Why Re is replaced by constant current source in differential amplifier?

To improve CMRR we need to increase Re. But Re cannot be increased to a great extent, since the dc current will decrease which will change the Q point of the circuit. Hence Re is replaced by constant current source which provide very high resistance.

What is the resistance of constant current source?

3 in the source). The INIC acts as a second current source passing “helping” current, I−R, through the load. As a result, the total current flowing through the load is constant and the circuit impedance seen by the input source is increased.

How can I increase my CMRR value?

To increase the value of CMRR, which circuit is used to replace the emitter resistance Re in differential amplifier? Explanation: Constant current bias offers extremely large resistor under AC condition and thus provide high CMRR value.

How does an emitter resistor improves stability?

The aim of an AC signal amplifier circuit is to stabilise the DC biased input voltage to the amplifier and thus only amplify the required AC signal. This stabilisation is achieved by the use of an Emitter Resistance which provides the required amount of automatic biasing needed for a common emitter amplifier.

Why does a current source have infinite resistance?

Current always takes the path of least resistance. Thus, in order for current to go to the path of least resistance, we must set up the source and load so that the source has much higher resistance than the load. This is why we construct the ideal current source to have infinite internal resistance.

What do you mean by constant current source and constant voltage source?

DC regulated power supplies are broadly classified into “DC constant voltage power supplies” whose output voltage is stable even when the load changes and “DC constant current power supplies” whose output current is stable.

What is constant current source and constant voltage source?

A constant voltage source is a power source which provides a constant voltage to a load, even despite changes and variance in load resistance. In other words, the voltage which a constant voltage source provides is steady, even if the resistance of the load varies.

Will the use of a constant current source increase CMRR?

Besides supplying constant emitter current, the constant current bias also provides a very high source resistance since the ac equivalent or the dc source is ideally an open circuit. High resistance RE will reduce the common mode gain thus improving the CMRR.

How can we increase the CMRR of a DC Circuit?

To improve CMRR we need to increase Re. But Re cannot be increased to a great extent, since the dc current will decrease which will change the Q point of the circuit. Since, we want constant current in the emitter terminal and also very high resistance in the emitter terminal.

Is there a common mode gain for q1/2 emitters?

There is a common mode gain, RC/2RE, but it’s much smaller than the differential gain, RC/2(Qx intrinsic Re), and for many purposes, this degree of CMRR is sufficient. You’ll notice there’s no resistor used between Q1/2 emitters to reduce the differential gain, so this gain stays very high.

What would happen if ordinary resistors were used as current sources?

If ordinary resistors were used then the current would vary considerably over the voltage range. One common example of the use of current sources is to drive a Zener diode in a regulator circuit.

Why is the current constant in a series pass transistor circuit?

Keeping the current constant regardless of the current taken by the series pass transistor in the circuit helps maintain a much better level of regulation. Also stand-alone current sources are also needed in processes ranging including electrochemistry and electrophoresis.