Audio power amplifiers, Single stage transistor amplifier & Multistage amplifier

Audio power amplifiers

Power amplifiers handles large signals (not very large). Size of power transistor is large, to dissipate heat. Tuned amplifier is used in RF application. Class A power amplifier uses one transistor, this transistor never turn off which is its main disadvantage.

Driver stage commonly uses class A. Class A amplifier is most commonly used, because of its simplicity. Class A has 360 conduction angle. Class A is also called single ended amplifier. Class A uses driver stage. Class B is introduced because of the heating problems of class A amplifier. The maximum efficiency of resistance loaded class A power amplifier is 25%. The maximum efficiency of transformer coupled class A power amplifier is 50%. When no signal is applied, the approximate collector efficiency of class A power amplifier is 0 %. In class A operation, the operating point is generally located at the midpoint of the d.c. load line

Class B has 180 conduction angle. Class B amplifier have higher efficiency than class A, but linearity is lesser than class A. Zero crossing or cross over distortion is a problem is class B. The maximum collector efficiency of class B operation is 78.5%. The basic class B amplifier uses two complimentary transistors (bipolar or FET) for each half of the waveform, they are arranged in push-pull form. No DC base bias current in class B power amplifier

Class AB is commonly used audio frequency power amplifier. Class AB conduction angle is slightly greater than 180. Class AB efficiency is between 25% and 78.5%

Class C power amplifier has the highest collector efficiency (80%) and worst linearity. Class C introduces a very heavy distortion of the output signal, so not used as AF amplifier. Conduction angle of class C is less than 180 degree. Class C amplifiers are used as RF amplifier. Output of class C amplifier can be made at a particular frequency by using a resonant LC circuit in collector circuit

Power amplifiers have comparatively less current gain. Power amplifiers handle large signals compare to voltage amplifiers. The size of a power transistor is made considerably large to Dissipate heat. Low efficiency of a power amplifier results in more battery consumption. Power amplifiers generally use transformer coupling because transformer permits impedance matching. The disadvantage of impedance matching is that it Gives distorted output

The output transformer used in a power amplifier is a step down transformer. The most important consideration in power amplifier is collector efficiency. The pulsating d.c. applied to power amplifier causes hum in the circuit. The most costly coupling is transformer coupling

Single stage transistor amplifier

In single stage amplifier load should be very high to derive maximum output. Current in any branch of transistor amplifier is sum of ac and dc. Complementary symmetry amplifier has one PNP and one NPN transistor. Frequency response of combined amplifier can be compared by AND gate. Buffer amplifier is used for minimum loading and minimum mismatch. Voltage gain of common emitter amplifier increases with increase in collector resistance. To get frequency response curve of an amplifier generator output is kept constant. An electrolyte capacitor of 10 micro F is used to couple input signal into the base of the transistor

A bypass capacitor of 100 micro F is used to provide low impedance path to the amplified ac signal. Output of the amplifier depends on frequency of input signal. Gain of amplifier falls at higher frequency because of internal capacitance of the device (miller capacitance). 180 degree phase shift provided in the midband region. At low frequency greater than 180 degree phase shift and high frequency have less than 180 degree phase shift. Low frequency response of RC coupled amplifier depends upon coupling capacitor and emitter by pass capacitor

In RC coupled amplifier, if emitter bypass capacitor is removed then midband gain decreases. When gain increases bandwidth decreases. Gain-bandwidth product of an amplifier is figure of merit. AF transformer is shielded to prevent induction due to stray magnetic field. Amplitude distortion is also called harmonic distortion. Transformer coupling causes frequency distortion. Impedance matching gives distorted output. Ear is not sensitive to frequency distortion. Time taken by charge carrier to flow from emitter to collector is transit time

For AC analysis capacitors are replaced with shorted wire and opened in DC analysis. Frequency response varies because reactance of capacitors varies with input signal. Bandwidth is the range of frequencies with voltage gain greater than 70.7 %. RC coupled amplifier is cheap and have high audio fidelity for wide range. RC coupled have low power and voltage gain and becomes noisy with ages (moist climate). Impedance matching of RC coupled is poor because output impedance is higher than input impedance

Radio receiver should have more than three stages. Transformer coupled used for power amplification. RC coupled used for voltage amplification. Coupling capacitor should be large to reduce attenuation at low frequency. Gain of the amplifier is expressed in db, because human ear response in logarithamic. RC coupling is low power amplification because of low efficiency. RC coupling cannot be used in amplification of very low frequency because size of coupling capacitor becomes very high

Multistage amplifier

If stages have only R and C component only then it is class A amplifier. Gain of multistage amplifier is product of individual stages (because of loading effect this total gain is actually less than the individual product), Because of low input impedance of common base, it is not used in cascaded AF amplifier. CE – CB configuration is called CASCODE amplifier

Impedance matching is required in multistage amplifier. Direct coupling of stages can amplify both AC and DC signals. Direct coupling is suitable for low frequency amplification. Direct coupling is suitable for wideband amplification. Direct coupling has good stability

Transformer coupling is suitable in RF amplification. It is used when load resistance is low. Transformer coupling can be used in both power and voltage amplifier. Transformer coupled have maximum voltage gain. Transformer coupling is used when load resistance is small,. Final stage of multistage amplifier uses transformer coupling. Initial stage of amplifier uses RC coupled. Final stage of multistage amplifier is power stage. Step down transformer is used for impedance matching

Frequency response of transformer coupled is poor. Feedback circuit is independent of frequency. Number of stages is limited, because of temperature instability. Bandwidth of single stage amplifier is greater than multistage. In multistage emitter bypass capacitor is 50 micro Farad