Differential Amplifier With Constant Current Source

In this lecture, we are going to see a differential amplifier with constant Current Source to increase the value of CMRR without affecting the Q-point. So let’s discuss the topic in detail.

Differential Amplifier With Constant Current Source

In the previous section, we have discussed the Effect of R_E on CMRR, where we have seen that it is necessary to increase R_E in order to increase that CMRR. But if R_E is increased then to keep I_E to an adequate level, we need to increase V_EE.

A constant current source has been added in place of R_E as shown in the below figure. The advantage of using the constant current bias is that it provides current stabilization and in turn, assures a stable operating point for the differential amplifier.

A dual-input balanced output differential amplifier is shown in the below figure. Note that the resistance R_E in the original circuit has been replaced by a constant current source.

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Also Read: What are the different methods to improve CMRR in Differential Amplifiers?

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Operation of Constant Current Source

Now let us see the operation of the constant current source of the figure shown above. The resistors R₁ and R₂ are used for biasing Q₃ properly.

• The DC collector current (I_C) transistor Q₃ is set up by the resistors R₁, R₂, and R₃. The voltage at the base of Q₃ is given by,

\mathbf{V_{B3}=\frac{-R_2V_{EE}}{R_1+R_2}}

\mathbf{V_{E3} = V_{B3}-V_{BE3} = \frac{-R_2V_{EE}}{R_1+R_2} - V_{BE3}}

\mathbf{\therefore I_{E3} \approx I_{C3}=\frac{V_{E3}-(-V_{EE})}{R_3} }

Substitute V_B3 from the above equation,

\mathbf{\therefore I_{C3}=\frac{V_{EE}-[R_2V_{EE}/(R_1+R_2)] - V_{BE3}}{R_3}}

As the two halves of the differential amplifier shown in the above figure are symmetrical, the current flowing through each half is I_C3/2

\mathbf{\therefore I_{E1} = I_{E2}=\frac{I_{C3}}{2}=\frac{V_{EE}-[R_2V_{EE}/(R_1+R_2)] - V_{BE3}}{2R_3}}

• The above equation shows that the emitter currents of Q1 and Q2 are independent of the AC signals applied at the transistor inputs. Also because all the quantities in the above equation are constants, the collector current IC2 remains constant. Thus the constant current source of the figure provides a constant current.

How does a constant current source circuit improve CMRR?

• In addition to supplying a constant emitter current, the constant current source of Figure 1 also provides a very high source resistance, since the AC equivalent of the DC current source is ideally an open circuit.

• Thus R_E will be ideal ∞ and the common mode gain A_c will be zero making CMRR = ∞.

• Thus when we replace resistor R_E with a constant current source, it improves CMRR, without disturbing the operating point of the circuit. The supply voltage -V_EE also need not be increased.

How to improve the Thermal stability of a constant current source?

The constant current source should maintain a constant emitter current in spite of variations in ambient temperature.

However, the parameters of transistor Q₃ such as V_BE and β are temperature dependent.

This will change the emitter current with changes in temperature and will eventually disturb the Q-point of the amplifier. Therefore thermal stability of Q₃ should be improved.

To improve the thermal stability of transistor Q₃, the resistor R₁ is replaced by diodes D₁ and D₂ as shown in the below figure.

The base of Q₃ is biased with the voltage divider which contains R₂, D₁, and D₂.

Now let us see how the arrangements of Figure 2 keep the emitter current constant. From the figure, it is clear that,

I₂ = I_D + I_B3

If the temperature of Q₃ increases, its base-emitter voltage V_BE3 will decrease. If Q₃ is made of silicon then the rate of decrease is 2 mv/^oC and if Q₃ is a germanium transistor the rate is 1.6 mv/^oC. This reduction in V_BE3 will tend to increase V_E3 and hence I_E3 will tend to increase.

But due to the presence of D1 and D2, the voltage drops across them also will decrease at the same rate with increased temperature.

This will cause a greater portion of I₂ to contribute to I_D, i.e. I_D will increase. As I₂ is a constant, increased I_D will cause I_B3 to decrease, which prevents any significant increase in I_E3.

Thus the compensating circuit of Figure 2 keeps the emitter current constant in the event of variations in temperature.

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Also Read: What is CMRR? | Common Mode Rejection Ratio

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