BJT amplifier

What is a common Base amplifier?

Common base amplifier is on in which the base terminal is common to both input and output circuit. Following are the some of the characteristic parameters of CB amplifier neglecting the output resistance of amplifier (which accounts for early effect) are

Rin = re

Av = gm*(Rc//Rl)

Rout = Rc

Gv = α*(Rc//Rl)/(Rsig+re)

Ais = α.

What is a Common emitter amplifier?

Common emitter amplifier is on in which the emitter terminal is common to both input and output circuit. Following are the some of the characteristic amplifiers of CE amplifier with emitter resistance neglecting the output resistance of amplifier (which accounts for early effect) are

Rin = Rb//(β+1)*(re+Re)

Av = -gm*(Rc//Rl)/(1+gm*Re)

Rout = Rc

Gv = -β*(Rc//Rl) / (Rsig+(β+1)*(re+Re))

Ais = -α*( Rb//(β+1)*(re+Re))/(re+Re).

What is a Common collector amplifier?

Common collector amplifier is on in which the collector terminal is common to both input and output circuit. Following are the some of the characteristic amplifiers of CC amplifier are

Rin = Rb//(β+1)*(re+(ro//Rl))

Av = (ro//Rl)/( ro//Rl+re)

Rout = ro//(re+(Rsig//Rb/(β+1)))

Gv = (Rb/(Rb+Rsig))*( ro//Rl/(Rsig//Rb/(β+1))+re+(ro//Rl)))

Ais = β+1.

What are the applications of transistor amplifiers?

a) The CE amplifiers are used in applications where high gain is required. It can be used as single stage or in cascade.

a) CB amplifiers have better frequency response than CE amplifier. It is used as high frequency amplifier combined with common emitter amplifier. Its low input resistance finds application in specific areas where the signal strengths are very low (or) when load on source should be very low.

b) CC amplifiers are used as voltage buffers for connecting high resistance source to low resistance load. It is used as output stage in multistage amplifier because of its low output resistance.

What is Darlington pair?

Darlington pair consists of two cascaded emitter followers used to achieve high input resistance. Darlington pair is a better buffer compared to single stage emitter follower circuit. The typical Darlington pair is shown below

Darlington pair amplifier

Darling-ton pair amplifier

Following are some of the characteristic parameters of Darlington pair

Rin = (1+β)^2 *Re/(1+β*Re/ro)

Ai = (1+β)^2/(1+ β*Re/ro)

Av = 1-β*(2+β*Re/ro)/((1+β)*Re)

Ro = Rsig/(1+β)^2+(2*β)/(1+β)

What is cascade amplifier?

To meet given amplifier specifications two or more stages are cascaded in series, each stage can be identical or different. Such amplifiers are called cascade amplifiers. Often amplifier with common emitter multi stages is referred to as Cascade amplifier.

What is cascode amplifier?

Cascode amplifier is a Common base amplifier stage in cascade with common emitter stage. It combines the high resistance, large Transconductance (high gain) achieved in common emitter amplifier with the superior high frequency response of common base amplifier. It is designed to achieve wider bandwidth but dc gain approximately equal o that of common emitter amplifier.

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