Frequency Response Of BJT Amplifiers

The voltage gain of an amplifier varies with signal frequency due to the effect of variations in circuit capacitive reactance with signal frequency in the output.

The curve drawn between the voltage gain and signal frequency of an amplifier is known as the frequency response.

If the input voltage is kept constant but its frequency is varied, it is observed that the amplifier gain.

  1. Remains practically constant over a sizable range of mid- frequency.
  2. Falls off at low as well as high frequencies due to effects explain below.

The performance of an amplifier depends upon its frequency response. In the design of an amplifier it is ensure that gain is essentially uniform over some specified frequency range.

Atypical graph of amplifier output voltage versus frequency is shown in figure below.

(Frequency response of curve)

It is found that the output normally remains constant over a middle range of frequencies &falls off at low & high frequencies. The gain over this middle range is termed as the mid frequency gain.

The low frequency & high frequency at which gain fall by falls by 3 dB are designed f1 & f2 respectively. This range is normally considered the useful range of operating frequency for the amplifier & the frequency difference f2-f1 is termed the amplifier band width.

The effect of coupling and bypass capacitors is the result of the fall-off in amplifier gain at low frequency.

At medium & high frequencies the capacitive reactance XC, being equal to 1/2πfc is very small & therefore all coupling & bypass capacitors behaves as short circuits. At low frequencies capacitive reactance of capacitors, XC increases & some of the signal voltage is lost across the capacitors as shown in figure below.

(Fig-1: Loss of signal voltage across coupling & Bypass capacitors at low frequency)
(Fig-2: Loss of output via stray capacitance at High frequency)

Thus, with the decrease in frequency the reactance of capacitance increases & the gain of the circuit falls.

All transistors have capacitances bet their terminals. As shown in figure 2, there are also stray capacitance Cs which are the capacitance bet” wiring & ground. All these capacitances are very small &therefore at low & medium frequencies their reactance is very high. With the increase in frequency the reactance of the stray capacitance falls & when this reactance became small enough, they begin to shunt away some of the input & output currents.

Also Read- Frequency response of OP-AMP

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