Exploring the Benefits of a Constant Current Circuit in Differential Amplifiers

• chaoseverlasting
In summary, a constant current circuit is added to the emitter of a differential amplifier to increase resistance and reduce common mode gain.
chaoseverlasting
In a differential amplifier in common mode, a constant current circuit is appended to emmitter part to increase the resistance Re. This results in a very low common signal amplification (which reduces noise in the ckt).

Why does a constant current circuit increase the resistance?

If you use a real resistor of high resistance, you get way too much voltage drop. Instead, you can use a current sink (or source) circuit to generate the bias current, but still present a high impedance.

Then what we're trying to do is control the resultant current, and its not the resistance but the current that causes the common mode gain to be small?

chaoseverlasting said:
Then what we're trying to do is control the resultant current, and its not the resistance but the current that causes the common mode gain to be small?

Not sure I understand the question. The current is the total bias current for the diff pair. The splitting of the current by the diff pair is what gives differential gain, as the current is split unevenly by the difference in input voltages. If the input voltages are the same but varying, the current should not be different in the two sides of the diff pair.

chaoseverlasting said:
In a differential amplifier in common mode, a constant current circuit is appended to emmitter part to increase the resistance Re. This results in a very low common signal amplification (which reduces noise in the ckt).

Why does a constant current circuit increase the resistance?

The dynamic resistance. The dynamic resistance of a current source is greater than a resistor.

$$R_{dyn} = \frac{dE}{dI}$$

For an ideal current souce, the voltage changes nothing for any change in current.

The common mode voltage gain is given by:

$$A_c=\frac{\beta R_c}{r_i +2(\beta +1)R_E}$$ (Boylestad, p.600)

To minimize this gain, its the resistor $$R_E$$ that we maximize.

The bias current also depends on this resistor:

$$I_c=\frac{1}{r_i+2(\beta +1)R_E}$$

If we use a constant current circuit to control this current and minimize it, we effectively increase the magnitude of the resistor $$R_E$$ and that causes the common mode gain to be small. Is that right?

That is what I meant to say when I said that we use the current to control the gain.

Looks good to me, chaos.

Finally!

What is a constant current circuit?

A constant current circuit is a type of electronic circuit that maintains a constant current, regardless of changes in voltage or other factors. This is achieved by using components such as transistors or operational amplifiers to regulate the current flowing through the circuit.

What is a differential amplifier?

A differential amplifier is a type of electronic amplifier that amplifies the difference between two input signals. This allows for greater precision and accuracy in measuring small changes in the input signals, making it useful in applications such as audio amplification, instrumentation, and signal processing.

How does a constant current circuit benefit a differential amplifier?

A constant current circuit can improve the performance of a differential amplifier by providing a stable and accurate source of current for the amplifier to work with. This helps to reduce noise and distortion in the output signal, resulting in a cleaner and more accurate amplification of the input signals.

What are some potential applications for a constant current circuit in a differential amplifier?

Constant current circuits are commonly used in differential amplifiers for various applications such as audio amplification, sensor interface, and precision measurement. They can also be used in communication systems, medical equipment, and industrial control systems.

Are there any drawbacks to using a constant current circuit in a differential amplifier?

One potential drawback of using a constant current circuit in a differential amplifier is that it may introduce additional complexity and cost to the circuit design. Additionally, if the constant current is not precisely regulated, it may affect the accuracy of the amplifier's output signal. However, these potential drawbacks can be mitigated by careful circuit design and component selection.

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