Direct coupled amplifier

Direct coupled amplifier

• 1.
  Direct Coupled Amplifier
• 2.
  The definition  Asno 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 figurebelow 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 transistorin 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 inputsignal 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 advantagesof 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 disadvantagesof 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 applicationsof direct coupled amplifier are as follows.  Low frequency amplifications.  Low current amplifications.
• 9.
  Comparisons Direct Coupling Transformer Coupling RC CouplingParticularS.No BestPoor Excellentin 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