PRACTICAL DESIGN OF CURRENT-SERIES FEEDBACK AMPLIFIER USING BJT

EDITOR: B. SOMANATHAN NAIR

1. INTRODUCTION

When high input and output impedances as well as finite gain are required we make use of current-series feedback. Figure 1 shows a current-series feedback amplifier (CSFBA) constructed using a BJT.

            It can be seen that a current-series feedback amplifier results in when the emitter bypass capacitor of the common-emitter amplifier is removed. Normally, a single stage of amplification will be sufficient for a current-series feedback amplifier to be used in a laboratory experiment. However, there will be some errors due to the finiteness of the open-loop gain of the amplifier. For a laboratory amplifier, this error can be neglected. The gain of the current-series feedback amplifier shown in Fig. 1 is given by

                                    A_f = ‒R_C/R_E                            (1)

where A_f is the gain with feedback, R_C is the collector resistor, and R_E is the emitter resistor.

Typical value of the gain for this single stage would be around 5 to 10 only, with open-loop gain ranging from 100 to 300. To get higher values of gain, single-stage amplifiers can be connected in cascade.  A method called partial feedback is also employed to get higher values of gain precisely. 

2. SPECIFICATIONS

                       

·         Voltage gain                         :           5 (sharp)

·         Voltage swing                       :           9 V (peak-to-peak)

·         Current swing                       :           1 mA (peak)

·         Input impedance                  :           High

·         Output impedance               :           High

3. DESIGN STEPS

Steps 1 to 6: Design of the Standard Amplifier

Follow the steps given for the design of the Standard Amplifier without feedback for getting the required voltage and current swings as well as gain. The already obtained values are repeated below for convenience (Fig. 2). Thus, R_C = 4.7 kΩ, R_E = 1 kΩ, R₁ = 12 kΩ, R₂ = 68 kΩ, V_CC = 10 V, I_C = 1 mA. The transistor selected is BC107.

Step 7: Modifications in the Design to get the desired Gain exactly

The voltage gain (magnitude) of the current-series feedback amplifier is given by

                                   

                             A_f = R_C/R_E = 4.7 k/1 k = 4.7                               

We want the gain to be exactly 5. To achieve this exactly, we must make slight changes in the values of R_C or R_E. As R_C is fixed according to the output swing requirement, we may better not touch it.  Instead, we may modify R_E. However, large changes in R_E are not desirable as it affects the biasing. Yet , some minor changes in the value of R_E are permissible. R_E, therefore, may be replaced with a fixed resistor of 560 W in series with a pot of 1 kW. The pot can then be adjusted to get the exact value of gain, as required. After R_E is adjusted, we must adjust R₁ and R₂ to fix the Q-point by using a potentiometer. Figure 2 shows the completely designed current-series feedback amplifier.