Classifications of amplifiers

It is useful classify the amplifiers into four types.they are,

1.Voltage amplifier

2.Current amplifier

3. Transconductance amplifier

4. Transresistance amplifier

This classification based on the magnitudes of the input and output impedance of an amplifier relative to the source and load impedance.

1.Voltage amplifier :

The figure given in below shows the thevenin's equivalent circuit of a two port network which is represents the voltage amplifier.

from above voltage amplifier circuit consists of 

Ri = Input resistance Rs = source resistance Vs = supply voltage 

Ro = output resistance RL = load resistance .

• If the amplifier input resistance Ri is large as compared with source resistance Rs. 

Ri >> Rs Then 

Vi = Vs

• If the external load  resistance RL is large as compared with the output resistance Ro of the amplifier.
• The amplifier provides a voltage output proportional to the voltage input,so the proportionality factor is independent of the magnitudes of the source and the load resistance.such a circuit is called a voltage amplifier.
• An ideal voltage amplifier must have infinite input resistance Ri and zero output resistance Ro.  

2.Current amplifier :

• An ideal current amplifier is defined as amplifier which provides an output current proportional to the signal current.

The figure given below shows the current amplifier circuit .

• from above figure consists of source resistance Rs and input resistance Ri.
• This amplifier circuit has low input resistance and high output resistance.The proportionality factor is independent of Rs and RL.
• An Ideal current amplifier must have zero input resistance Ri and load resistance RL. 
• If the amplifier input resistance Ri is less as compared with source resistance Rs. 

                                        Ri << Rs 

3.Transconductance amplifier :

The figure given below show the transconductance amplifier circuit.

• A transconductance amplifier is represented by a thevenin equivalent in its input circuit and a norton equivalent in its output circuit.
• The ideal transconductance amplifier supplies an output current which is proportional to the signal voltage.
• The amplifier must have an infinite input resistance Ri and infinite output resistance Ro.
• From the figure transconductance amplifier has a large input resistance Ri as compared with source resistance Rs.

Ri >> Rs

4.Transresistance amplifier :

The figure given in below shows the Transresistance amplifier 

• A transresistance amplifier is represented by a norton equivalent in its input circuit and a thevenin's equivalent in its output circuit.
• From the equivalent circuit of an amplifier which ideally supplies an output voltage Vo in proportion to the signal current Is independently of Rs and RL.
• This amplifier has both the input and output resistance are very low relative to the source and load resistance.
• If the amplifier input resistance Ri is less as compared with source resistance Rs. 

                                        Ri << Rs