NEGATIVE FEEDBACK AMPLIFIERS-BASIC PRINCIPLES

EDITOR: B. SOMANATHAN NAIR

1. INTRODUCTION

Figure 1 shows an amplifier whose output is connected back to its input through a network called B at a point called the summing point such that the fedback voltage V_f gets subtracted from the input voltage V_s. This amplifier configuration is called as negative feedback amplifier (NFA). We shall prove that, in general, negative feedback stabilizes the performance of an amplifier.

2. GENERAL ANALYSIS OF NFA

Consider Fig. 1, which shows the fed-back voltage at the summing point being negative. We can, with respect to the summing point, write the following relation:

                                         V_i = V_s ‒V_f                      (1)

where V_s is the source (externally applied input) voltage to the amplifier, V_i is the  input voltage (or error voltage) to be amplified, and V_f is the voltage fed-back from the output to the input.

From Fig. 1, we find that

                                             V_f = BV_o                      (2)

                                                           

where B = feedback factor or gain of feedback (B) network, and V_o is the amplifier output voltage. Combining Eqs. (1), and (2),

                                                                                                                                    

                                        V_i = V_s ‒ BV_o                      (3)

Now, all amplifiers amplify signals that are applied across its input terminals. Therefore, we find that

                                             V_o =AV_i                         (4)

                                                             

From Eqs. (3), and (4), we have

                                          V_o = A(V_s ‒BV_o)                 (5)

                                                          

Rearranging Eq. (5), we get the voltage gain of the amplifier with negative feedback as

                                      A_f = A/(1+AB)                     (6)                                                

In Eq. (6), let us assume that loop-gain

                                                              AB >> 1                                   (7)                                                                     

Then, from Eq. (7), we find that

A_f = A/AB = 1/B                         (8)

                                  

From Eq. (8), we find that the gain has become totally independent of transistor parameters and is now only dependent on feedback factor B. This means that, if the B network is made entirely up of fixed resistors, we can fix the gain at any desired value (of course, within reasonable limits!), as well as stabilize it against temperature variations. Thus we find that negative feedback introduces many desirable properties in an amplifier, two of which have been mentioned above.

3. CLASSIFICATION OF NEGATIVE FEEDBACK                 

Negative feedback is classified into four categories:

·         Voltage-series feedback

·         Current-series feedback

·         Current-shunt feedback

·         Voltage-shunt feedback

            Figures 2 to 5 show the basic structures defining these feedback configurations. As can be seen from the Fig. 2, in voltage-series feedback, we find that a portion of the output voltage (BV_o) is fed back in series with the input voltage V_s.

            In the case of current-series feedback, shown in Fig. 3, a portion of output current BI_o is fed back in series with the input voltage V_s.

In the current-shunt feedback, which is shown in Fig. 4, a portion of output current BI_o is fed back in shunt with the input voltage V_s.

        Finally, in the voltage-shunt feedback, shown in Fig. 5, a portion of output voltage is fed back in shunt with the input.

In the forthcoming blocks, analysis and design of all these four categories of NFA will be done.