PRACTICAL DESIGN OF VOLTAGE-SERIES FEEDBACK AMPLIFIER USING BJTs

EDITOR: B. SOMANATHAN NAIR

1. INTRODUCTION

In our previous blog, we had obtained the expressions of gain, input impedance, and output impedance of voltage-series feedback amplifiers. In this article, we give the step-by-step practical design of VSFB amplifier using BJTs. Figure 1 shows a two-stage voltage amplifier constructed using BJTs in which we have introduced voltage-series feedback. Voltage-series feedback is used when high input impedance and low output impedance are the additional requirements. Usually, two-stage amplifiers are used in voltage-series feedback circuits. Assuming each stage to have an approximate gain of 100, the overall gain of the cascaded amplifier can be seen to be more than 10,000.

Consider the expression

                                                            A_vf = A_v/(1+A­_vB)        (1)                                                                                                                                 

where A_vf is the gain with feedback, A_v is the open-loop gain, and B is the feedback factor. With A_v = ∞,

                                                                  A_vf = 1/B               (2)                                                                                    

Equation (2) shows that the gain with feedback is dependent only on the B network. As shown in Fig. 1, the output voltage V_o is developed across the voltage-divider network consisting of resistors R₃ and R₄. A portion of V_o developed across R₃ (= BV_o) is fed back in series with the input voltage V_i as shown to make the circuit a voltage-series feedback amplifier.

2. SPECIFICATIONS

·         Gain Required                      :           25 (strictly)

·         Output Swing                       :           9 V (peak-to-peak)

·         Input Impedance                  :           Very high

·         Output impedance               :           Very low

3. DESIGN PROCEDURE

Steps 1 to 6: Design of the Standard Amplifier without Feedback

In voltage-series feedback, an amplifier with very high open-loop gain is required for the feedback to be effective. Usually, a two-stage amplifier with identical sections is employed in such a situation. The design of each stage of the amplifier is done according to our Standard-Amplifier design procedure given in an earlier blog. Therefore, for this amplifier we shall use the same values of V_CC, I_C­, R_C, etc., which are obtained in the case of the single-stage Standard Amplifier. For simplicity, both the stages are assumed to have identical values because, with negative feedback, loading effect of the second stage on the first stage is negligible. Therefore, cascading of two identical stages is justified in this case. The two-stage completely designed amplifier without feedback is shown in Fig. 2.

Step 7: Design of the Feedback Network Consisting of R₃ and R₄

With voltage-series feedback, the gain A_vf is given by

                                                   A_vf = 1/B      = (R₃+ R₄)/ R₃    (3)  

In our problem, we want the gain to be exactly equal to 25. As the gain is dependent on pure resistors only, we can get the exact gain, as desired. Substituting this in Eq. (3)

                                                            25 = (R₃+ R₄)/ R₃       (4)                                                      

Solving  Eq. (4) yields

                                                                    R₄ = 24 R₃                (5)

This is an equation in two unknowns. So, we require one more equation to solve for R₄ and R₃. Referring to Fig. 1, we find that R₃ is a part of the emitter resistor of the first stage, and hence forms part of its biasing network. Any large deviation in the value of R_E will produce a total displacement of the Q-point. Therefore, R₃ must be chosen so that biasing is not affected. A simple solution to this is to choose

                                                                    R₃ =  R_E/10                (6)

                                                           
It can be seen that the drop across R₃ would be only one-tenth of that across R_E. This is negligible and hence will not cause any change in the Q point. Here, since R_E = 1 kW, we choose

                                                             R₃ = 1 kW/10 = 100 W

  

Solving for R₄ now yields

                                                            R₄ = 24 R₃ = 2.4 kW

            To get 2.4 kW exactly, we add a 2.2 kW pot in series with 1.8 kW fixed resistor, as shown in Fig. 3, so that the gain can be adjusted correctly to the required value.

4. CONCLUSION

It may be noted that the exact values of input and output impedances are difficult to calculate. In our previous blog, we had obtained the expressions of input and output impedances as Z_if = Z_i (1 + BA_v) and Z_of = r_o/(1+ a_vB), respectively. from these expressions, we find that to compute the exact values  Z_if and Z_of requires the computation of parameters like A_v ,Z_i and r_o. However, our only requirement is that for a voltage amplifier, the input impedance must be high and the output impedance must be low. This will be ensured by voltage-series feedback connection. Therefore, we stop our computation any further.