Microwave: Properties, Advantages, and Disadvantages

What is Microwave?

Electromagnetic waves having frequencies between 1GHz to 300GHz (it may be 300MHz to 300GHz) are called microwaves.

Properties of Microwaves:

Microwave are unidirectional. This means that when an antenna transmits microwaves, they travel in one direction. That is why microwave propagation is line of sight (LOS) propagation. They are not reflected by ionosphere. High frequency microwaves can not penetrate an object like wall.

Advantages and disadvantages of microwaves: 

Due to above properties,  microwaves have following advantages and disadvantages:

• Due to line of sight propagation, a pair of antennas can be  aligned without interfering with another pair of aligned antennas.
• Due to wide frequency bandwidth, high data transfer rate is possible.
• Due to line of sight propagation, it can be transmitted long distance. But a problem is faced due to its penetration properties. It can not be transfer inside a bound area. Also its transformation has obstacles like hills trees, mountains, earth curvature etc. That is why we need repeaters or tall towers for long distance transmission.
• As they are not reflected by ionosphere, they can be used for space transmission.

Read more ...

Properties of Negative Feedback: An Explanation

What is Negative Feedback?

When the feedback energy is out of phase with the input signal, it is called negative feedback. Negative feedback reduces the gain of the amplifier. That is why, it is sometimes called degenerative or inverse feedback. However, the advantages of negative feedback are:

• Higher input impedance
• Lower output impedance
• Better stabilized gain
• Reduction of noise
• Improved frequency response

Properties negative voltage series feedback: 

Considering the following feedback circuit, we will explain the mentioned properties of a negative feedback circuit.

Fig: Negative Feedback

Decreased Gain & increased Stability:

The gain of the amplifier without feedback is A_v . Negative feedback is then applied by feeding a fraction m_v of the output voltage e_o back to amplifier circuit.

Therefore, the actual input to amplifier = e_g -m_ve_o

The output e_o must be equal to the input voltage multiplied by gain A_v of the amplifier.

                           ( e_g – m_ve_o ) A_v = e_o

                 Or,   e_g A_v  - m_vA_ve_o = e_o

                Or,   e_o ( 1 + m_vA_v ) = e_gA_v

                Or,   e_{o /} e_g  = A_v /( 1 + m_vA_v )

But   e_o |e_g   is the gain of the amplifier with feedback. Therefore-

                           A_vf = A_v /( 1 + m_vA_v )

So due to negative voltage feedback, the gain is reduced by a factor  ( 1 + m_vA_v )

For effective design  m_vA_v >> 1   

 then  A_vf = A_v / m_vA_v  =1/m_v

As the gain of the negative feedback depends on only feedback ratio or feedback circuit which is actually a resistive network that is why the negative feedback gain is unaffected from the variations of transistor parameters, temperature, frequency. Therefore it will be extremely stable.

Increased in Input Impedance: 

Consider the above circuit parameters. We also assume that:

                       Z_in  = input impedance without feedback

                       Z_in^/ = input impedance with feedback

                       i₁  = input current

So from above fig we get,   e_g – m_ve_o = i₁Z_in     --- (1)

Now,         e_g = e_g – m_ve_o + m_ve_o

                        = (e_g – m_ve_o) + (e_g – m_ve_o)m_vA_v         
                                                [ as ( e_g – m_ve_o ) A_v = e_o ]

                    = (e_g – m_ve_o) ( 1 + m_vA_v )

                    = i₁Z_in ( 1 + m_vA_v )       [ From 1]

             Or, e_{g /} i₁ =  Z_in ( 1 + m_vA_v )      

But   e_g|i₁  is the input impedance with feedback.

             So,    Z_in^/ = Z_in ( 1 + m_vA_v )      

Therefore the input impedance of the negative feedback is increased by a factor ( 1 + m_vA_v ). This is an advantage, the amplifier will now present less of a load to its source circuit.

Reduction in Output Impedance: 

Apply an voltage  e  at the output circuit which causes to flow current I and short the input voltage e_g .

Now consider,     Z_o  = output impedance without feedback

                           Z_o^/ = output impedance with feedback

We get,       e = Z_oI + A_vV_i     

       where V_i = input voltage across the amplifier

But,           V_i = e_g - e_f                      

       where  e_f = voltage across the feedback

                     =  - e_f

                     =  - m_ve                 Here  e = e_o

Putting this into above relation we get,

                  e = Z_oI - A_vm_ve  

            or, e( 1 + m_vA_v ) = Z_oI 

            or, e/I  =  Z_{o  /}( 1 + m_vA_v )     
      

Therefore output impedance with feedback, 
             Z_o^/ = e/I  =  Z_{o /}( 1 + m_vA_v )   

So the output impedance is reduced by a factor   ( 1 + m_vA_v ) .

