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Amplifier is a circuit which increases the strength of weak input signal without any change in its shape.

Faithful amplification is the process of increasing the strength of the signal without any change in its shape.

CB amplifier is best suited for low input impedance and high output impedance.

(i) CC amplifier is used as buffer amplifier to deliver maximum power to the load.

(ii) It is used as current amplifier

Since reactance (X_L and X_c) depend on frequency, RC and transformer coupling is avoided for very high and low frequencies.

The coupling capacitors block DC and allow AC signal.

(i) Loudness heard by human ear is according to the logarithm of the intensity of sound.

(ii) In multistage amplifiers, total gain in decibel is equal to the sum of the decibel gains of individual stages.

Cascading of amplifiers is required to get high overall gain.

(i) The operating point should be at the centre of load line.

(ii) Collector current should be kept constant against temperature variations.

(iii) The operating point should not change when transistor is replaced by another of same type.

Efficient, \(n = \frac{P_{out}(AC)}{P_{in}(DC)}\)

The transistor does not conduct until the input signal voltage exceeds 0.7V for silicon and 3V for Germanium. Due to this, there is time interval between positive and negative cycles of input signal. The distortion when neither of transistors is conducting in push pull amplifier is called cross over distortion.

Power amplifiers are used to increase the power level of the input signal. Voltage amplifiers are the amplifiers that amplify small input signals whose amplitudes are very much smaller than DC operating point value.

Power amplifiers drive the loud speakers in audio systems, radio and TV receivers.

CC amplifier is used to drive the loud speaker system of a record player.

Class A is more suitable for faithful amplification. Class C power amplifier produces maximum. distortion. In Class AB amplifier, Q point is located in between X-axis and the centre of load line.

(i) Class A power amplifier: The Q point is located exactly at the centre of the load line. The current flows for full cycle of input signal.

(ii) Class B power amplifier: The Q point is located in between X-axis and the centre of load line. The output current flows for more than half cycle and less than full cycle of the input signal.

(iii) Class C power amplifier: The Q point is located beyond cut off region below the X-axis. The output current flows for less than half cycle of input signal.

The capacitive reactance of coupling and by pass capacitors is large at low frequencies. As this reactance with is in series with source resistance, the input signal is attenuated and the gain is less.

At high frequencies, the reactance offered by junction capacitance of transistor and wiring capacitance is less. As this is in paralleled with output resistance, gain decreases with increase in frequency.

CC amplifier = Emitter follower, Buffer amplifier.
CB amplifier = Radio frequency amplifier.
CE amplifier = Audio frequency amplifier.

(i) Class A

(ii) Class B

(iii) Class AB

(iv) Class C

Power amplifiers are used to boost the power level of the input signal.

If the gain falls to 70.7% of maximum gain, the ear can not detect the change in the intensity of sound. If the gain falls below 70.7%, the ear will hear distortion. Hence the range of frequencies above 70.7% of the maximum gain is called as bandwidth.

Output voltage V_CE = V_CC - I_CR_C. During positive half cycle forward bias increases. Hence I_C increases and as I_CC is constant, V_CE decreases.

The variation of the gain of an amplifier with the frequency of the input signal is called frequency response of the amplifier.

The zero signal collector current and collector to emitter voltage is called operating point.

(i) Replace DC sources by short circuit.

(ii) Replace capacitors by short circuit.

(i) Distortion is minimum.

(ii) Overall gain is high.

(iii) It has good frequency response.

(iv) It costs less.

(i) Reduce all the AC sources to zero.

(ii) open all the capacitors.

The phase difference between input and output in CE amplifier is 180°.

The collector current is more than base current and collector base junction has more resistance as it is reverse biased. Hence collector base junction gets heated more.

Buffer amplifier is an amplifier with unit gain which is used to deliver the maximum power to the load.

(i) High frequency signals cannot be amplified.

(ii) It has poor temperature stability.

The disproportionate amplification of frequencies in complete signals the reason for poor frequency response.

Cascaded amplifier is number of amplifiers connected in series. Cascading of amplifiers is required to get high gain.

Magnetostriction (extension and contraction of magnetic sheet steel when magnetised) is the reason for noise in transformer-coupled amplifier.

The phase difference between input and output in CC amplifier is 0°.

CC amplifier is called emitter follower because the emitter voltage follows the base voltage.

Base current decreases.

The phase difference between input and output in CB amplifier is 0°.

(i)Voltage gain is nearly equal to 1.

(ii) Input and output signals are in phase.

Class C power amplifier has maximum efficiency.

Coupling capacitors are used to block dc and allow only ac to pass through. The emitter bypass capacitor bypasses the alternating current at the emitter to the ground by providing easy alternate path.

A₁ = 100, A₂ = 200, A₃ = 400.
A_Total A₁ x A₂ x A₃
= 100 x 200 x 400 = 8 x 10⁶
Gain in dB = 20 logA
= 20 log 8 x 10⁶
= 20 x (6.9031)
= 138.02dB.

The emitter by pass capacitor bypasses the alternating current at the emitter to the ground by providing easy alternate path.