Multistage Amplifiers
In multistage Amplifiers two or more single stage amplifier are used to achieve more voltage and current amplification.In multistage amplifier output of one stage acts as input for next stage.Multistage Amplifier are also divided in to two types
- Cascaded Amplifier:-In this type same configuration is used in all stage e.g if first stage is in CE(common emitter) configuration than each all next stages should be in CE configuration.
- Compound Amplifier:-In this type configuration may vary from stage to stage.
The overall gain for the cascaded amplifier is equal to the product of the gain of individual stage.
Av=Av1×Av2+Av3..........
Av is total gain and Av1,Av2,Av3 is each stage gain.
Amplifier Coupling
In multistage amplifiers coupling is most important because it will join the one stage to other respectively.In multistage amplifier different types of coupling used which is given as below
Resistance Capacitance coupling:- This coupling is used in RC coupled multistage amplifier.
Impedance Coupling or inductive coupling:- It is also known as choke capacitance coupling.
Transformer coupling:- In this type of multistage amplifier no coupling capacitor is needed because secondary of transformer is already only given AC component.
Direct Coupling:- As the name suggest in this case direct coupling is done without use of any capacitor or resistor.
Multistage amplifiers types
- RC Coupled amplifier
- Impedance coupled
- Transformer coupled
- Direct coupled Amplifier
RC Coupled Amplifier
RC coupled amplifier consist a resistor and capacitor for coupling so thats reason it is named as RC coupled amplifier. In this case CE configuration is used in which resistors R2 and R3 form a coupling network. R2 is acts as output resistor for Q1 and output of Q1 is coupled to Q2 through it.Circuit is given as below
Working :- The input signal Vi is amplified by Q1 and its phase get reversed by 180 degree because we use CE configuration.The amplified output is comes across resistor R2 and this output is coupled to next stage through coupling capacitor to next stage.This coupling capacitor is also called as blocking capacitor because it blocks DC.Q2 is also amplified the signal and net output came across R4 resistor. Phase of the signal also reverse twice.
Advantage of RC coupled amplifier
- Overall amplification higher than other coupling
- Less distortion
- Suitable for audio frequencies
Impedance coupled Amplifier
This type of amplifiers consist inductor and capacitor and resistor. Inductor is placed at output of first stage.The working operation is same as per the RC coupled amplifier. Circuit is given below
to overcome the disadvantage of RC coupled amplifier inductor is used in this circuit which is low down the dc drop across L and result low collector supply voltage is used.This is only advantage of this amplifier.
Disadvantage of Impedance coupled amplifier
- High Cost.
- Low frequency response
- High distortion
Transformer coupled Amplifier
In this case we use transformer for coupling the output of first stage to next stage.Here T1 is coupling transformer and T2 is output transformer C1 is input coupling capacitor and C2,C3,C4 are the bypass capacitors.Circuit is given as below
Working:-Input signal is coupled through C1 to the base of Q1 after amplification it appear at primary of T1 and after due mutual induction it is passed to secondary and secondary transfer that signal to Q2 and it will again amplified the signal.The net output came across R7.
Advantages of Transformer coupled Amplifier
- Higher voltage gain
- Impedance matching
- More efficient
Disadvantages
- High cost
- Large size
- Poor frequency response
- Noise
Direct Coupled Amplifier
These type of amplifier operate without the use of resistor,capacitor,inductor. They are specially used for amplification of low frequencies.In this circuit is consist two transistor Q1 and Q2 collector of Q1 is directly connected to Q2 input.Total output is came across resistor R4.
working operation is simple input Vi is fed to Q1 base then amplified signal is directly fed to input of Q2 and same in case of Q2.
Advantages of Direct Coupled Amplifier
- Low cost
- No coupling is required
- Simple Circuit
Disadvantages
- Not amplify high frequency
- Poor temperature stability due to proper biasing is not used
so friends hope you will understand multistage amplifier if you have any question please comment.
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