Solution for Question 3 : IES Conventional 1998 : Electronics & Telecommunication Engineering : Paper II

Q.3. (a): Develop the voltage transfer function V0/VS for the amplifier shown.

Solution: 
Consider two voltage nodes V1 and V2, as shown in figure below,

The given circuite if negatively feedbacked, hence due to virtual ground principle, 

V1 = 0 V

Now, apply Kirchoff's Current law at node V1, we get

Hence, 

Now, by applying apply Kirchoff's Current law at node V2, we get

by solving, 

This is an required solution.

Q.3. (b): Explain the current foldback characteristics of voltage regulators. Draw a circuit to realize the current foldback characteristic in a low voltage regulator using 723 IC chip. Explain the working of the circuit.

Solution: 

The current foldback characteristics of voltage regulators:

As seen from characteristics of voltage regulators, beyond the rated value, if the load increases, the voltage folds backward and finally becomes zero at Isc which is less than the rated load current. Hence, the protection circuit is called foldback protection circuit.

The disadvantages of constant current limit is relatively large power dissipation in the series pass transistor when the load terminals are shorted. Thus a large power rating transistor is required. The foldback limiting technique allow us to provide the necessary load current at rated voltage but reducing the short circuit current. Thus the series pass transistor gets utilized efficiently.

Here, the key point is, when the output terminals are shorted, the current decreases rather than increasing. Thus series pass transistors get protected automatically.

 Low voltage regulator using 723 IC chip

The internal working can be explained by dividing it into two blocks, the reference voltage generator and the error amplifier. In the reference voltage generator, a zener diode is being compelled to operate at fixed point (so that zener output voltage is a fixed voltage) by a constant current Source which comes along with an amplifier to generate a constant voltage of 7.15V at the Vref pin of the IC.

As for the error amplifier section,it consists of an error amplifier, a series pass transistor Q and a current limiting transistor. The error amplifier can be used to compare the output voltage applied at Inverting input terminal through a feedback to the reference voltage Vref applied at the Non-Inverting input terminal.These connections are not provided internally and so has to be externally provided in accordance with the required output voltage. The conduction of the transistor Q is controlled by the error signal. It is this transistor that controls the output voltage.