Important Short Questions and Answers: JFET and MOSFET Amplifiers

JFET AND MOSFET AMPLIFIERS

1. State the reason for fall in gain at low frequencies.

The coupling capacitance has very high reactance at low frequency. Therefore it will allow only a small part of signal from one stage to next stage and in addition to that the bypass capacitor cannot bypass or shunt the emitter resistor effectively. As a result of these factors, the voltage gain rolls of at low frequency.

2. State the reason for fall in gain at higher frequencies?

At high frequency the reactance of coupling capacitor is very low. Therefore it behaves like a short circuit. As a result of this the loading effect of the next stage increase which reduces the voltage gain. Hence the voltage gain rolls off at high frequencies.

3. Why the electrolytic capacitor is not used for coupling?

Electrolytic capacitor is a polarized capacitor. So it cannot be used for coupling and also in electrolytic capacitor, the dielectric is not an insulating material but it conducting material which will change the capacitance effect.

4. Write a note on effects of coupling capacitor.

The coupling capacitor Co transmits AC Signal. But blocks Dc. This prevents DC interferences between various stages and the shifting of operating point.

It prevents the loading effect between adjacent stages.

5. What is the significance of gain bandwidth product?

It is very helpful in the preliminary design of a multistage wideband amplifier. This can be used to setup a tentative circuit, which is often used for this purpose.

6. Why N-channel FET’s have a better response than P-channel FET’s?

N- channel FET have a better high frequency response than P-channel FET due to the following reason. Mobility of electrons is large in N-channel FET whereas the mobility of holes is poor in P-channel FET. The input noise is less in N-channel FET that that of the P-channel FET. The trains conductance is larger in N-channel FET that that of P-channel Fet.

7. Define Miller effect in input capacitance?

For any inverting amplifier, the input capacitance will be increased by a miller effect capacitance, sensitive to the gain of the amplifier and the inter electrode capacitance connected between the input and output terminals of the active device.

CMi = (1-Av) Cf ;

CMo = Cf

Cf = Inter electrode capacitance between input and output.

8. What is a Darlington connection in the amplifiers?

A Darlington transistor connection provides a transistor having a very large current gain, typically a few thousand. The main features of the Darlington connection is that the composite transistor acts as a single unit with a current gain, that is the product of current gains of the individual transistors.

9. Give the Applications of JFET

FET is used as a

(1) Buffer amplifier  

(2) Low Noise Amplifier                              

(3) Cascaded Amplifier

(4) Analog Switch

(5) Chopper

(6) Phase Shift Oscillator circuits

(7) Voltage Variable Resistors in Operational Amplifiers and tone controls etc.,

(8) For Mixer operation on FM and TV receivers

10. Give the Applications of MOSFET

MOSFETs can also be used for most applications where JFET is used. MOSFETs have become very popular for digital logic circuits due to high density of fabrication and low power dissipation.

(1) MOSFET is used in Sample and Hold circuit as a switch.

(2) P-MOSFET and N-MOSFET are used in digital logic circuits

(3) C-MOSFET is very popular in fabricating of MSI and LSI technology.

11.            Draw a single stage amplifier circuit using JFET

The circuit of a Single Stage Common Source N-channel JFET amplifier using self bias is shown in fig

12.  What is the purpose of input capacitor, Cin in single stage common source JFET amplifier?

An ac signal is supplied to the gate of the FET through an electrolytic capacitor called input capacitor Cin. This capacitor allows only ac signal enter the gate but isolates the signal source from RG. If this capacitor is not used, the signal source resistance will come across the resistor RG and thus changing the biasing conditions.

13. What is the purpose of Biasing Network (Rs and Cs) in single stage common source JFET amplifier?

The JFET is self-biased by using the biasing network Rs- Cs. The desired bias voltage is obtained when dc component of drain current flows through the source-biasing resistor Rs. whereas, the capacitor Cs bypasses the ac component of drain current.

14.            What is the purpose of Coupling Capacitor (Cc) in single stage common source JFET amplifier?

It is an electrolytic capacitor used to couple one stage of amplification to the next stage or load. It allows only amplified ac signal to pass to the other side but blocks the dc voltage. If this capacitor is not used, the biasing conditions of the next stage will be drastically changed due to the shunting effect of Rd.

15.            Give the expression for I_D for E-MOSFET.

I_D  = (K(V_GS  - V_T)2

16.            What are the different biasing circuits of FET?

•      Fixed bias circuits

•      Self bias circuits

•      Voltage bias circuits