TYPES OF’ NEGATIVE FEEDBACK CONNECTIONS:

There are four different combinations in which negative feedback may be accomplished, as given below:

•  Voltage-series feedback

•  Voltage-shunt feedback

•  Current-series feedback Current-shunt feedback

The effect of negative feedback on the input and output resistances differ with the methods of sampling and mixing. The above mentioned four classification are done on that basis only.

I is observed that:

•     Negative feedback using output voltage sampling regardless mixing, tends to decrease the output resistance.

•     Negative feedback using output current sampling regardless mixing tends to increase the output resistance.

•     Negative feedback using series mixing, regardless of sampling, tends to increase the input resistance.

•     Negative f using shunt mixing, regardless of the method of sampling, tends to decrease the input resistance.

VOLTAGE SERIES FEEDBACK:

A block diagram of a voltage series feedback is shown below:

•     The effect of negative feedback on the input and output resistances differ

with the methods of sampling and mixing.

•     The above mentioned four classification are done on that basis only.

 VOLTAGE SERIES FEEDBACK:

A block diagram of a voltage series feedback is shown below:
I is observed that:

•     Negative feedback using output voltage sampling regardless mixing, tends to decrease the output resistance.

•     Negative feedback using output current sampling regardless mixing tends to increase the output resistance.

•     Negative fcedback using series mixing, regardless of sampling, tends to increase the input resistance.

•     Negative f using shunt mixing, regardless of the method of sampling, tends to decrease the input resistance.

•     Here, the input to the feedback network is in parallel with the output of the amplifier.

•     A fraction of the output voltage through the feedback network is applied in series with the input voltage of the amplifier.

•     The shunt connection at the output reduces the output resistance R the series connection at the input increases the input resistance.

•     In this case, the amplifier is a true voltage amplifier The voltage feedback factor is given by fi = V / V Output resistances.

    From the above diagram, we have,

Example of voltage series feedback

    The common collector or emitter follower is an example of voltage series feedback since the voltage developed in the output is in series with the input voltage as far as the base emitter junction is concerned.

VOLTAGE SHUNT FEEDBACK:

It is called shunt-derived, shunt-fed feedback connection. Here, a fraction of
the output voltage is supplied in parallel with the input voltage through the feedback network. The feedback signal I is proportional to the output voltage V therefore, the feedback factor is given by, fi = I / V this type of amplifier is called a trans resistance amplifier.
A voltage shunt feedback circuit is shown below                          
                                                                                                                     

Example of voltage shunt feedback:

    The collector feedback biased common emitter amplifier is an example of voltage-shunt feedback circuit.

CURRENT-SERIES FEEDBACK:

•     In current series feedback, a voltage is developed which is proportional to the

     output current.

•     This is called curre feedback even though it is a voltage that subtracts from the input voltage.

•     Because of the series connection at the input and

•     One of the most common methods of applying the current-series feedback is to place a resistor Re between the emitter lead of a common emitter amplifier and ground.
•     As the common emitter amplifier has a high gain, this is most often used with series negative feedback do that it can afford to lose some gain.
•      Such a circuit is shown below.
•     When R is properly bypassed with a large capacitor Ce, the output voltage is V and the voltage gain without feedback is A. resistor R provides d.c. bias stabilization, but no a.c. feedbaêk.
•     When the capacitor Ce is removed, an ac. voltage will be developed across R due to the emitter current flowing through R and this current is approximately equal to the output collector current.













        





•     This voltage drop across R will serve to decrease the input voltage between b
and emitter, so that the output voltage will decrease to V the gain of the amplifier with negative feedback is now A Simplified diagram:








































•     Therefore, we find that there is a large decrease in voltage gain due to negative
     feedback. output resistance (R)

•     The expression for the output resistance R looking back into the collector
     involves R and all the h-parameters.

•     For values of Re in the order of R and h an approximate expression for R is
     R 1+hfe/hoe = 1/h

•     This has usually a large value in the range of Mg, so the overall output resistance R taking load resistance into consideration.
    Examples of current-series feedback:

•     The CE amplifier with Re in the emitter lead and FET CS amplifier stage
     with source resistor R are the best example for current series feedback circuit   
     input.

• It is called a series-derived, shunt feedback

•     The shunt connection at the input reduce F the input resistance and the series connection at the output increases the output resistance. This is a true current amplifier.
•     The current feedback factor is given by fl L/L
•     Input and output resistances:
•     The below circuit diagram shows the: current-shunt feedback circuit used to calculate input and output resistances I


















































•     Thus this type of feedback decreases the input resistance and increases
     output resistance

•     As this type of feedback has the least desirable effects, this connection
    not be considered at all for practical applications