Direct Coupled Amplifier

Why is it called as direct coupling?

• The output of the first stage is connected to input of the second stage without any coupling device therefore it is called as direct coupling amplifier.

Circuit Diagram

• The circuit diagram of three stage direct coupled amplifier is shown in the Figure. 
• It consists of three back to back connections of NPN transistor and PNP transistor. 
• The collector output of the first NPN transistor is connected to the input of the PNP transistor. 
• Similarly the collector output of the PNP transistor is connected to the input of the NPN transistor. 
• The coupling transformer and capacitor is not used at the output otherwise the amplifier becomes bulky at low frequencies.

Working

• The weak ac signal is applied at the base of the NPN transistor, this signal is amplified and output is available at the collector terminal of the NPN transistor. 
• The amplified output voltage is available at the base of the PNP transistor. 
• The output of the PNP transistor is available at the collector load.

Advantages

• Circuit design is very simple because it requires minimum use of resistors
• Low cost due to absence of coupling capacitor and coupling transformer

Disadvantages

• The operating point may be shifted due to temperature variations which lead to distortion in the output waveform
• This amplifier cannot be used for high frequencies applications.

Applications

• Low frequencies applications such as photo electric current, thermal electrical current

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