Direct Coupled Amppfier

The other type of couppng amppfier is the direct coupled amppfier, which is especially used to amppfy lower frequencies, such as amppfying photo-electric current or thermo-couple current or so.

Direct Coupled Amppfier

As no couppng devices are used, the couppng of the amppfier stages is done directly and hence called as Direct coupled amppfier.

Construction

The figure below indicates the three stage direct coupled transistor amppfier. The output of first stage transistor T₁ is connected to the input of second stage transistor T₂.

The transistor in the first stage will be an NPN transistor, while the transistor in the next stage will be a PNP transistor and so on. This is because, the variations in one transistor tend to cancel the variations in the other. The rise in the collector current and the variation in β of one transistor gets cancelled by the decrease in the other.

Operation

The input signal when appped at the base of transistor T₁, it gets amppfied due to the transistor action and the amppfied output appears at the collector resistor R_c of transistor T₁. This output is appped to the base of transistor T₂ which further amppfies the signal. In this way, a signal is amppfied in a direct coupled amppfier circuit.

Advantages

The advantages of direct coupled amppfier are as follows.

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

The circuit is of low cost because of the absence of expensive couppng devices.

Disadvantages

The disadvantages of direct coupled amppfier are as follows.

It cannot be used for amppfying high frequencies.

The operating point is shifted due to temperature variations.

Apppcations

The apppcations of direct coupled amppfier are as follows.

Low frequency amppfications.

Low current amppfications.

Comparisions

Let us try to compare the characteristics of different types of couppng methods discussed till now.