Amplifier Basics

What is an amplifier?

An amplifier is an electronic device which increases the strength of input signal fed to it. For example if A is the amplitude of input signal, after amplification the signal amplitude increases to g*A, where g is gain of amplifier.

What are the typical characteristic parameters of an amplifier?

Following are some of the characteristic parameters of amplifier

• Input resistance with no load:  Ri = Vi/Ii (load open circuited)
• Input resistance: Ri = Vi/Ii
• Open circuited voltage gain: Avo = Vo/Vi (load open circuited)
• Voltage gain: Av = Vo/Vi
• Short circuit current gain: Ais = Io/Ii (load short circuited)
• Current gain: Ai = Io/Ii
• Short circuit Transconductance: Gm = io/Vi (load open circuited)
• Output resistance of amplifier proper: Ro = Vx/ix  where Vx is voltage applied at the load by open circuiting the load and not considering the effect of signal input resistance on ix , ix is the current delivered by voltage source Vx.
• Output resistance: Ro = Vx/ix (input voltages short circuited) where Vx is voltage applied at the load by open circuiting the load and considering the effect of signal input resistance on ix , ix is the current delivered by voltage source Vx.
• Open circuit overall voltage gain Gvo = Vo/Vsig with load open circuited.
• Overall voltage gain Gv = Vo/Vsig.

Draw typical frequency response characteristics of an amplifier?

The typical frequency response of an amplifier resembles that of a band pass filter and is shown below

frequency response characteristics of amplifier

What is invariant in amplifiers?

Gain bandwidth product is invariant in amplifier. By providing the feedback the gain will be reduced by same factor with which the bandwidth is increased.

What is gain band width product?

The name itself suggests that it is the product of bandwidth and gain at which bandwidth is measured. It allows trade off between gain and bandwidth by introducing feedback in the system.

Which transistor technology is dominating in amplifiers?

BJT’s are more dominant technology used in amplifiers. Theoretically MOSFET’s are ideal amplifiers due to infinite impedance provided by them at the input terminal.  This is true for only low frequencies but at high frequencies internal capacitances introduce negative feedback which reduces the gain to a large extent.

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