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### Coupling of Amplifiers- Direct coupled amplifier

• 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.
• 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!!
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