Improved Frequency Response:

The gain of the negative feedback depends on only feedback ratio or feedback circuit which is actually a resistive network. That is why the gain is independent of signal frequency. The result is that voltage gain of the amplifier will be constant over a wide range of signal frequency. The negative voltage feedback therefore improves the frequency response of the amplifier.

Increase in the Bandwidth: 

When the gain decreases by a factor  ( 1 + m_vA_v ) by providing negative feedback. It is seen that the lower cut off frequency is also lowered by this factor  ( 1 + m_vA_v ) and upper cut off frequency is raised by the same factor. As a result the difference between the frequencies means bandwidth is increased.

Fig: Bandwidth Increased by Negative Feedback

 

Decreased distortion: 

Let the harmonic distortion voltage generated within in the amplifier change from D to D^/ , when negative feedback is applied to the amplifier.

Suppose     D^/ = xD

The fraction of the output distortion voltage which is feedback to the input is:   m_vD^/ = m_vxD

After amplification, it becomes m_vxDA_v and is antiphase(due to negative feedback) with orginal distortion voltage D.

Hence the new distortion voltage D/ which appears in the output is:

               D^/ = D – m_vxDA_v

       Or,  xD = D – m_vxDA_v

      Or,  x = 1 -  xm_vA_v

      Or, x ( 1 + m_vA_v ) = 1

      Or, x = 1/  ( 1 + m_vA_v )

      Or, xD = D/( 1 + m_vA_v )

     Or, D^/ = D/( 1 + m_vA_v )

Therefore, the negative feedback reduces the distortion.

Reduce Noise Effect:

Negative feedback can help reduce the impact of noise on electronic circuits by reducing the gain of the amplifier and increasing the signal-to-noise ratio. This makes negative feedback a useful tool in designing circuits that are stable, reliable, and perform optimally in the presence of noise.

Read more ...

Understanding the Different Types of Feedback in Electronic Circuits

Feedback: 

The process of injecting a fraction of output energy of some device back to the input is known as feedback. The gain with feedback is called closed loop gain. The gain without feedback is sometimes called open loop gain.

Feedback is a crucial concept in the world of electronics, and understanding the different types and classifications of feedback can be instrumental in designing high-performance circuits. In this blog post, we will explore the different types of feedback and their applications in electronic circuits.

Types of Feedback:

Feedback can be classified based on the nature of the feedback, electrical quantities of feedback, and circuit arrangement.

 

Based on the nature of the feedback, feedback can be classified into two types:

• Positive feedback: When the feedback energy is in phase with the input signal, it is called positive feedback. Positive feedback increases the gain of the amplifier. That is why, sometimes it is also called ‘regenerative/direct’ feedback. However it has the disadvantages of increased distortion and instability. Therefore, positive feedback is seldom employed in amplifiers. One important use of positive feedback is in oscillators. If positive feedback is sufficiently large, it leads to oscillation.

• Negative feedback: When the feedback energy is out of phase with the input signal, it is called negative feedback. Negative feedback reduces the gain of the amplifier. That is why, it is sometimes called degenerative or inverse feedback. However, the advantages of negative feedback are:

1.Higher input impedance

2. Lower output impedance

3. Better stabilized gain

4. Reduction of noise

5. Improved frequency response

6. More linear operation

 

Based on electrical quantities of feedback, it can be classified into two types: 

• Voltage feedback: When the feedback is proportional to the output voltage, it is known as voltage feedback.
• Current feedback: When the feedback is proportional to the output current, it is known as current feedback. 

These types of feedback are commonly used in operational amplifiers and other electronic circuits to control gain and stability.

Based on the circuit arrangement, feedback can be classified into four types:

• Voltage series feedback: Voltage series feedback occurs when the feedback signal is applied in series with the input signal.
• Current series feedback: Current series feedback occurs when the feedback signal is applied in series with the output current.
• Voltage shunt feedback: Voltage shunt feedback occurs when the feedback signal is applied in parallel with the input signal.
• Current shunt feedback: Current shunt feedback occurs when the feedback signal is applied in parallel with the output current.

Each type of feedback has its advantages and disadvantages, and the selection of the appropriate feedback type depends on the specific requirements of the circuit.

Fig: (i)Voltage series Feedback (ii) Current series Feedback (iii) Voltage shunt Feedback (iv) Current shunt Feedback

Feedback is a crucial concept in the world of electronics, and understanding the different types and classifications of feedback can be instrumental in designing high-performance circuits. By selecting the appropriate type of feedback for a specific application, designers and engineers can create circuits that are stable, reliable, and perform optimally.

Read more ...