Direct coupled amplifiers amplify lower frequencies by directly connecting the output of one transistor stage to the input of the next without any coupling components. They have a simple circuit arrangement with minimal components, making them low cost. However, they cannot amplify high frequencies and are susceptible to operating point shifts from temperature variations. Direct coupled amplifiers are well-suited for applications requiring low frequency or low current amplification such as photoelectric or thermoelectric sensors.

1. Direct Coupled
Amplifier

2. The definition
 As no coupling devices are used, the coupling of the amplifier
stages is done directly and hence called as Direct coupled
amplifier.
 It is especially used to amplify lower frequencies, such as
amplifying photo-electric current or thermo-couple current or
so.

3. 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.

4.  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.

5. 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 T2 which
further amplifies the signal. In this way, a signal is amplified in
a direct coupled amplifier circuit.

6. 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.

7. Disadvantages
 The disadvantages of direct coupled amplifier are as follows.
 It cannot be used for amplifying high frequencies.
 The operating point is shifted due to temperature variations.

8. Applications
 The applications of direct coupled amplifier are as follows.
 Low frequency amplifications.
 Low current amplifications.

9. Comparisons
Direct Coupling
Transformer
Coupling
RC CouplingParticularS.No
BestPoor
Excellent in audio
frequency range
Frequency
response
1
LeastMoreLessCost2
LeastMoreLessSpace and Weight3
GoodExcellentNot good
Impedance
matching
4
For amplifying
extremely low
frequencies
For Power
amplification
For voltage
amplification
Use5