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How to design an Amplifier using given Gain, Input Impedance and output Impedance.

Updated on November 8, 2010


  • Amplifier is the device which provides amplification.
  • Amplification is the increase the strength of a signal, the signal being of any type: voltage, current or power of a circuit.
  • There are many forms of electronic circuits classed as amplifiers, from Operational Amplifiers and Small Signal Amplifiers up to Large Signal and Power Amplifiers. Amplifiers can be thought of as a simple box or block containing the amplifying device, such as a Transistor, Field Effect Transistor or Op-amp, which has two input terminals and two output terminals (ground being common) with the output signal being much greater than that of the input signal as it has been "Amplified". An amplifier’s performance is defined as high only if the amplified signal contains none or minimum number of distortions or alterations when comparing with the original input signal. Amplifiers are of different types.
  • They are used in radios, television transmitters and receivers and even in most of the electronic instruments. An ideal amplifier has three main properties, Input Resistance, Output Resistance and of course amplification known commonly as Gain. No matter how complicated an amplifier circuit is because a general amplifier model can be used to show the relationship of those three properties.

Learning by Doing

Lets take this as an example to design a transistor amplifier which achieves these three basic requirements.

  • Gain of 50
  • Input impedance of 10kΩ
  • Output impedance of 10Ω


Selecting A transistor

  • In an example like this, we have to follow given parameters and when we make assumptions we have to mention them clearly in order to get better results. In an example like this,
  • We hope to use BC107, in separated biasing methods. The first transistor is in voltage-divider bias and the second one is in emitter follower bias.Because we cant get all three parameters in a single transistor bias.
  • From the data sheet of BC107, we can get the Input impedance and Gain of transistor, and also we can get VBE value as 0.7v.

    We used the first transistor to match the input impedance which is 10kΩ and to give the needed gain to the amplifier. The emitter follower is used here as its gain is 1 and this gain will not affect the overall gain of the circuit. The second transistor used to get the needed output impedance. By this addition the overall gain of the amplifier can be reduced in small values. But according to our mathematical calculations we can prove that given parameters are already achieved.

Hope this article would be useful for anyone who is interested in electronics specially for the students.


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      NTLANGANO S 5 years ago

      This is so informative, thnx 2 u. thumbz up!!

    • profile image

      great 5 years ago


    • lakshanhere profile image

      Lakshan Thiranagama 6 years ago from Colombo, Sri Lanka

      Thank you Yousef, Thank you Xhunanh and Thank you sir, the Engineer...!!!

      @Xhunanh: Yes friend. Hope to add something to here soon. Actually I'm quite busy these days. Else give me your mail address I'll send you a pdf about PIC programming. Thank you..:)

    • profile image

      Xhunanh Nguyen 6 years ago

      thnx loads for sharing this....please post an article about PIC programming. Hopefully waiting for it. I like your simple english very much. Thanks again.


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      Yousef dannan 6 years ago

      thx a lot for this useful information great job and i hope to see more thank you again

    • lakshanhere profile image

      Lakshan Thiranagama 7 years ago from Colombo, Sri Lanka

      Wow...lots of interesting stuffs....thanks for sharing. keep up good works...!!!!!

      hugs and cheers!!

    • profile image

      EIDESK 7 years ago

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      EID 7 years ago

      see this

    • lakshanhere profile image

      Lakshan Thiranagama 7 years ago from Colombo, Sri Lanka

      ;) :)

    • profile image

      Janaka Bandara 7 years ago

      report, edited version. ;)