Lecture - 25: Frequency Response of Amplifier

Frequency curve of an RC coupled amplifier:

A practical amplifier circuit is meant to raise the voltage level of the input signal. This signal may be obtained from anywhere e.g. radio or TV receiver circuit. Such a signal is not of a single frequency. But it consists of a band of frequencies, e.g. from 20 Hz to 20 KHz. If the loudspeakers are to reproduce the sound faithfully, the amplifier used must amplify all the frequency components of signal by same amount. If it does not do so, the output of the loudspeaker will not be the exact replica of the original sound. When this happen then it means distortion has been introduced by the amplifier. Consider an RC coupled amplifier circuit shown in fig. 1.

 

Fig. 1
Fig. 2

Fig. 2, shows frequency response curve of a RC coupled amplifier. The curve is usually plotted on a semilog graph paper with frequency range on logarithmic scale so that large frequency range can be accommodated. The gain is constant for a limited band of frequencies. This range is called mid-frequency band and gain is called mid band gain. AVM. On both sides of the mid frequency range, the gain decreases. For very low and very high frequencies the gain is almost zero.

In mid band frequency range, the coupling capacitors and bypass capacitors are as good as short circuits. But when the frequency is low. These capacitors can no longer be replaced by the short circuit approximation.

First consider coupling capacitor. The ac equivalent is shown in fig. 3, assuming capacitors are offering some impedance. In mid-frequency band, the capacitors are ac shorted so the input voltage appears directly across br'e but at low frequency the XC is significant and some voltage drops across XC. The input vin at the base decreases. Thus decreasing output voltage. The lower the frequency the more will be XC and lesser will be the output voltage.

Fig. 3

Similarly at low frequency, output capacitor reactance also increases. The voltage across RL also reduces because some voltage drop takes place across XC. Thus output voltage reduces.

The XC reactance not only reduces the gain but also change the phase between input and output. It would not be exactly 180o but decided by the reactance. At zero frequency, the capacitors are open circuited therefore output voltage reduces to zero.

The other component due to which gain decreases at low frequencies is the bypass capacitor. The function of this capacitor is to bypass ac and blocks dc The impedence of this capacitor in mid frequency band is very low as compared to RE so it behaves like ac short but as the frequency decrease the XCE becomes more and no longer behaves like ac short. Now the emitter is not ac grounded. The ac emitter current i.e. divides into two parts i1 and i2, as shown in fig. 4. A current i1 passes through RE and rest of the current passes through C. Due to ac current i1 in RE, an ac voltage is developed i1 * RE. With the polarity marked at an instant. Thus the effective VL voltage is given by

Vbe = Vs RE.

Fig. 4

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