmetrical, each has half of the current I_C3. Thus,

The collector current of transistor Q₃(I_C3) is fixed and must be invariant because there is no input signal into either the base or emitter of transistor Q₃. Thus the transistor Q₃ is a source of constant emitter current for transistors Q₁ and Q₂ of the differential amplifier [Fig. 20.19].

We have seen, in the analysis of differential amplifier circuits with emitter bias, that R_E must be much larger than r′_e i.e., R_E >> r′_e. The constant current bias, in addition to supply of constant emitter current, also provides a very high source resistance. This is because the ac equivalent of the dc current source is ideally an open circuit. Thus, all performance equations derived for the different configurations of differential amplifiers with emitter bias are also valid in case of differential amplifiers with constant current bias.

The thermal stability of the constant current transistor Q₃ can be improved if resistor R₁ is replaced by diodes D₁ and D₂ as depicted in Fig. 20.20. The base of transistor Q₃ is biased with the voltage divider consisting of components resistor R₂, and diodes D₁ and D₂. Diodes D₁ and D₂ assist in holding the emitter current I_E3 constant even though there is large variation in temperature.

From the circuit shown in Fig. 20.20, it is seen that current I₂ flows to the node at the base of transistor Q₃ and then divides into diode current I_D and base current I_B3 of transistor Q₃. In case the temperature of transistor Q₃ goes up, its base-emitter voltage V_BE3 reduces (in silicon transistors, V_BE reduces by 2 mV/°C, and in germanium transistors, V_BE reduces by 1.6 mV/°C). Because of reduction in V_BE3, the voltage drop across R₃ tends to rise and, therefore, emitter current I_E3. On the other hand, the voltage drops across diodes D₁ and D₂ also reduce, causing a greater portion of current I₂ to contribute to diode current I_D. This causes the base current I₃ to fall, which prevents any significant increase in emitter current I_E3.

The transistor Q₃ emitter current I_E3 may be determined as follows :

Voltage at the base of transistor Q₃,

Assuming equal voltage drop across the diodes and denoting it by V_D, voltage at the emitter of transistor Q₃,

and emitter current through transistor Q₃,

Assuming same characteristics for transistor Q₃ and diodes D₁ and D₂ i.e., V_D = V_BE3, we have

Thus the emitter current through transistor Q₃ depends upon V_D and resistor R₃. The voltage drop across the diodes depends upon the diode current I_D and diode current is a part of current I₂ which depends upon the value of resistor R₂. Thus emitter current I_E3 can be varied by varying either R₂ or R₃.

Performance can be made better, if we use a transistor array like CA3086 as a constant current source [Fig. 20.21]. Here an isolated transistor is employed, and required diodes are formed by employing transistors connected for diode operation. No doubt, discrete diodes will work as well. Figure 20.21 shows the functional diagram of CA3086.

For designing a constant current bias circuit depicted in Fig. 20.20, we may proceed as follows :

First of all let us choose a transistor Q₃ emitter current I₃.

The values of resistor R₃ and R₂ now can be determined from Eqs. (20.43) and (20.44) respectively by assuming V_D1 = V_D2 = 0.7V and I₂ = I_E3

Often we use a zener diode in place of diodes as depicted in Fig. 20.22, because zeners are available over a wide range of voltages and can have a matching temperature coefficient of voltage to those of transistors.

From circuit diagram given in Fig. 20.22.

and voltage at the emitter of transistor Q₃,

and so the current through resistor R₃,

The value of R₂ should be selected so as to provide I₂ = 1.2 I_ZT where I_ZT is the minimum current required to cause the zener diode to conduct in the reverse region (i.e., to block the rated voltage V_Z) and its value is specified on the data sheet of a zener diode.

The zener diode is quite useful for maintaining a constant base voltage and, therefore, the constant emitter current in a constant current bias circuit.