Direct Coupled Amplifier


The other type of coupling amplifier is the direct coupled amplifier, which is especially used to amplify lower frequencies, such as amplifying photo-electric current or thermo-couple current or so.

Direct Coupled Amplifier

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

Construction

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

Direct Coupled

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 applied at the base of transistor T1, it gets amplified due to the transistor action and the amplified output appears at the collector resistor Rc of transistor T1. This output is applied to the base of transistor T2which further amplifies the signal. In this way, a signal is amplified in a direct coupled amplifier circuit.

Advantages

The advantages of direct coupled amplifier 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 coupling devices.

Disadvantages

The disadvantages of direct coupled amplifier are as follows.

Applications

The applications of direct coupled amplifier are as follows.

Comparisions

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

S.No

Particular

RC Coupling

Transformer Coupling

Direct Coupling

1

Frequency response

Excellent in audio frequency range

Poor

Best

2

Cost

Less

More

Least

3

Space and Weight

Less

More

Least

4

Impedance matching

Not good

Excellent

Good

5

Use

For voltage amplification

For Power amplification

For amplifying extremely low frequencies