RC-COUPLED AMPLIFIERS USING BJTS FOR HIGHER GAINS AND PRECISE GAINS

EDITOR: B. SOMANATHAN NAIR

I. AMPLIFIERS FOR HIGHER GAINS

1. INTRODUCTION

For very large gains, multistage amplifiers are used. A two-stage amplifier can be constructed by cascading two single stages of common-emitter amplifier. Assuming each stage to have a gain of 125, the overall gain will be 15625. Such large gains, however, will make the amplifier unstable and may lead to oscillations. This is the major difficulty in designing a multistage amplifier.

            To have a stable gain, we must incorporate negative feedback into this system. This, however, will reduce the gain to a much lower value. Normally, voltage-series feedback is employed to get the desired characteristics. With the open-loop gain equal to 15625, we may get a maximum gain of 500 to 1000 without oscillations by using voltage-series feedback. The amplifier so obtained will have the desirable characteristics of high input impedance and low output impedance.

2. MULTISTAGE AMPLIFIER FOR A GAIN OF 500

The design given in an earlier blog for the voltage-series feedback amplifier is used here to design a multistage amplifier that will produce a gain of 500. We use the two-stage amplifier consisting of two standard single-stage amplifiers in cascade, as shown in Fig. 1.

            From Fig. 1, we find that resistors R₃ and R₄ are the feedback resistors that are to be used for the precise adjustment of gain. The design of these resistors are carried out using the expression for the gain with feedback

A_vf = 1+(R₄/R₃)

Substituting the given values, we get

500 =  1+(R₄/R₃)

Therefore                                                      

499 = R₄/R₃

We have seen that, since R₃ forms part of the emitter-biasing network, it should not be of too high a value to upset the bias setting. We therefore choose R₃ as one-tenth of R_F. Thus

R₃ = 1 kΩ/10 = 100 Ω

Using R₃ = 100 Ω, we obtain

R₄ = 499 x100 = 49.9 kΩ

Choose a 47-kΩ resistor in series with a pot of 4.7 kΩ as R₄.

The completely designed amplifier is shown in Fig. 1.

 

3. AMPLIFIER FOR PRECISE GAIN USING PARTIAL FEEDBACK

We have seen that by employing negative feedback, gain can be precisely controlled. The main disadvantage of this scheme is that the gain obtainable is very low. To increase the gain, we have to cascade a number of single-stage amplifiers with feedback. Another simple, yet very convenient, method is to employ a technique called partial feedback.

In the partial feedback technique, first the emitter resistor R_E is replaced with a pot of the same value. Then the bypass capacitor C_E is removed from the emitter and connected to the wiper-arm of the pot, as shown in Fig. 2. Now, by adjusting the wiper-arm, the gain can be controlled to the desk level. In this process, we see that only a certain portion of the emitter resistor is bypassed through the capacitor. 

This will increase the gain. The remaining unbypassed portion of R_E provides the required negative feedback to control the gain. By suitably adjusting the wiper-arm, the reduction in gain due to the negative feedback can be counter-balanced by the increase in gain due to the bypass action of the capacitor. This will provide the desired control over the overall-gain of the amplifier.

In a single-stage amplifier, we can get precise control over the gain up to about 75% of b_min.  Beyond this, generally, transistor characteristic will dominate the procedure and we may not get the required control. In the construction of RC phase-shift oscillators, partial-feedback technique can be used to obtain the required gain of 29 to produce sustained oscillations.