amp circuit

Multi-Stage Transistor amp circuit:

In the field of amp circuit ,one stage in not capable to amplifying weak signal to required strength or level .Therefore multi-stage amp circuit is necessary to amplifying up to desire level signal stage amp circuit is generally insufficient to drive or run at output of electronics equipments .Therefore an extra amplification is necessary .For this other level or stage is needed .In order to accomplished this ,the output of one stage amp circuit stage is coupled to another or next input of amp circuit .The whole construction of this amp circuit is Multi-stage amp circuit.It has wide range of application in this field .For example ,Radio communication system , number of amplification stage may be greater than five stages.

According to coupling devices amp circuit are classified as followings;

• Direct-coupled amp circuit :In the case of direct coupling (i.e d.c coupling the one stage of the amp circuit are directly connected to another stage of amp circuit without d.c isolation.This type of amplifier widely used in the low frequency (i.e less than 10Hz) signal to be amplified .For example ,amplifying thermo-couple current ,photo-electric current etc.There is no coupling devices (such as capacitor ,transformer etc) because at low frequency the electrical size of these elements become large .Therefor output of one stage is directly connected to the input of the next stage of an amp circuits with out coupling devices.

Circuits Details:

There are two stage direct-coupled amp circuit.It consists of two NPN transistor and next so on It is very simple amp circuit .The first transistor output from collector is connected to the input of the next stage of the transistor base .And similar way for the other additional stage .This circuits is stable even temperature is change .

The weak a.c signal is given to the base of the first transistor .The amplified a.c signal appears across the output or collector of the same transistor .This amplified a.c signal is also give to the base of the next transistor of further amplification and amplified a.c signal is appears across the collector load or out put of the same transistor . For other additional stage similar way ,this type of amp circuit amplified the weak signal which is less than 10Hz.

Advantage :

• The construction of this type of amp circuit is very simple.
• It contains minimum number of resistor.
• This amplifier circuits does not contains coupling devices .So it has low cost .

Limitations:

• This amp circuit can not be used at high frequency (i.e greater than 10Hz) for amplification.
• Because of temperature change ,the operating point of amp circuit is shifted.
  
  

1. RC-coupled amp circuit : A capacitor is used as the coupling devices ,in the RC -coupled amp circuit of electronics equipment.Here capacitor connects the output of the first stage of amp circui to the input of the an next stage of amp circuit.By the same way for other required stage.Here the capacitor is used allow to pass the a.c signal where as block the d.c biasing voltages.This type of circuits is most useful circuits.It is simple in construction.and it has low cast .It give well audio fidelity over wide rage of frequency.Capacitor is used as coupling device which connect the output of the first stage of transistor to the next stage input base of next stage of other transistor in amp circuit and soon for other stage.This type of amp circuit is generally employed for voltage amplification.

Circuits details:

Here weak a.c signal is given to the input base of the first transistor .Amplified a.c signal is appears across its collector resistor or load (Rc).This amplified a.c signal is also given to the input base of the next transistor for further amplification with the help of coupling capacitor .This amplified a.c signal is appears across the collector load (Rc).This stage does further amplification of the applied a.c signal.In this same way ,cascaded (i.e one stage after another stage) stage s amplify the a.c signal and as a result overall gain of amp circuits is increase .But here total gain of the amp circuit is less than the gain of the individual stages because output of first stage drives the second stage and soon .Due to the shouting effect of the input resistance of second stage of the amp circuit ,the effective load resistance of first stage of amp circuit is reduced. But in the case of 3-stage amplifier circuits total gain of the amp circuit is equal to the product of the individual stage of amp circuit .

Advantages:

• It contains the resistors and capacitors as coupling devices .So it has lower cost .
• It provided excellent audio fidelity over wide range of frequency.So it has excellent frequency response.
• This amp circuits is easy to make and compact.

Limitations:

• This type of amplifier has low voltages gain and as well as power gain.
• It has noise in the circuits with age.
• It has poor impedance matching.
  

Transformer-coupled amp circuit :A transformer is used as coupling devices in transformer coupled amp circuit .It also allow to pass the a.c signal whereas block the d.c signal .In this circuits additional impedance matching circuits is necessary.The main disadvantage of RC-coupled amplifier is low voltage or power gain .This is over come by this transformer-coupled amplifier circuits.In RC-coupled amp circuit ,effective load resistance (Rc) of each individual stage is decrease because of low resistance presented by input of each individual amplifier stage and previous stage .That means impedance of input of an amplifier is low whereas its output impedance is large.In the case of multi-stage amp circuit ,the large output impedance of one stage of an amplifier circuits become in parallel with the low input impedance of next other stage.Therefore effective load resistance is decreased.Transformer has a properties of impedance change with the frequency ,the low impedance of a one stage of amplifier circuits load can be reflected as high load impedance to the preceding stage of an amplifier circuits.This type of amplifier is generally employed when the load resistance or impedance is small.RC coupled amplifier is used for voltage amplification whereas Transformer-coupled amplifier is used for power amplification.

Circuits details:

This figure shows two amplification stage of transformer coupled amp circuits .Here transformer is used as coupling devices which connect the output of one stage to the input of next amplification stage of amp circuit. In this type of amplifier circuits primary coil of this transformer is made the collector load where as secondary coil is connects as input to the next stage.

In this amp circuit an weak a.c signal is given to the input base of first transistor ,amplified a.c signal is appears across primary coil of the coupling transformer .By transformer action, the voltage is developed across primary coil is transferred to the input of the next of next transistor by the secondary coil of transformer,amplified a.c signal is shown by similar way across the next collector load or resistor.

Advantages:

• In this amplifier circuits ,no a.c signal power is lost in base resistor or collector.
• It provides an excellent impedance matching due to transformer-coupled.
• It provides higher voltage and power gain.

Limitations:

• It is bulky and fairly expansive at audio frequency.
• The gain is change considerably with frequency .So it has a poor frequency response.
• It has high frequency distortion.
• This amp circuit tends to introduce hum in the output stage.