Working, Advantages and Disadvantages of Direct Coupled Amplifier

Direct Coupled Amplifier

The direct coupled amplifier suits well for very low frequency applications. Since the circuit especially works at low frequencies, the coupling capacitor and transformers are not used because the electrical size of these components become very large at these frequencies.

The above fig shows the circuit of three stage direct coupled amplifier. It uses complementary transistor. Thus, the first stage uses npn transistor, the second stage uses pnp transistor and so on. In this circuit the output of first transistor is directly connected to the input of second transistor.

Working: The weak signal is applied to the input of first transistor T₁. Due to transistor action, an amplified output is obtained across the collector load R_C of transistor T₁. This voltage drives the base of the second transistor and amplified output is obtained across its collector load. In this way, direct coupled amplifier raises the strength of weak signal.

Advantages and Disadvantages of Direct Coupled Amplifier

Advantages:

1.       The circuit arrangement is simple because of minimum use of resistors.

2.       The circuit has low cost because of the absence of expensive coupling devices.

Disadvantages:

1.       It cannot be used for amplifying high frequencies.

2.       The operating point is shifted due to temperature variations.

Applications of Direct Coupled Amplifier

These are used to amplify low frequency (<10 Hz) signals. For example amplifying photoelectric current, thermocouple current etc.