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 RE on CMRR, where we have seen that it is necessary to increase RE in order to increase that CMRR. But if RE is increased then to keep IE to an adequate level, we need to increase VEE.

A constant current source has been added in place of RE 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 Note that the resistance RE in the original circuit has been replaced by a constant current source.

Differential Amplifier With Constant Current Source

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

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 (IC) transistor Q3 is set up by the resistors R1, R2, and R3. The voltage at the base of Q3 is given by,


\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 VB3 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 IC3/2

\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 RE will be ideal ∞ and the common mode gain Ac will be zero making CMRR = ∞.
  • Thus when we replace resistor RE with a constant current source, it improves CMRR, without disturbing the operating point of the circuit. The supply voltage -VEE 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 VBE 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.

Constant Current source with diode compensation

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

I2 = ID + IB3

If the temperature of Q3 increases, its base-emitter voltage VBE3 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 VBE3 will tend to increase VE3 and hence IE3 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 ID, i.e. ID will increase. As I2 is a constant, increased ID will cause IB3 to decrease, which prevents any significant increase in IE3.

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

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

Hello friends, my name is Trupal Bhavsar, I am the Writer and Founder of this blog. I am Electronics Engineer(2014 pass out), Currently working as Junior Telecom Officer(B.S.N.L.) also I do Project Development, PCB designing and Teaching of Electronics Subjects.

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