The topic is part of electronics and deals with the coupling of two-stage or multi-stage amplifiers without the use of any resistors, capacitors or transformers.

1. Department of physics
Electronics Seminar
Direct Coupled Amplifier
Presented By- Tanisha Shukla
MSc Physics Sem-1
Rashtrasant Tukadoji Maharaj nagpur university, nagpur

2. Outline
 Why do we needcouple devices?
 Introduction
 Circuit Diagram
 Construction
 Working
 Current & drift
 Frequency response
 Meritsand demerits
 applications

3. Why we need to couple devices?
 The basic purposes of a coupling device are :
To transfer the AC from the output of one stage to the
input of next stage
To block the DC to pass from the output of one stage to
the input of next stage, which means to isolate the DC
conditions.

4. direct couple amplifier
The output of first stage transistor is directly coupled with
input of second stage transistor
“since no coupling device is used here hence, called as
direct coupling amplifier”
It is especially used to amplify lower frequencies, such as
amplifying photo-electric current or thermo-couple current .
It was invented by Harold J Paz and Francis P. Keiper Jr.
in 1955.

5. circuit Diagram

6. construction
1. In this two stage direct coupled transistor amplifier output of first stage transistor
Q1 is connected to the input of second stage transistor Q2 which are of npn type.
2. R1 and R2 are the voltage divider biasing
3. Resistor Rc1 ,Rc2 and Re1 , Re2 across collector and emitter terminal of
transistor Q1 and Q2 is fabricated respectively.
4. Collector voltage Vcc is supplied to each transistor and load resistor Rl is
connected across output .
5. Input ac signal Vin (low frequency) is supplied to transistor Q1 and output Vout
signal (amplified) is obtained across Q2.

7. working
 A very low frequency (below 10 hz
or 0 frequency ) signal is applied
 There is no use of inductor,
capacitor, resistor for coupling in
the amplifier
 Voltage divider biasing is done to
the first stage of 2- stage direct
coupled amplifier for keeping Q-
point at the centre of Dc load line.

8.  Output of 1stage is feeded to input of input of 2
 CE Transistor-amplifier will give 180 phase shift across Rc and
this is again experience a shift by 180 phase
 Output Vout is obtained across Rl
 Observation
 Amplified signal is obtained across o/p
 It can amplifiy signal of very low frequency even 0.

9. Multi stage (Three- stage)
 Consequtive npn and
pnp transitor are
used
to cancel out the
variations came in B
due to increase in Ic
and temperature

10. Current
 A direct-coupled amplifier is
also called a "low pass amplifier"
because its frequency response
resembles that of a low pass
filter.
 Amplification of DC (zero
frequency) is possible only with
this amplifier, and later it
becomes a building block of
differential amplifiers and
operational amplifiers.
Drift
 Direct-coupled amplifiers
constructed in the form of
cascaded traditional single-
ended amplifiers suffer from
output voltage drift over time
and temperature.
 Many high-gain applications
require offset adjustment to
combat drift. The drift
problem can be solved by
using a difference amplifier.

11.  The frequency response curve is flat up to upper cutoff
frequency f2 which is determined by stray wiring capacitance
and internal transistor capacitances
 Above this, the gain decreases due to interelectrode
capacitance of device and wiring capacitance.
Low –pass filter It only passes signals below cutoff frequency while attenuating
above signals.
Frequency
Response

12. Merits
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.
Demerits
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.

13. Applications of direct coupled amplifier
 Pulse amplifier
 Differential Amplifier
 Regulator circuits of
electronic power supply
 Computer circuitry
 Jet engine control
 Electronic instruments
Thank You!!