**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comparing the two circuits (circuits shown in Figs. 20.2 and 20.19) it is noticed that the emitter resistor R_{E} has been replaced by a constant current transistor Q_{3} circuit. The dc collector current in transistor Q_{3} is established by resistors R_{1}, R_{2} and R_{3} and may be determined as follows :

Voltage at the base of transistor Q_{3},

By voltage divider rule, neglecting base loading effect

Voltage at the emitter of transistor Q_{3},

Collector current,

Substituting the value of V_{E3}Â 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 Q_{3}(I_{C3})Â is fixed and must be invariant because there is no input signal into either the base or emitter of transistor Q_{3}. Thus the transistor Q_{3} is a source of constant emitter current for transistors Q_{1} and Q_{2} 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 râ€²_{e}Â i.e., R_{E} >> râ€²_{e}.Â 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 Q_{3} can be improved if resistor R_{1} is replaced by diodes D_{1} and D_{2} as depicted in Fig. 20.20. The base of transistor Q_{3} is biased with the voltage divider consisting of components resistor R_{2}, and diodes D_{1} and D_{2}. Diodes D_{1} and D_{2} 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 I_{2}Â flows to the node at the base of transistor Q_{3} and then divides into diode current I_{D} and base current I_{B3}Â of transistor Q_{3}. In case the temperature of transistor Q_{3} goes up, its base-emitter voltage V_{BE3 }reduces (in silicon transistors, V_{BE}Â reduces by 2 mV/Â°C, and in germanium transistors, V_{BE} reduces by 1.6 mV/Â°C). Because of reduction in V_{BE3}, the voltage drop across R_{3} tends to rise and, therefore, emitter current I_{E3}. On the other hand, the voltage drops across diodes D_{1} and D_{2} also reduce, causing a greater portion of current I_{2} to contribute to diode current I_{D}. This causes the base current I_{3} to fall, which prevents any significant increase in emitter current I_{E3}.

The transistor Q_{3} emitter current I_{E3}Â may be determined as follows :

Voltage at the base of transistor Q_{3},

Assuming equal voltage drop across the diodes and denoting it by V_{D}, voltage at the emitter of transistor Q_{3},

and emitter current through transistor Q_{3},

Assuming same characteristics for transistor Q_{3} and diodes D_{1} and D_{2} i.e., V_{D} = V_{BE3}, we have

Thus the emitter current through transistor Q_{3} depends upon V_{D} and resistor R_{3}. The voltage drop across the diodes depends upon the diode current I_{D} and diode current is a part of current I_{2} which depends upon the value of resistor R_{2}. Thus emitter current I_{E3 }can be varied by varying either R_{2} or R_{3}.

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 Q_{3} emitter current I_{3}.

The values of resistor R_{3} and R_{2} now can be determined from Eqs. (20.43) and (20.44) respectively by assuming V_{D1}Â = V_{D2}Â = 0.7V and I_{2}Â = 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 Q_{3},

and so the current through resistor R_{3},

The value of R_{2} should be selected so as to provide I_{2}Â = 1.2 I_{ZT}Â where I_{ZT}Â 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.